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And Tobari1969a, 196913; Kojimaand YARBROUGH1967), and Fecundity (Andersonand WATANABE1974) Components of Fitness

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And Tobari1969a, 196913; Kojimaand YARBROUGH1967), and Fecundity (Andersonand WATANABE1974) Components of Fitness MODES OF SELECTION MAINTAINING AN INVERSION POLYMORPHISM IN DROSOPHILA PAULZSTORUM MARK H. GROMKO AND ROLIJN C. RICHMOND Department of Zoology, Indiana University, Bloomington, Indiana 47401 Manuscript received July 6,1976 Revised copy received October, 1977 ABSTRACT The possibility that fitness relationships associated with an inversion poly- morphism in D. paulisforum were frequency dependent was investigated. Using allozymes of tetrazolium oxidase to inark inversions, the effects of genotype frequency, larval density, and culture conditions on fitness were assessed. The proportions of genotypes among egg-laying females were varied, thus changing the expected proportions of progeny produced in the absence of fecundity or viability selection. The genotypes of progeny were determined by electrophoresis and comparisons of the ratio of the numbers of the different genotypes produced to the expected ratio was used to evaluate fitness relation- ships. Fitness relationships were dependent on genotype frequency, larval density, and culture conditions. Selection was either absent, directional, frequency dependent (favoring rare types), or heterotic depending on density and culture conditions. It is implied that the adaptive value of genetic variants need not be apparent in all environments, or may change with changing con- ditions. There is evidence for different criteria for selection in the two sexes. These results add to the evidence supporting the importance of frequency- dependent selection. It is argued that for frequency dependence to be of general importance, selection must act on genes in groups, either as an inversion or as lengths of chromosome with integrity maintained by disequilibrium. FREQUENCY-dependent selection has been demonstrated to be effective in maintaining variability in experimental populations of Drosophila (EHRMAN 1966; KOJIMA1971), Tribolium (SINNOCK1970), Mormoniella (GRANT,SNY- DER and GLESSNER1974), and Poecilia (FARR1977). In Drosophila frequency dependence has been shown in mate selection (EHRMAN1969; SPIESS1970), viability (KOJIMAand TOBARI1969a, 196913; KOJIMAand YARBROUGH1967), and fecundity (ANDERSONand WATANABE1974) components of fitness. BUND- GAARD and CHRISTIANSEN( 1972) have demonstrated frequency dependence in mate selection and viability in the same system. Studies of the viability component of fitness in Drosophila have demonstrated frequency dependence for frequency classes defined by inversion karyotypes (KOJIMA and TOBARI1969b), fourth chromosomes (BUNDGAARDand CHRIS- TIANSEN 1972), and polymorphisms marked by allozymes at the ADH (KOJIMA and TOBARI1969a) and at theesterase-6 (YARBROUGHand KOJIMA1967; KOJIMA and YARBROUGH1967) loci of D. melanogaster. The frequency dependence of Genetm 88: 357-366 February, 1978. 358 M. H. GROMKO AND R. C. RICHMOND fitness values has been clearly shown to depend on density (KOJIMAand HUANG 1972; NASSAR,MUHS and COOK1973). A mechanism for its operation among larvae has been proposed that involves a partitioning of the environment by individuals of different genotypes (HUANG,SINGH and KOJIMA1974). The term frequency-dependent selection is used here to refer to a type of balancing selection (GROMKO1977). In a system out of genic equilibrium, the operation of frequency-dependent selection defined in this way is characterized by large differences in fitnesses between the common and rare genotypes, the latter having the highest fitness. As the rare type becomes more common. fitness differences among the genotypes lessen until, at equilibrium, the genotypes are equally fit. Since there are no fitness differences among genotypes at equilibrium, the population has no genetic load. The importance of this conclusion cannot be evaluated, however. until the generality of this mode of selection and its capabil- ity of maintaining large amounts of variability have been discovered. In the experiments to be described, the possibility that frequency-dependent selection plays a role in maintaining an inversion polymorphism in Drosophila paulistorum has been investigated. The results show that both frequency-depen- dent and heterotic selection are important in maintaining this polymorphism in experimental populations. MATERIALS AND METHODS A two year old cage population of D. pzulistorum (cage IS-18 of POWELLand RICHMOND 1974) was used as the starting material for these experiments. The population was at apparent gene-frequency equilibrium for alleles at the tetrazolium oxidase (To) locus. To is a sex-linked locus with two codominant electrophoretic alleles, designated slow (S) and fast (F) (RICHMOND 1972). The cage population was segregating at approximately 60 io 65 percent of the slow allele. Eleven lines homozygous for the fast and twelve lines homozygous for the slow allozymes were isolated from the cage population by Fair matings and sib matings. Lines carrying the same allele were combined to make two homozygous stock populations from which virgin flies were collected to begin experiments. Examination of the salivary gland chromosomes of several female larvae produced from each of eight reciprocal crosses between the fast and slow lines all showed inversion heterozygosity in the right arm of the X chromosome. The intersion seen is apparently that described by DOBZHANSKYand PAVLOSKY(1962) and drawn in their Figure 7. That inversion hcterozygosity was seen in all of the preparations examined suggests that one gene arrangement was associated with the To slow allele stock and the other with the To fast allele stock. If each of the eight crosses made sampled a minimum of three X chromosomes from the two homozygous lines, then the probability that all eight crosses would show inversion lieterozygosity is less than 0.05 (= 0.8824) if the inversion and To alleles are associated 88 percent of the time. A consideration of the origin of the founder cage population also leads to this conclusion: the cage from which lines were isolated was initiated with the progeny of only two isofemale lines. This cage then is likely to have contained a maximum of six X chromo- somes. Thus the association between the To alleles and gene arrangements is not necessarily surprising (POWELLand RICHMOND1974). Virgin flies one to three days old were combined in mass matings in which the To genotypes of flies of each sex were known. Four different types of mass matings were made (FF x F, FF x S, SS x F, SS x S), each having only one type of hemizygous male and one type of homozygous female. After three days females were separated from males and held for one additional day. Females from the various matings were then combined, without etherization, in known proportions and left to lay eggs in bottles of standard cornnieal-molasses medium. SELECTIOhT IN A POLYMORPHIC SYSTEM 359 TABLE 1 Scheme for uarying proportions of parental females and the expected genotypic ratios among progeny, assuming no selection % S input Genotype of input females and mates ss x s FF X S SS X F FF X F 85 75 10 10 5 65 55 10 10 25 35 25 10 10 55 15 5 10 10 75 Zygotic input and expected genotypic ratios among progeny if each female lays 2x eggs. (Le ,z female eggs and x male eggs). % S input Females Males SS SF FF S F 85 751 20x 5x 85x 152 65 552 202 252 65x 352 35 252 202 55x 352 65x 15 52 20x 752 152 852 These mated females comprised the input of the experiment and young imagoes hatching from the culture bottles the output. The To genotypes of output flies were determined by electro- phoresis, using the techniques of RICHMOND(1972). Selection in either fecundity or viability will be evident in a comparison of the genotype frequencies of flies eclosing with the frequencies expected on the basis of no selection. Three parameters were varied: (3 ) proportions of premated females, (2) larval density, and (3) culture conditions. Each experimental series consisted of four cultures differing in the proportions of premated females and thus in the expected ratios of eclosing progeny. These four input frequency conditions, represented as 85, 65, 35, and 15 percent of the S allele, and the ratios of progeny genotypes expected undcr conditions of no fecundity and no viability selection are given in Table 1. Larval density was varied by allowing the females in each series to lay first for four days, producing a “crowded larvae” series. Females were then transferred to fresh culture bottles for one day to produce an “uncrowded larvae” series. Culture conditions were varied with respect to the kind and extent of microbial growth. “Clean” cultures were main- tained by putting 100 males (50 F and 50 S) into the bottles immediately after the egg-laying females were removed. These males were removed the day before the F, progeny were first expected to eclose and made no genetic contribution to the experiment. “Bacterially contami- nated” cultures invariably resulted when these males were not introduced. Media in these bottles became very dark in color and dried up by the time eclosion was expected. Few or no flies eclosed in nine of ten trials maintained in this way. Media innoculated with excess yeast produced the third type of culture condition. Two series of experiments have been comFleted with clean media conditions, while there have been one of each of the contaminated series. Samples of the media from contaminated cultures were struck for isolation on a complex peptone-yeast extract agar medium, and isolates were gram stained. The bacterial contaminant present most abundantly was Bacillus. Both the bacterial and yeast contaminants
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