Indian Journalof Experimental Biology Vol. 43, July 200 pp. 575-595

Review Article

Genetics of sexual behaviour in Drosophila

Seema Sisodia & B N Singh* Genetics Laboratory, Department of Zoology, Banaras Hindu University, Varanasi . India

The analysis of genetics of behaviourwithin and between speciesprovides important clues aboutthe forces shaping the evolution of behavioural genes. In· Drosophila. a number of key processes such as emergence from the pupal case, locomotor activity, feeding, olfaction and aspects of mating behaviour are under circadian regulation. Genes controlling sexual behaviour are likely to control species specific differences in courtship that are involved in reproductive isolation of closely related species. Courtship in Drosophila is characterized by a series of stereotyped behaviours that lead to copulation and more than 30 genes have been identified through mutations that affect one or more of these elements. Although curiosity about behavioural differences between the sexes undoubtedly predates recorded history, little efforts have been made to uncover the molecular basis of male and female courtship. The brain and nervous system functions that underlie sex-specific behaviour are of obvious importance to all animals including humans. To understand behaviourrelated to sex it is important to distinguish those aspects that are controlled genetically. The isolation and analysis of Drosophila mutants with altered sexual orientation lead to the identification of novel branches iii the sex-determination cascade, which govern the sexually dimorphic developmentof the nervous system.

Keywords: Drosophila, Sexual behaviour

Sexual behaviour in Drosophila is among the more this review, some aspects of sexual behaviour in important of these fruitflies' actions, both from the Drosophila arefo cused. organism's perspective and in terms of how its nervous system can be investigated. Reproductive Courtship behaviour in Drosophila has long been a heavily Mating behaviour of Drosophila species consists of genetic subject. In fact, some of the first behavioural species-specific fixed action patterns, which are · genetic experiments involved Drosophila courtship accompanied by orientation movements. In 1915 and mating, and these genetic studies in turn were the Sturtevant first described the courtship of first behavioural ones involving this organism. Since D. melanogaster and attempted to identify the stimuli that time a number of further genetic variants have involved. Subsequently, he described the courtship of 2 been applied to analyze the fruitfly's reproduction, 22 species.-within Drosophila and related genera • including to some extent, the neural substrates of the . Two basic genetic approaches have been taken to re levant behaviours. Thus, reproductive "behaviour study courtship behaviour in Drosophila. The first genetics" became in part "neurogentics". The pioneer approach is to dissect the behavioural phenotype into Drosophila geneticist Alfred Sturtevant was the first its simplest genetic components and uses the tools of to study the genetic basis of pre-mating behaviour of molecular and developmental genetics to unravel the D. melanogaster. He observed the behaviour of four . biochemical basis that permits both the flexibility and mutants (yellow, curved, white and vermillion) and rigidity of behaviour. This is the singh� gene approach found that yellow, curved and white males were all or neurogenetic approach. Second approach is through less vigorous than wild type or vermillion males I . quantitative genetics, which seeks the partitioning of Courtship in Drosophila is influenced by a wide phenotypic variability into genetic and environmental variety of genes, in that many different kinds of sources. With the possibility of obtaining accurate and pleiotropic mutations lead to defective courtship. In rapid information on male courtship behaviour using a time-sampling technique3, it is now possible to * Correspondent author investigate the genetic architecture of individual Phone : 91-542-2307 149 ( 0). 2575185 (R) elements of male courtship sequence. These Fax : 91-5452-2368179 individual elements are shown to be under different E-mail�bnsingh @bhu.ac.in [email protected] genetic controls. It should now be possible to use this 576 INDIAN J EXP BIOL, JULY 2005

model of male courtship behaviour to investigate in by the malesl l . Ewing and Bennet-Clark l2 examined greater depth the role of genotype-envir:onment the sound produced by members of the melanogaster interaction in the evolution of behavioural characters and obscura species groups within the subgenus

closely related to fitness4. Sophophora . .The results suggest that males of many Bennet-Clarke and Ewing5 reported pulse interval Drosophila species perform wing displays during as a critical parameter in the courtship song of courtship and there is evidence that the stimuli D. melanogaster. D. melanogaster males produce provided by these are primarily acoustic. The songs courtship song which sexually stimulate the females. consisted of regularly repeated pulses of sound. The 6 Hoikkala and Lumme studied genetic basis of frequency within the pulse and interval between them evolution of the male courtship sounds in D. virilis varied between species but was constant and group. The results indicate that an X-chromosomal characteristic for each species. It is suggested that the maj or change allowing variation in IPI has occurred songs are codes, which allow females to recognize during the separation of the two D. virilis group conspecific mates. Most animals communicate phylads, the long IPI allowing variation also in length acoustically using a transducer, which causes of pulse (PL) and number of cycles in a pulse (N). vibrations in the medium around them. These The evolution of sounds in the virilis phylad has variations can be received at a distance by a vibration probably gene towards longer and denser pulse train, sensitive receptor on another animal. By producing a while in montana phylad the sounds have evolved in patterned train of vibrations, the animal producing the different directions. The interpulse interval (IPI) of sound can provide a species-specific signal, which courtship song in the D. auraria complex is the only may be important in the sexual or social behaviour of parameter that is consistently species -specific among the animal. Insects have many different types of several courtship elements examined within the auditory organs. The simplest of these are the sensory 7 complex. Tomaru and Oguma examined the genetic hairs, which respond to displacement of the D. auraria basis of species-specific courtship song of surrounding air. These receptors are fo und on the anal complex by analysing the song of interspecific cerci and on other parts of the body and they have a hybrids and of backcross progeny. low frequency response, giving a synchronous Courtship songs in many species are species­ nervous discharge at frequencies up to about 300 Hz. specific and play a role in sexual isolation between In Drosophila the antennal arista can be set in closely related speciesB• Pugh and Ritchie9 reported vibration by sounds of 180 to 220 Hz and the that IPI (an acoustic mating signal), which plays an amplitude cif vibration is greatest when sounds arrive important role in courtship, has a polygenic from 90° to the plane of arista about 30° away fro m determination. The genes determining the difference sagittal axis. According to Ewingl3 presence of the in mean IPI between D. simulans and D. mauritiana "low-repetition rate song" in hybrids of D. are distributed evenly throughout the genome, with pseudoobscura and D. persimilis is determined by genes on each major chromosome are contributing genes on the X- chromosome. Some mutant genes, approximately equally to the difference between · affecting among other things the male courtship et al. 10 period species. Alt reported that gene controls sounds, have been mapped to the X chromosome of D. melanogaster. courtship song cycles in They D. meianogaster. Von Schilcher14 has isolated a applied wavelet decomposition and discrete mutant called cacophony in which interpulse intervals Fourier transform techniques for analysing the pulse and the pulse lengths of the courtship sound are very portion of the mating song of D. melanogaster. Using long and the pulses polycyclic. Kyriacou and Hall15 these techniques, they were able to confirm the have found that sex linked circadian rhythm mutations presence of cycles in the IPI on pulse song of some of D. melanogaster affect the short-term fluctuations individuals, and found a significant effect of per gene of the inter pulse intervals in the male courtship on cycling in IPI. sound. Aldinger-von Kleist16 investigated the genetic All species of Drosophila perform fairly complex basis of light independence (lin) in the courtship and and stereotyped courtship. While in some species mating behaviour of D. subobscura. By the use of such as D. nannoptera and D. willistoni ' sexually chromosomes marked with different alleles of enz,yme receptive fe males show an acceptance posture most cif · loci, the genetic effect of each of the fo ur autosomes the overt sexual behaviour one observes is carried but · was determined. SISODIA & SINGH : SEXUAL BEHAVIOUR IN DROSOPHILA 577

Males of many large Drosophila species of Hawaii deficient (olf D) flies of D. melanogaster. Virgin characteristically show elaborate, species-specific females respond to male courtship by slowing and courtship pattern. Male characters related to courtship finally stopping their movements, which appears to attract particular attention since they frequently enhance the probability of copulation. Neurogenetics constitute the most conspicuous taxonomic is a promising approach for studying problems in differences between Hawaiian species. Carson 17 neurobiology and behaviour26-28. Von SchiIcher and studied genetic variation in a courtship-related male Hale9 studied neural topography of courtship song in character in D. silvestris from a single Hawaiian sex mosaics of D. melanogaster. The control of locality, displaying a brush of large dorsal cilia on the courtship song in D. melanogaster was mapped to the tibia of the foreleg of the male fly. This is used to central nervous system using. sex mosaics. Some stimulate the female during courtship. The results male-female mosaics follow females and extend their indieate the existence of ample genetic variance for wings at them. This behaviour is associated with male this character, existing as a balanced polymorphism in brain tissue only about 2/3 of the courting mosaics the natural population. The character appears to be have a normal pulse song when courting; the rest under stabilizing selection. . produces abnormal sonic output when extending and Information is there about courtship and the genetic vibrating their wings, or no sound at all. basis of courtship · in D. melanogaster and Weibergen et al. 30 compared the courtship D. simulans. However, very little work has been done behaviour of the sympatric species D. melanogaster on the hybrids of these two s cies. Manning18 and and D. simulans. The results suggest that both D. Von SchiIcher and Manning1 Freported that hybrid simulans males and females are less active in males preformed to court D. simulans females. They performing their behaviour tharrD. melanogaster. . also measured the courtship song and characterized The courtship sounds may act as pre mating the mating speed for hybrid males. Wood et al.20 isolation mechanisms maintaining species integrity. analyzed the courtship sequences of the hybrids By using autosomal recessive markers, Hoikkala and between D. melanogaster and D. simulans. The Lumme31 made a detailed genetic analysis of the hybrid males from special reciprocal crosses did not difference between the male courtship sounds of D. differ. While D. melanogaster and D. simulans males virilis and D. lummei. The results demonstrated that differed markedly for the majority of transition genes affecting the species difference in the number studied, hybrid males showed no consistent patterns of pulses in a pulse train and located on each of the with the parent species, being indistinguishable from' large autosomes but not on the X-chromosome and D. simulans males, indistinguishable from that, the nonadditive interactions between genes on D. melanogaster or intermediate between them, different autosomes are rather weak. Averhof f and depending on the trait observed. This suggests ' Richardson32 studied pheromone system controlling independent genetic control of these transitions during mating in D. meianoga5ter. The variation among male courtship. pheromones is a common requirement for stimulation The courtship sound of D. littoratis is quite unique of either sex. The pheromone which emitted by in the D. virilis group21. Hoikkala22 studied the courting male to female involves both volatile as well genetic variation in the male courtship sound of as nonvolatile component. The volatile components D. littoralis. The results suggest that there exists are associated with loci on the second and/or third genetic variation in· different traits of the courtship chromosomes while the nonvolatile components are sound of D. littoralis. Mutations affecting behavioural associated with Xlor fourth chromosomes. These are characters could provide the key to a better loci associated with female signals on the X and/or understanding of the structural and functional fourth chromosomes that induce males to court. complexities of the nervous system23•24. Olfactory The first single-gene mutation that was isolated stimuli play a significant role in the reproductive based on specific effects on the courtship song is interactions of males and females and that they very cacophony 14. Kulkarni and Hale3 carried out a likely help to "trigger" the initiation of courtship as detailed analysis of X-chromosomal mutation well as accentuate the chances of successful cacophony (cae) on courtship song and other

copulation. Gailey et al.25 . compared the courtship behaviours. The mutation cacophony causes the behaviours of normal and genetically olfaction- production of polycyclic pulses readily 578 INDIAN J EXPBIOL, JULY 2005

distinguishable from those in wild .type, which are D. melanogaster by using spectral treatments of the mono- or bicyclic. Kulkarni et al. 34 studied courtship fluctuating rates of tone pulse production that occur song of dissonance mutant of D. melanogaster. The during courtship. The results suggest that courtship dissonance mutation of courtship song was induced songs are strongly rhythmic except for courtships OJ by chemical mutagenesis. The X-chromosomal performed by per males. Homosexual courtship has mutation causes the D. melanogaster male's acoustical described as a "male mistake" that occurs because output, resulting from his wing vibrations directed at a males are so motivated to mate that they fail to female, to include very long and loud tone "pulses". discriminate between conspecific males and females. Konopka imd Benzer35 reported that courtship song Wheeler et al. 37 applied . spectral analyses to the rhythm of D. melanogaster was changed by mutation courtship song of the ' mutants cacophony (cac), at the period (per) locus, which also controls a dissonance (diss), fruitless (jru) and period (per) as circadian eclosion and locomotory rhythm. crosslei6 well as to the double mutant cac diss. Aberrant could not confirm the existence of rhythm in D. intervals between song pulses were observed in diss, melanogaster courtship song. cae, cac diss, and fru songs. diss males displayed a et al 7 defect in song hums manifested by an irregular sine Wheeler .3 applied spectral analysis to the . courtship songs of the mutants cacophony (cac), wave, although the fundamental frequencies were dissonance (diss), fruitless (jru), and period (per), as normal. Sine song frequencies and intrerpulse well as to double mutant cac diss of D. melanogaster. frequencies were aberrant in cac diss males. Two per l' ' fru is a homozygous-viable, male-sterile mutant that mutant alleles (pe and pet ) were associated with was induced with X-rays38.. The fru courtship normal song pulses and hums. mutation was dissected into three defects of male Sexual behaviour in fruitflies has been dissected in reproductive behaviour, which were separable as to part by analysis of the behaviour of genetic mosaics their genetic etiologies by application of existing and that are part male, part female; anatomical 'foci' newly induced chromosomal aberrations. Gailey and, controlling various components of male behaviour Hall 39 studied the behaviour and cytogenetics of have been identified42.43,. Male courtship precedes as a fruitless in D. melanogaster. They reported that sequence of behaviour involving, progressively, the different courtship defects caused by separate, closely animal's head to its abdomen (for references see 44 46 linked lesions. Thus, uncovering the fru distal Siegel et al. ; Hall 45). Gailey et al. reported that a breakpoint with deletion usualfy led to males with two genetically defined element of the fruitless (jru) locus of the fru courtship abnormalities: no copulation in D. melanogaster regulates the development of a attempts with females and vigorous courtship among male-specific muscle spanning the fifth abdominal males, including the formation of "courtship chains". segment in adult males, the Muscles of Lawrence However, certain genetic changes involving region (MOL). Elements of the fruitless courtship locus, 91B resulted in males who formed courtship chains when genetically aberrant in a variety of ways, not but also mated with fe males. Uncovering the fru only block a male's ability to court the abdomen proximal breakpoint led to males that passively elicit during courtship (which causes behavioral steri lity: inappropriately high levels of courtship. This Gailey & Hall 39). elicitation property was separable genetically from the Tompkins47 studied the genetic control of sexual sterility and chain formation phenotypes and behaviour in D. melallogaster and concluded that in provisionally mapped to the interval 89F-90F, which D. melanogaster, the male's ability to perform includes the fru proximal breakpoint. The lack of courtship and the females ability to elicit these basiconic antennal sensilla in the mutant lozenge was behaviour are controlled by the number of X­ used by Stocker and Gendre40 to assess the role of chromosomes in specific tissue, a gene called sex­ these olfactory receptors in the courtship behaviour of lethal that respond to the number of X-chromosomes, D. melanogaster. Under normal light conditions, and several genes that are regulated by sex-lethal. lozenge males courted virgin females much less, than Genetic analysis of species-specific characters can wild type males did. reveal the evolutionary significance of species 41 Kyriacou et al. studied courtship songs of normal differences. If a species-specific characteristic males and those expressing short period, long period involved reproductive isolation, the process of and arrhythmic mutations at the period (per) locus of speciation should be traceable by means of genetic SISODIA & SINGH : SEXUAL BEHAVIOUR IN DROSOPHILA 579

analysis. Hybrid and chromosomal analyses were of the sex-determination hierarchy functioning used to clarify the genetic basis of species-specific specifically in the central nervous system (CNS) courtship. Zouros480bserved that species-specific (reviewed by McKeown55). courtship behaviour of different sexes between The phenotypes of new fru mutants encompass D. arizonensis and D. majavensis were controlled by nearly all aspects of male sexual behaviour. different chromosomes; male behaviour was affected Alternative splicing of fr u transcripts produces sex­ by the Y chromosome and one autosome, whereas specific proteins belonging to the BTB-ZF fa mily of female behaviour was affected by two other transcriptional regulators. The sex-specific fr u 5 chromosomes. In D. melanogaster and D. simulans products are produced in only about 500 of the 10 the X chromosomes'9 or the autosomes49-51 play a neurons that comprise the CNS. The properties of significant role in the determination of courtship song neurons expressing these fru products suggest that fru and the X-chromosome influences the mating success specifies the fates or activities of neurons that carry of females49• out higher order control fu nction to elicit and Villella and Hall52 examined the role played by the coordinate the activities comprising male courtship sex-determining gene double sex (dsx) and its behaviouy6 The courtship song of male influence on D. melanogaster courtship. Several D. melanogaster influences mating success and may investigations have concluded that a sex-specific have been involved in speciation within the muscle (MOL) is unaffected by genetic variation at melanogaster subgroup. Ritchie and Kyriacou57 this locus53. Analyses of several reproductive examined variability of two important parameters of behaviours and control for genetic background effects courtship song, inter-pulse interval and hum-song indicated that the XY dsx mutants are impaired in frequency of a population of D. melanogaster. The their willingness to court females. However, these re·sults suggest that D. melanogaster song shows mutants never produced courtship-humming sounds. relatively low phenotypic and additive genetic Mature XY dsx flies elicited anomalously high levels variability, which is consistent with most expectations of courtship; that this occurs merely because of a for an important mating signal. The interpulse interval delay in imaginal development. of the courtship song of D. melanogaster is a Courtship in Drosophila is influenced by a wide character, which may play a significant role in mating 8 variety of genes, in that many different kinds of success and reproductive isolation. Ritchie et al. 5 pleiotropic mutations lead to defective courtship. studied genetic variability of the inter-pulse interval "Courtship genes" that seem to play more particular of courtship song among some European pop�lations roles were originally identified as sensory, learning or of D. melanogaster. The results suggest that the IPI is rhythm mutations, their reproductive abnormalities subject to strong selection, as the populations are have been especially informative fo r revealing known to differ for other characters. Reciprocal F1s components of male or female actions that might and backcrosses implied that the variance was otherwise have gone unnoticed. Further, behavioural predominantly additive and autosomal. mutations seemed originally to be courtship-specific, Tomaru et at. 59 studied the effect of courtship song turned out not to have that property and have led to a in interspecific crosses among the species of the D. broadened perspective on the nature and action of auraria complex. The results suggest that the sound Drosophila's sex-determination genes. . produced by wing vibration plays an important role A fe male indicates her receptivity by allowing or and that the wing movement itself is less important. helping the male to fertilize her eggs. Thus, Barnes et at. 60 examined flightless mutations that fertilization seems at least partly controlled by female affect wing-beat frequency of D. melanogaster fo r behaviour. Females play an active role in their effect on male courtship. The results suggest that reproduction (for review see Ring054). Like other mutation drastically reduces wing beat frequency aspects of sexual differentiation, sexual behaviour is during tethered flight; they do not have any similar governed by a hierarchy of sex-determining major effect on courtship. regulatory gene. The Sxl, tra and tra-2 genes control Chemical signals from males play an important all aspects of somatic sexual differentiation. Sexual role in stimulating D. melanogaster females to mate orientation and courtship behaviour in Drosophila an� and male-dominant pheromones may influence a regulated by fruitless (jru), the first gene in a branch female's choice of mater. Scott and Richmond61 580 INDIAN J EXP SIOL, JULY 2005

described a genetic analysis of male-predominant attributes of leks are the mechanisms of sexual pheromones based on. crosses between strains in selection. Leks have been emphasized as model which the predominant male hydrocarbon is either systems for studying sexual selection. 7-tricosene or 7-pentacosene. The results suggest that Besides courtship and copUlation, males of some the expression of both the compounds is controlled by species show other types of sexual behaviour. These X-linked loci and by at least two different groups of behaviours are agonistic displays (aggregation, autosomal loci. Finally, X-linked loci also play a defense and submission), sexual advertisement, significant role in the production of these territoriality and communal displays (for review see 6 65 hydrocarbons. Spieth & Ringo 4). Ring0 studied the sexual and Cobb and Ferveur62 reviewed the behavioral non-sexual behaviour of D. grimsilawi, a lek species. genetic and evolutionary studies of courtship He found that only sexually mating males show fo ur behaviour, its fu nction and its bases for the four lek behaviours (courtship, sexual advertisement, species of the D. melanogaster species complex contact aggression and communal displays). The (D. melanogaster, D. simulans, D. mauritiana and factor that determines the specific male dispersion D. sechellia). They emphasized on the role of sexual patterns on lek may vary greatly among species and selection and of sensory mechanisms in shaping are of particular relevance to sexual selection 66 courtship and particularly the role of chemosensory theory . factors. In Drosophila male courtship, behaviour is The factor primarily responsible for the fo rmation governed by the action of a set of regulatory genes of leks and the pattern of dispersion on leks may that control all aspects of somatic sexual reveal a great deal about the nature of sexual selection differentiation. The wild type fu nction of one of these operating in a species. If leks are male-initiated, regulatory genes, transformer-2 (tra-2) is necessary intense male competition may be an important for fe male sexual differentiation in the absence of tra- evolutionary force within that species. However, if 2 fu nciton XX individuals differentiate as males. leks form for female-initiated reasons, such as to Belote and Baker63 studied sexual behaviour and its allow better discrimination of potential mates, males genetic control during development and adulthood in should be distributed in ways that allow for effective D. 1Jlelanogaster. The results suggest that the choice. D. grimshawi is lek forming member of the regulatory hierarchy controlling sexual differentiation picture-winged group of Hawaiian Drosophila. Male is functioning in the adult central nervous system. mating signal and female response to male signals is More importantly, these results suggest that the adult important for animal communication and sexual central nervous system has some functional plasticity selection. Drone/7 studied environmental influences with respect to the innate behavioural pattern of male on male courtship and implications for female choice courtship and is maintained in a particular state of in a lekking Hawaiian Drosophila, D. grimshawi. It differentiation by the active control of gene was observed that males fe d high-protein diets were expression in the adult. in better physical condition, courted more vigorously and mated more often and sooner than males fed low­ Lek behaviour protein diets. Leks can be defined generally as groups of D. planitibia, D. heteroneura and D. silvestris are displaying males that are visited by fe males for the the three most recently evolved species of the sole purpose of mating. If females do not form social Hawaiian picture-winged D. planitibia sub group. group, then males may aggregate at a traditional site Hoikkala and Welbergen68 studied signals and for the purposes of display and courtship. This area is . responses of fe males and males in successfu l and known as lek. Females visit lek and choose among the unsuccessful courtship of three Hawaiian 1ek mating males. Details of lek organization vary among taxa Drosophila species. Lek polygyny is an uncommon 6 but general behavioural characteristics of males and mating system 9, which may . be defined as an fe males of lek species are similar, males are highly aggregated male display that fe males attend primarily competitive and often possess extreme morphological for the purpose of ferti lization. D. mycetophaga is one traits and fe male appear highly discriminating in their of the few Drosophila species in which a lek mating choice of mates. Male competition and female choice system has been reported. Males of this species tend are the characteristic features of leks and these two to form mating aggregations on the white areas SISODlA & SINGH : SEXUAL BEHAVIOUR IN DROSOPHILA 581

underneath hard bracket fu ngi and fe males visit those shaping persp.ectives of the role of female choice in aggregations for the purpose of fertilization. Aspi and sexual selection. Variation in male mating success is ' Hoffaman 70 examined male costs and benefits most pronounced in leks in which 10-20% of males associated with male display size in field populations often obtain 70- 80% of the mating (for review see of an Australian lekking Drosophila species Wilei\ D. mycetophaga. Results suggest that male mating Mackenzie et al.76 present simple models to success is positively correlated with display size since quantity the potential contributions to variation in matings appeared to be more common in large male mating success from a variety of processes: (1) displays, Several mechanisms to explain the average male mating success, (2) variation in male development and maintenance of lek system have territory quality, (3) males attendance times at lek, 7 been discussed (for review see Hoglund & Alatal0 1 ). and (4) degree of fe male synchrony, aggregation and In general these mechanisms fall into classes of copying. Variation in mating success is fundamental female and male initiated models although some to sexual selection and readily ql.nntifiable in fi eld models incorporate elements from both sexes. Female studies. initiated models suggest that male clustering is Genetic fa ctors affe cting sexual behaviour produced by dominant effect of female choice, Inversion polymorphism and sexual activity whereas in male initiated models, active fe male It is known that inversion polymorphism in the choice has no effect on male settlement. However, in genus Drosophila is of freq'uent occurrence and has D. mycetophaga aggregation size could provide some adaptive function 77,78. The correlation between sexual information about male quality since larger males activity and chromosomal polymorphism has been may provide genetic or non-genetic benefits like found in many species of Drosophila. Spiess and better fertilization ability or resources in the fo rm of 79 7 Langer found mating speed differences for different seminal fluids (see Markow & Ankney 2). homokaryotypes in D. pseudoobscura such as AR The degree of territoriality seems to vary in karyotypes mated the most rapidly followed in order different drosophilids fo rming aggregations. In a by ST, CH, TL and PP. Kaul and Parsons80 fo und the South American Zygotricha species, the males defend karyotypes of the mate to be critical importance in territories against con specific males and males of a determining both mating speed and duration of Hawaiian species, D. conformis defend a single leaf copulation for all possible combinations between the 3 against other males7 • However, in D. grimshawii, three karyotypes ST/ST, SXICH and CHICH. another Hawaiian species forming aggregation Many species of Drosophila exhibit chromosomal territoriality seems to be mating arenas67• In polymorphism due to inversions of chromosome. D.

D. mycetophaga, males did not defend spatially fixed pavani IS with a stable polymorphism for territories but behaved aggressively and maintained a chromosomal inversions. Brncic and Koref­ distance from other males. This suggests that males Santibanez81 studied mating activity of homo- and defended moving territOlies similar to those reported heterokaryotypes in D. pavani. The results indicate in some vertebrates 74 in which the males maintain a that the greater mating activity is an expression of the zone within which other males are not tolerated. heterotic properties of the heterokaryotypes. It may be 73 Shelly studied male spacing, aggression and female an important factor in the maintenance of balanced visitation in a Hawaiian Drosophila, D. conformis. polymorphism in natural population. It has become The results indicate that at the lek studied. fe males D. apparent that the male reproductive component of conformis preferentially visited and mated with males fitness, virility may play a more important role than occupying the lowest territories within a particular fitness variables in preadult stages and female leaf cluster. In D. conformis, competition among fertility. Kosuda82 studied aging effect on m,ale mating males was more intense for low territories than for activity in D. meLanogaster under chromosomally higher ones. Furthermore, it appears that a male's homozygous and heterozygous conditions. position within the lek and his status in aggressive Differences among , homozygous lines were highly interactions were strongly size dependent with larger significant for old and young males, indicating a 3 males usually occupying lower territories and chasing genetic basis for the trait. Clarke and Feldman8 smaller males. Studies of variation in male mating investigated interactions between ' genes on specific success within populations have been instrumental in inversions marked by cy and pm in the second 582 INDIAN J EXPBIOL, mL Y 2005

chromosome of D. melanogaster in a heterozygous populations of living organisms can change condition for various fi tness components. The results genetically through time. Usually, it is assumed that suggest that there is no interchromosomal interaction such changes are gradual and slow. Pascual et al. 95 between two major autosomes in male mating studied genetic changes in mating activity in activity. Kosuda84 studied genetic variability in laboratory strains of D. subobscura. The results mating activity of D. melanogaster male. Male mating indicate that mating activity increases with the time of activity was measured for 29 lines of D. melanogaster maintenance under laboratory conditions. In certain made homozygous fo r second chromosome. Genetic cases, mating activity is also affected due to enzyme differences between lines were found to be highly variants. In D. melanogaster, males heterozygous at significant. Mating activity of homozygous males was the Adh locus show higher mating ability as compared 6 7 much lower than that of heterozygous ones. to homozygotes9 . Gilbert and Richmond9 reported Spiess and Yu85 studied relative mating activity of that D. melanogaster males with active esterase-6 the sexes in homokaryo types of D. persimilis from a mate sooner, copulate for shorter duration and Redwoods population. Results suggest that mating produce more progeny per mating than do esterase-6 activities of the two sexes were uncorrelated null males at 16°C. However, there is no correlation indicating that either sexes have independent genetic between Est- l polymorphism and mating activity in systems ' controlling mating activities or that the certain species of the D. bipectinata species 9 expression of the' same genetic system is influenced complex98.9 • by sex. Since the hybrid males of the two karyotypes Genetic analysis by artificialselection experiments displayed different courtship activities, while the Selection for mating propensity was successful in fe males were 'at about an equal level of receptivity, D. melanogaster, D. pseudoobscura, D. ananassae intrasexual selection among males is likely to be and D. bipectinata but some obvious differences were important in nature. That genetic structure within found among these four species. A response to naturally occurring karyotypes has influence on the selection for mating propensity was obtained by . 1 mating s eed has been shown by Spiess and 100 · 01-102 · M anmng III D . me 1anogaster an d by K ess I er III Langer79. R6,87 in D. persimilis and D. pseudoobscura, 88 D. pseudoobscura. ManninglOO established fast and by Kaul and Parsons80 and Spiess et al. in D. slow mating strains of D. melanogaster when the pseudoobscura, by Brncic and Koref-Santibanez81 in sexual behaviour of both the sexes was affected. D. pavani and by Prakasb.89,90 in D. robusta. Prakash Selection was less successful when. it was practiced 91 presented evidence for chromosome interaction . Jln on I y on one sex at a tIme 10 D . me I anogaster . effecting mating speed in D. robusta. Singh ' and although a response was obtained ' towards slow 9 Chatterjee 2•93 investigated the mating ability of mating in the male lines. In D. simulans selection was homo-and heterokaryotypes of D. ananassae. The effective for slow mating speed and females, but not results suggest that there is a correlation between males were affected by selection 104. KesslerlOI-I02 mating propensity and chromosome arrangement showed that the selection for mating speed has frequency and chromosomal polymorphism in D. differential effects in the two sexes: the fe males ananassae may have a partial behavioural basis and responded to selection for slow mating whereas the males are more subject to intra-sexual selection than males responded to selection for rapid mating. An fe males. The males heterozygous for inversion show analysis of variance showed that the contribution of greater mating propensity as compared with females of the total variance of mating speed was homokaryotypic males, which provides evidence for greater than that of malesIOI-102. heterosis associated with AL inversion in D. Singh and Chatterjee1 05 conducted artificial ananassae with respect to male mating activity. selection experiment for high and low mating Partridge et at. 94 studied the relationship between propensity and fo und a positive response to selection male mating success, male fertility and 40 third in both directions. Males were much more affected by chromosome homozygous lines of D. melanogaster. selection than females. Further, significant differences There was significant between-line differentiation for in mating propensity of hybrids produced by fast and both characters and comparison with a heterozygous slow males indicated the possibility a Y -linked stock indicated inbreeding depression and hence influence on mating propensity in D. ananassae. dominance variation for them. Natural and artificial Singh and SisodialO6 found positive response to SISODIA & SINOH : SEXUAL BEHAVIOUR IN DROSOPHILA 583

selection for mating propensity in both directions in neural, hormonal and behavioural control mechanisms D. bipectinata. Both sexes were affected by selection has been done via a thoroughly different approach, but the effect of selection was greater for males than that of analyzing the role of single genes on the one females. hand and of dissecting physiological processes by manipulating the genotype on the other. There are a Female receptMly number of studies providing evidence for selection for Females are generally reluctant to mate and do so fast and slow maters in different species of only after appropriate stimulation by males. The IOO lOl l04 I06 1 4 1 15 Drosophila . . - . 1 . . Analysis of each line chances of successful fertilization generally depend after selection suggests that some alteration of female upon a female's receptivity or willingness to receive receptivity occurred in all these experiments. sperm. Receptivity is a major feature of the 6 lI7 reproductive biology of most insects54. A female Hudak and Gromko1 l , Pineiro et al. and l indicates her receptivity by allowing or helping the Sherwinl 4 reported that the onset of receptivity male to fertilize her eggs. Thus, fertilization seems at differed in the selected lines from that of control. 1 I7 last partly controlled by female behaviour. Pineiro et al. conducted bidirectional selection Receptivity develops in one of three patterns in the experiment for female receptivity in D. melanogaster. adult female (a) she is receptive during a brief period, Selection was successful in both directions. They also (b) her receptivity is cyclic, and (c) she is fo und a clear relationship between high or low continuously receptive. In D. melanogaster, primary receptivity and high or low frequency of hybridization receptivity increases rapidly during the first 48 hr. of with D. simulans males, emphasizing the role of adult life. At first it appears to be off but then it female receptivity in maintaining isolation with its 18 constantly changes107. Receptivity usually declines in sibling species D. simulans. Izquierdo et al. 1 . also aged virgin insects108-IIO and may decrease as a reported that selection for increasing hybridization function of the number of matings. The cAMP was accompanied by an increase in receptivity. The responsive transcriptional factor, CREB, is highly results suggest that female receptivity in conserved among animal species and its activity D. melanogaster may be genetically correlated with affects their behaviour. In D. melanogaster, one of the rate of sexual maturation. Comparison of several alternatively spliced products of CREB gene, dCREB2 strains and lines has revealed genetic variation for I 1 -1 -a is a transcriptional activator while another isoform, receptivity within species 9 21 . Isofemale lines of dCREB2-b is repressor of dCREB2-a. Sakai and D. melanogaster and D. simulans varied significantly 122 Kidokora III demonstrated that overexpression of in receptivity . Receptivity of D. melanogaster was dCREB2-b in virgin females enhances their sexual positively correlated with the speed of sexual 2 receptivity and CREB may be involved in female maturationl 3 and with willingness to hybridize with 2 sexual behaviour of animals. D. simulans malesl 4. Directional dominance for the The X-linked recessive mutation icebox of D. speed of female onset of receptivity was fo und in melanogaster lowers the sexual receptivity of crosses between isofemale lines of D. melanogaster. 1 13 fe males. The probability of mating with mature wild Veuille and Mazeau sampled chromosomes from a type males is reduced in ibx homozygotes, and the wild population and constructed heterozygotes from a frequency of rejection behaviour towards courting random set of these genes; so that the resulting males is increased. Males and females mutant for ibx individuals were highly heterozygous, yet genetically have normal egg-to-adult survival and appear normal identical with respect to any given chromosome being in several general behavioural traits including manipulated. Two kinds of genes affect sexual olfaction, phototaxis, locomotor activity and receptivity, those essential to the development or heartbeat. ibx males court normally and are successful functioning of sensory systems and those acting on in mating. Kerr et al. 112 suggest that ibx does not some other physiological substratum of receptivity. In cause sensory or motor defects. Despite the many genetic experiments mutations can have pleiotropic powerful methods available for detecting genetic effects, i.e. a mutant may have both a sensory defect variation, most geneticists studying primary and low receptivity. Mutations known to lower or receptivity have restricted themselves to two of the increase receptivity and that probably do not act via less effective approaches: artificial selection and the sensory system, comprise a short list: yellow, crossing mildly inbred strainsll3. The genetic study of ebony, hypoactiveC, inactive, apterous, fs (2)B and 584 INDIAN J EXP BlOL, JULY2005

54 fs ( 1 )M72 (for references see Ring0 ). The gene located in the X chromosome and in the left arm of encoding sex peptide cannot be approached with ease chromosome II. The inheritance mode of both genetic genetically because attempts to find variant alleles systems is different. The genes in the X-chromosome have not succeeded. Rather, its chromosomal location show dominance for high level of hybridization and its structure have been discovered via reverse whereas those in chromosome II show dominance for genetics, in which a cDNA copy of the sp gene was low levels. The type and the action of the . genes isolated using a synthetic DNA probe whose sequence involved in sexual isolation may be different was deduced from the sex peptide primary depending on the particular evolutionary forces that structure 125 Sex peptide-induced unreceptivity is act in each speciation process for instances females . characterized by the same rejection . behaviour of D. melanogaster bear genes for isolation from normally mated females namely high rates of D. simulans male in all the three major 126 128 oviposition extrusion . chromosomes , whereas females of D. simulans Casares et al. 127 studied genetic architecture of carry genes for isolation from D. sechellia and D. mauritiana males in chromosome II and III and the female receptivity by 5 x 5 diallel crosses in '3o '31 X chromosome having no effect . . Some species D. melanogaster. The overall picture emerging from 1 have genes with additive effects 24.132 the analysis provides evidence for genetic control of . female receptivity characterized by additive genes The behaviour of gynandromophs which are part with dominance for female receptivity in one of the female, part male offers clues to which portion of the 1 diallel crosses. Carracedo et al. 23 studied receptivity insect control the decision to mate or not. The fe male and sexual maturation of D. melanogaster females in portion of a gynandromorph exhibits female characteristics including receptivity while the male relation to hybridization with D. simulans males. 43 Genetic variability within the population was revealed portion exhibits none . In nature gynandromorphs for the three traits and significant positive correlation are rare, but they can be produced in D. melanogaster between them was found. Females that matured through genetic manipulation. A portion of dorsal earlier had greater receptivity indicating a common brain must be female (have two X chromosomes) f�r physiological basis for both characteristics. Notable the fly to be receptive which suggests that a high female receptivity was correlated with high "receptivity centre" is located in the brain of tendency to hybridize with D. simulans males. D. melanogaster. Gynandromorphs that were males 8 on either the left or right side of the critical group of Carracedo et a . 1 studied receptivity and sexual l 2 cells and female on the other, were unreceptive, i.e. maturation in D. melanogaster females to investigate primary receptivity requires female specific genes the contribution of the X, II and III chromosomes to 33 functioning on both sides of their structurel . female receptivity and to sexual isolation with its sibling species D. simulans. Genes involved in Duration of copulation receptivity were located in chromosome III whereas It is known that male activity and female those affecting sexual isolation were located in all the receptivity are the main factors responsible for 134 three chromosomes. These results indicate direct successful mating in Drosophila . A quantitative relationship between female receptivity and sexual trait having some genetic basis should respond to isolation in D. melanogaster because of the selection, since by selecting extreme phenotypes, occurrence of gene common to two traits in extreme genotypes would be selected. Duration of chromosome III. D. simulans and D. melanogaster copulation seems to be less sensitive to environmental are sibling cosmopolitan species with imperfect variation than mating speed in D. melanogaster. Thi s ethological isolation. Hybridization is easy between means that it could be a useful trait for further genetic D. melanogaster females and D. simulans males, but analysis in attempt to locate genetic activity to 135 the reciprocal cross has been traditionally considered chromosomes and even to regions of chromosomes . as very rare and little is known about the Evidence for the genetic control of duration of environmental and genetic factors that affect it. copulation in D. melanogaster has been given by 1 9 13 137 Carracedo et al. 2 studied genetics of hybridization Merrell 6 and Hildreth who found differences 138 between D. simulans females and D. melanogaster between strains and by Hosgood and Parsons who males. The results suggest that at least two genes or found differences between strains derived from single two groups of genes are responsible for hybridization inseminated females taken from natural populations. SISODIA & SINGH : SEXUAL BEHAVIOUR IN DROSOPHILA 585

13 MacBean and Parsons 9 selected for long and short the determination of copulation duration and at least duration of copulation in D. melanogaster which gave one of the loci governing the male property was responses in both directions. Duration of copulation is probably located on the X chromosome. Genes almost entirely male-determined and is not controlling male copulation duration are at least partly determined by an interaction between the sexes. linked with a gene controlling body colouration. 4 Kaul and Parsons88 studied genotype control of Sisodia and Singhl 7 found evidence for positive mating speed and duration of copulation in correlation between duration of copulation and D. pseudoobscura. They fo und a negative correlation fertility in D. bipectinata. A negative correlation between mating speed and duration of copulation for between male mating success and productivity has l 1 different karyotypes in the male. In D. robusta males, been reported in certain strains of D. simulans 5 with \) Prakash9 observed an association between fast the lower productivity of males which are more mating, repeat mating and the number of offspring successful in mating perhaps being caused in part by produced. At 16°C, D. melanogaster males with an exhaustion of rapidly synthesized components of '52 active esterase-6 mate sooner, copulate for a shorter the ej aculate . The mating speed arid duration of duration of copulation and produce more progeny per copulation may be regarded as two components of an 4 mating than males without esterase-697. Fulkerl 0 integrated system controlled mainly by the male. The investigated the relations between speed of mating, sum of the components is less for the duration of copulation, the number of copulation heterokaryotypes than the homokaryotypes in males, resulting in fertilization and the number of progeny which therefore represents an example of l4 produced in D. melanogaster. Parsons 1 has heterokaryotype advantage restricted to male. 1 3 suggested that the individual completing mating and , Gromko 5 reported that duration of copulation had copulation rapidly would most readily have genes in significant heritability. There was a large negative ' subsequent generations and thus there is some genetic correlation between copulation duration and selective advantage in completing mating and courtship vigour and a positive genetic correlation l54 copulation rapidly. A positive correlation between between copulation duration and fertility. Grornko male mating activity and fertility has been found in D. studied courtship and reproduction in Drosophila; l42 143 pseudoobscura and D. ananassae • , Singh and only the lag time from first courtship to copulation l44 Singh found significant variation among the strains was significantly 'heritable. The genetic correlation tested with respect to courtship time, duration of between fertility and lag time to first courtship , was copulation and fertility in D. ananassae. negative and significant. However, this genetic Copulation duration considerably varies in correlation is expected to have little impact on the Drosophila from 40 sec in D. robusta to 40 min. in D. relation of additive genetic variance or on response to 142 immigrans . In D. melanogaster, D. pseudoobscura, selection because it involves two attributes with low D. athabasca, D. ananassae, D. bipectinata, heritability. Heritable variation affecting mating copulation duration also varies among geographical or behaviour has been demonstrated to be present in 88 136-139 144 146 147 148 several species of Drosophila. The nature of the inbred strains . . . . . Patty carried out · ' hybridization experiments using geographical strains genotypic variance affecting the mating speed in of D. athabasca and suggested that loci controlling D. melanogster has been studied utilizing a diallel 155 '40 copulation duration located on the X-chromosome design by Parsons and by Fulker . 3 and autosome(s). In addition MacBean and Parsons 1 9 14 Remating and Gromko et al. 9 ,carried out selection for Precise gene localization is essential for increased and decreased copulation duration in D. understanding the character and function of genetic melanogaster and revealed that copulation duration­ 135 l56 systems of quantitative traits . Pyle and Gromko has a low heritability and genetically correlated with obtained a response to selection that was apparently courtship vigour or fertility. due to a relatively small number of genes. However, 150 Hirai et al. studied copulation duration and its these results were based on a single line selected for genetic control in D. elegans. The genetic analysis fast remating in both sexes. Grornko and Newportl57 using FI hybrids and recombinant inbred lines suggest carried out selection for fast and slow remating speed that two or more loci were responsible for the based on the behaviour of one sex. Response was l5 differences in both of male and female properties for limited largely to females. Gromko and Newport 8 586 INDIAN J EXP BIOL. JULY 2005

were able to demonstrate correlated responses to single locus with two alleles and fo und that a stable selection in virgin mating behaviour, early fecundity, polymorphism in a sperm displacement locus can be and courtship elicited by mated females and the maintained if the sperm displacement parameters are relation between sperm use and remating interval. either over-dominance or non-transivity of alleles. Fukui and . Gromko159 studied genetic basis of Prout and Clarkl67 have shown that pleiotropic effects remating in D. melanogaster and found that ' on fecundity and male mating success can increase chromosomal effects on direct responses to selection the opportunity for stable polymorphism for alleles were distinctively different between selection lines affecting sperm precedence but in this case induced derived from two different base populations. over-dominancel68 or by antagonistic pleiotropic

Significant interaction between chromosomes II and interactions with other components of fi tness. Clark et the other chromosomes were also fo und in both the aI. 169 demonstrated that sperm competitive ability is selection lines. Fukui and Gromkol60 studied not a property of sperm haplotype but rather of the recombination analysis. The major loci affecting the diploid male's genotype. They tested whether the difference between the fast and the slow remating relative sperm competitive ability of males can be speed map to the right arm of chromosome II. ranked on a linear array or whether competitive 1 W ahlsten 61 has argued that the conceptual foundation ability instead depends on particular pair-wise of methods often used to study the genetic basis of contests among males by using a set of six behavioural traits emphasizes chromosomal chromosome extracted lines, they found strong inheritance unduly. An analysis of variance with departure from transitive relations among a set of six planned comparisons and a biometrical analysis genotypes of males as the pairwise tests were not previously showed that autosomal loci contribute compatible with a single linear ranking of sperm significantly to the difference in remating speed in competitive ability. Further, they proposed that non­ these selected lines 162. transivity seen in sperm precedence theoretically should increase the opportunity for polymorphism in Stamencovic-Radak et al. 163 reported that selection genes that influence this phenotype. for females that remate readily could result in relaxed selection for male vigor in D. melanogaster. Sgro et Mutations associated with sexual behaviour 64 aL. 1 fo und a significant direct response to selection The question of whether either sex in for time to remating in D. melanogaster females for D. melanogaster is selective with regard to mutants or both selection lines (fast and slow) and suggested that abnormalities of the opposite sex in mating activity remating time is heritable and that remating time in was first investigated by Sturtevantl . Since 19 15 females is under polygenic control. Singh and genetic changes of various sorts, single mutants, 6 Singh 1 5 studied the genetic basis of female remating inbread lines and strains differing in several ways speed in D. ananassae. Their findings suggest that have been analyzed in varying degrees of genetic or fe male remating time in D. ananassae is under behavioural detail 170. Mutants support behavioural polygenic control. The remating frequency of females analysis at four levels: (1) they highlight properties of showed a correlated response in both fast and slow the cellular basis of behaviour, (2) they help to dissect lines, which suggests that remating time is heritable complex behavioural systems, (3) they contribute to and rapid remating is pleiotropically associated with structure function mapping in the brain and (4) they frequent remating in D. ananassae. Several studies can be used to investigate the ecological context and have suggested that genotype variation for sperm significance of behaviour. precedence or sperm displacement exists both within An elegant new technique to localize brain l66 and between populations. Prout and Bundgaard structures for sexually dimorphic behaviour is based l demonstrated that different laboratory strains of on the GAL 4 enfiancer trap technique 7l. In this case, D. melanogaster have different degrees of sperm a feminizing splice variant of the gene transfonner competitive ability. There are some models of the (tra) under the control of a UASGAL4 element is used. population genetics of sperm displacement, which This transgene is crossed into lines with GAL 4 explain the maintenance of polymorphism of genes expression in certain parts of the brain including parts that influence sperm compehtlOn. Prout and of antennal lobes and MBs (mushroom bodies). The 66 Bundgaard 1 presented a population genetics theory partially feminized males may be expected to be less for sperm displacement, which is determined by a motivated to perform male courtship but this is not SISODIA & SINGH : SEXUAL BEHA VIOUR IN DROSOPHILA 587

1 2 observed. Instead, they are less selective in choosing splicing) 8 . Both the female type and the male type their courtship target i.e. the frequency of courting dsx transcripts encode functional transcription factors 2· adult males goes upl7 173. each of which activates or represses transcription of a The sexual behaviour of females is less evident different set of "realizer genes" required for the than that of ml!les. In respOnse to an attempt at development of male specific or female specific copulation by a male, the female displays either structures. receptive or rejection actions. The existence of such Sexual behaviour, and thus the underlying neural an overt sexual differences in behaviour implies the mechanisms are induced under the control of the sex sexual dimorphism in the central nervous system determination cascade. The neural system for male (CNS) areas responsible for mating behaviour. behaviour and that for female behaviour exists in the Gynandromorph has been used to determine the brain, and that one of these is acti vated locations of anatomical foci controlling male type and (or inactivated) by the constitutive action of the sex 63 fe male type behaviour. A sex linked enzyme marker determination gene product (e.g. tra and tra_2) . The distinguishes male tissue from female tissue in sex-determination gene products could determine the sexually mosaic flies. The results indicate that the sex by the selective elimination in adequate neural posterior dorsal brain near the mushroom body is circuits or the remodeling of synaptic connections. An indispensable for generating male type behaviourl74, approach complementary to the use of loss-of whereas the dorsal anterior brain is essential for fu nction mutations in the sex-determination genes is fem�e type behaviour133• Thus, according to the artificial sexual transformation from male to fe male gynandromorph studies, the brain center for male type by means of the forced expression of wild type copies behaviour is anatomically distinct from that for of these genes. female type behaviour. However, subsequent An elegant experiment was performed by Ferveur 2 histological analysis of the respective brain regions and his colleagues17 who induced trans sexualism in did not reveal any obvious sexual differences in restricted regions of the brain with tra+, using the cellular compositions or in neural connectivity (for targeted gene expression system. The findings were references see Yamamoto et al. 175). that the male flies with trans sexual nervous display The analysis of the developmental biology of bisexuai courtship only whep a given brain region is sexual traits in D. melanogaster leads to the feminized. The critical region in the brain for the hypothesis that most of the sexually dimorphic induction of bisexuality is the antennal lobe, which is characteristics are under the control of sex­ 'the primary olfactory centre in the insect's CNS, They determination cascade of genes 176-178. The master were able to identify the gIo1V-eruJi responsible for switch gene for female development, sex lethal (Sxl) changes in the sexual orientation: when ,';��� DMZ, can be activated only when XlA value exceeds 1, the DA3 and DA4 glomeruli are feminized by the tra+ condition in which transcription factors coded by . action, the male flies express bisexuality in courtship, genes linked to the X chromosome are accumulated to regardless of the sex of other glomeruli in the the level sufficient for trans activation of Sxl179• The antennal lobe. Expression of tra+ in a part of the Sxl gene encodes a splicing inhibitor, which binds to n.ushroom body, to which the antennal lobe neurons 172 the splice junctions of its own primary transcript, project, also induces bisexual courtship in males . preventing the use of the exon that contains a stop Bisexual courtship activity by males is known to codon 1BO. The Sxl protein also binds to its downstream result from ubiquitous expression of the white (w) target, transformer (tra) mRNA. Tra is an RNA­ gene under the control of the heat shock promoter 18 binding protein that enhances splicing at the site it (hs-mini-w+). Zhang and Odenwald 3 suggested that . binds to in concert with Tra-2 and RBPI 1B 1 . A classic deprivation of tryptophan, the precursor Of target of Tra is the double sex (dsx) primary monoamine neurotransmitters, may occur as a transcript, of which the fourth exon bears the binding consequence of ectopic.expression of w+ in the CNS, sites for Tra : In the presence of a related protein, resulting in homosexuality as behavioural phenotypes. Tra-2, tra induces the female type splicing which There are mutations known to alter flies' sexual connects the fo urth exon to the third. In the absence of orientation a classic example of which is fruitless these proteins, the fourth exon ' is spliced out and the (fru) discovered by K. S. Gill in his screen for male fifth exon is connected to the third (the male-tvoe steri le mlltants38 Thp. malp." from thE" (YIJ "toclc ('()J lrt 588 INDIAN J EXP BIOL, JULY 2005 not only females but also males, although they never interactions are to an extent operationally separable copulate with females. Since 1963, several alleles of from courtship behaviour. Mc Robert and fr u have been isolated fru sat males court only Tompkins 192 reported ' the effect of transformer, females 184, whereas males carrying the other fru doublesex and intersex mutations on the sexual alleles court both males and femalesI85-187. Tissue behaviour of D. melanogaster. They identified the localization of the fru transcripts common to both effects of genes that regulate sex determination in sexes are detected in many cells in the eNS and female specific tissues in the abdomen that produce various non-neural tissues, the expression of the sex phenomones and parts of the central nervous sexually dimorphic transcript is restricted to a fraction system that function when a male performs courtship. of eNS cell 188. The male antennal lobe contains a Male homolbisexuality is induced by mutations of fr u structure that induces homosexual courtship when satori gene or by sexual transformation of certain feminized by means of GALA-driven brain cells in Drosophila, These studies unequivocally tra+expression172. The tra+ action of inducing showed that sexual orientation is heritable and can be homosexual courtship is mediated by the fru­ changed by manipulation of a single gene. Another expressing cell in the antennal lobe 189. In addition to important findings is that sexual orientation could be the behavioural phenotype, the fru mutant exhibits a determined by a certain class of cells localized in a particular muscle defect. In wild type male flies a pair defined structure of the brainl89. Sex is one of the of large dorsal muscle bundles known as the Muscle most conspicuous outcomes of convergent evolution of Lawrence (MOL) is present in the fifth abdominal and the sex determination process utilizes diverse sets segment MOL is absent in females and in larvae of of mechanisms that differ from species to species. both sexes. Male flies carrying some of the fru alleles This implies that the roles of sex-determination genes (e.g. fru sat, fr u3 and fru4) lack the MOL. In flies in one species do not necessarily give an immediate carrying fru I fru 2 alleles, the muscle fiber length of answer to the question as how the homologous genes MOL is shortened to half, and the number of nuclei in function in other species. Nevertheless, it is the cells of the MOL fibersis greatly reduced. conceivable that genetic dissection of sexuality in Drosophila will provide conceptual frameworks, Satori (sat) is a Drosophila mutant in which male which assist our search for determinants for sexual sexual . orientation changes from heterosexual to orientation in different organisms. homosexual. Another phenotype of the satori mutant is the absence of the male-specific MOL. The A new mutation he's not interested (hni) was formation of MOL is repressed in females by the identified based on the low level of the courtship of action of a female determinant protein, Transformer females by males 193. Interestingly, hni . is associated (Tra), while a tra target doublesex (dsi) has no role with a muscle phenotype as is fru: the number of MOL formation. Molecular cloning of the fr uitless fibers comprising the MOL is decreased whereas the (jru) gene responsible for the sat mutation revealed fiber number is increased in other abdominal that it encodes a putative transcription factor with a longitudinal muscles hni muscle phenotype is more BTB domian and Zn finger motifs, and that the marked in heterozygotes than in homozygotes second exon of this gene contains three repeats of the implying that the hni mutation is tra-binding consensus sequences. These observations antimorphicl94. Genetic experiments need to be suggest that fr u is a novel target of Tra in the sex carried out to determine whether the hni mutation determination cascade of Drosophila 190. Lee and genetically interacts with the fru mutant. Ryner and Hall 191 demonstrate that fru mutations cause a Swain178 demonstrated that the sex-determination previously unappreciated behavioural anomaly: high cascade has a bipartite pathway downstream of Tra: level of head to -head interactions between mutant one uses dsx and the other fru as the transcriptional . males. Mutant dissatisfaction (dsJ) males also exhibit regulator of target genes. However, Finley et al. 195 1% this behaviour at higher than normal levels, but it was suggests a third branch in the sex determination barely displayed by doublesex (dsx) or intersex (isx) cascade downstream of tra. The gene functioning in mutants. They suggested that head to head the postulated third branch is dissatisfaction (dsj), interactions, especially those performed by fru males, which was recognized because of the aberrant mating are instances of putative aggression analogous to behaviour of the flies carrying mutations in this gene. those exhibited by wild type males and that head The dsf males vigorously court in additions to SISODIA & SINGH : SEXUAL BEHAVIOUR IN DROSOPHILA 589

fe males, other male� forming a short chain of gene of Drosophila. The cacophony (cae) locus of courtees. The dsf males even show attempted Drosophila melanogaster which encodes a calcium­ copulation towards male flies, time to copulation is channel subunit has been mutated to cause courtship vastly prolonged. song defects or abnormal responses to visual stimuli. The dsf mutant females exhibit subnormal The fu nction of the central nervous system as it receptivity. When a dsf female is placed with a wild controls sex specific behaviours in Drosophila has type male in an observation chamber, the female been studied with renewed intensity in the cont�xt of displays active resistance to courting male, with genetic faCtors that influence the development of decamping, wing flicking and kicking. A similar sexually differentiated aspect of this insect. Taylor et 1 enhancement of male refusal in females has been al. 87 discussed three categories of genetic variations reported to occur in flies carryin mutations in the that cause anomalies in courtship and mating � sp inster (spin) 197 and Chaste (cht)1 8 loci, the 'epistasis behaviours: (1) mutants isolated with regard to of which with dsfremains to be established. Although courtship defects, of which putatively courtship­ the neurobiological bases for homosexual tendencies specific variants such as fr uitless are a subset, (2) in male and reduced receptivity in female dsf mutants general behavioural and " neurological variants are not determined, insufficient bending of the male (including sensory and learning mutants) whose abdomen and the oviposition defect in females are defect include subnormal reproductive performance known to have anatomical correlates in the peripheral and (3) mutations of genes within the sex­ nervous systeml95. determination regulatory hierarchy of Drosophila, the Transplantation of a paragonial gland or injection analysis of which has included studies of reproductive of its secretion into the abdomen of virgin females behaviour. results in the repression of female sexual receptivity Conclusion and stimulates their oviposition to level similar to The aim of this review was to summarise the · those of mated femalesl99• 200 Furthermore, studies revolving around the genetical aspects of biochemical and · physiological studies in 1 sexual behaviour of different species of Drosophila. D. meianogaster20 and D. fu nebrii02 indicated the The genetic analysis remains the best means to presence of pep tides in glandular secretion that affect define the mechanisms and to begin the process of reproductive behaviour after injection into virgin assigning the contributions of genes to behaviour. The fe males. Chen et al. 203 reported the isolation and observation is that behavioural mutants isolated in the amino acid sequence determination of a peptide from l�boratory are mostly pleiotropic, mild alleles relative male D. meianogaster paragonial glands that after to the null phenotype links them mechanistically with purification by HPLC reversephase chromatography the kinds of genetic variation that exits in nature. and subsequent injection into the female abdominal Moreover, it highlights the interconnectedness cavity, represses sexual receptivity and stimulates between the genetic systems necessary for behaviour oviposition. The results of . obigonucleotide-directed like courtship and those taking part in the many other cDNA cloning and of RNA analysis provide aspects of the fly 's biology. This fundamental important information on the accessory gland peptide pleiotropy of the behavioural genes suggests that we with regard to biosynthesis and' its regulation. Sex­ need to think in terms of overlapping networks, rather determination in Drosophila is controlled by a 4 than simple pathways, in order to do · justice to the cascade of regulatory genes. Pultz et al. 20 described complexity of the system. The findings that fr u hermaphrodite (her), a new component of this mutations lead to males courting females and males regulatory cascade with pleiotropic zygotic and indiscriminately, whereas wild-type D; meianogaster maternal fu nctions. Zygotically, her+function is males normally recognize only females as appropriate required for female sexual differentiation: when courtship objects; implies that one function of fr u+is zygotic her+function is lacking females are ' to provide males with the ability to discriminate transformed to intersexes. Zygotic her+function may fe males from males as appropriate individuals to also play a role in male sexual differentiation. court Thus, sexual orientation in flies is controlled by 2 5 Chan et ai. 0 studied the courtship and other the same hierarchy of genes that control all other behaviours affected by a heat sensitive molecularly aspects of sex. As in many other areas of research, the novel mutation in the cacophony calcium-channel surprising similarity of basic biologic principles 590 INDIAN J EXP BIOL, JULY 2005

between the little fly and our own species boosted the 2 Sturtevant A H. The North American species of Drosophila. Carnegie Inst Washington Publ, 301 (192 1) 1. progress of unraveling the genetic foundation of 3 Collins M F, Hewitt J K & Gogarty J F, Validating time mammalian clock mechanisms. Question regarding sampled observations of courtship in Drosophila the evolution and the proximal causes of fe male melanogaster fo r behaviour genetic analysis, Behav Genet, receptivity can best be answered by studying the 15 (1985) 31. genetics of the fly. 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