Concentration of Chromosomal Aberrations on Chromosome 3 of Drosophila Nasuta
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Heredity 69 (1992) 263—267 Received 12 November 1991 Genetical Society of Great Britain Concentration of chromosomal aberrations on chromosome 3 of Drosophila nasuta A. KUMAR & J. P. GUPTA* Department of Biological Sciences, University of Notre Dame, Notre Dame, U.S.A. and *Center of Advanced Study in Zoology, Benaras Hindu University, Varanasi, India Drosophilanasuta harbours a total of 85 inversions in its natural populations. Among them, eight inversions are distributed on the X chromosome, 24 are located on chromosome 2 and the remaining 53 are concentrated on chromosome 3. In order to test the possibility of chromosome 3 being intrinsically more vulnerable to breakage in this species, an irradiation study was carried out on the mitotic chromosomes of the neural cells. The results show random distribution of induced aberrations among the X chromosome, chromosome 2 and chromosome 3, which suggests that these chromosomes of D. nasuta have similar radiosensitivity. Keywords:Drosophilanasuta, concentration of inversions, X-ray-induced aberrations. plement of this species consists of four pairs of Introduction chromosomes: a pair of metacentrics (chromosome 2), Naturalpopulations of many Drosophila species are a pair of large rods (chromosome 3), a pair of dots often polymorphic for chromosome inversions (chromosome 4) and a pair of small rods representing (Lemeunier et al., 1986). Inversion polymorphism in the X chromosomes while the Y chromosome is sub- Drosophila is maintained by selective superiority of metacentric (Kumar & Gupta, 1988). Cytologically, the inversion heterozygotes (Dobzhansky, 1970; Sperlich species is highly polymorphic and harbours a total of & Pfriem, 1986). Based on the distribution pattern of 85 inversions and three chromatin band deletions in inversions among different chromosomes, three natural populations examined across its distribution categories have been recognized: (i) species in which range (Rajasekarasetty et a!., 1979; Wakahama & the gene sequences are extremely stable showing very Kitagawa, 1980; Kumar & Gupta, 1986a,b, 1988). Of 85 few or no inversions; (ii) species in which inversions are inversions detected, eight are on the X chromosome, about equally distributed among different chromo- 24 are on chromosome 2, and the remaining 53 are somes; and (iii) species in which there is a marked con- concentrated on chromosome 3 (Kurnar & Gupta, centration of inversions on a particular chromosome 198 6a, 1988). Moreover, only three inversions, IIL-2 (Ward, 1952). The species which show the last category on the left arm of chromosome 2 and 111-2 and 111-35 of pattern include D. kohkoa (Mather & Thongmeear- on chromosome 3, are shown to be heterotic in both kom, 1973), D. nebulosa (Pavan, 1946), D. tetracha eta laboratory and natural populations of the species (Angus, 1968), D. athabasca (Novitski, 1946), D. (Kumar & Gupta, 1988, 1989). melanica (Ward, 1952), D. pseudoobscura and D. In order to test whether chromosome 3 of D. nasuta persimilis (Dobzhansky, 1944), D. guaramunu (Sal- is intrinsically more vulnerable to breakage, an irradia- zano, 1954), D. bocainensis (Carson, 1954),D. para- tion study was performed on the third instar larvae melanica and D. euronotus (Stalker, 1960, 1964), and with different doses of X-rays and their neural cells D. jambulina (Singh & Gupta, 1980). were scored for different types of aberration on each Drosophila nasuta is a member of the immigrans chromosome. This method of Gatti et al. (1974) has an species group of the subgenus Drosophila and is distri- advantage over others, in which genetic damage has buted in several countries of south-east Asia and been analysed in the F1 generation, in that the genetic Africa (Kitagawa et al., 1982). The chromosome corn- damage can be easily assessed in the same generation. Moreover, there is a little or no chance of the genetic damage remaining undetected by this method unlike Correspondence: Dr A. Kumar. Department of Biological Sciences, University of Notre Dame, Notre Dame, In 46556,U.S.A. others in which inviability of gametes will give failure of 263 264 A. KUMAR & J. P. GUPTA detectionin the F1 generation. An analysis of variance (AN0vA) was carried out to test if there was a marked I concentration of breaks on chromosome 3 or if the C breaks were randomly distributed among the X '4 chromosome, chromosome 2 and chromosome 3. Materialsand methods 'I In all the irradiation experiments performed, third 0 instar larvae were obtained from a laboratory stock of I D. nasuta. The stock was founded by a single naturally inseminated female collected from the Botanical I Garden on the Benaras Hindu University campus. The a cytological analysis of the polytene chromosomes of V third instar larvae from this stock showed that the V larvae were homozygous for standard gene arrange- K 'g: ments on all the chromosomes. St Mitotic chromosome preparations were made from Si the neural cells. A group of third instar larvae contain- i ing a similar number of males and females of about I similar age were placed in a plastic Petri dish with blot- ting paper soaked in insect saline (0.67 per cent solu- 'p tion of NaC1) on its bottom. Larvae were irradiated with either 625 rad or 1250 rad of X-rays (110 kV, '0 S 4 mA, 1 mm aluminium filter, 10 cm distance, 420 rad 'I mm - 'I bythe Picker's X-ray machine). Larvae in the Petri dish were divided into two groups. Larvae in one group were immediately dissected in Poels' salt tissue culture medium (Mukherjee & Lakhotia, 1981) for Fig. 1 X-ray-induced aberrations in mitotic chromosomes brain ganglia which were then treated with 1 g m1' of the neural cells of Drosophila nasuta (see arrows): (a) (d) of colchicine for 1 h and 25 mm. The brain ganglia dicentric in chromosome 2; (b) (d) chromosome break at the were transferred to a hypotonic solution (0.67 per cent centromere of chromosome 2; (c) (d) symmetrical chromatid solution of tn-sodium citrate) for 35 mm before fixing exchange between both homologues of chromosome 2; (d) in aceto-methanol (1:3) for 15 mm with three changes (d) chromosome break at the centromere of the Y chromo- of 5 mm each. The brain ganglion from each larva was some; (e) (d) Asymmetrical chromatid exchange between both homologues of chromosome 2; and, (f) (d) chromatid teased out in a drop of 60 per cent acetic acid on a deletion in chromosome 2. warm slide to facilitate the dissociation of neural cells. These cells were later fixed on the same slide by adding a few drops of fixative and the slide was air-dried (2 h aberrations. These aberrations were chromatid dele- of the post-irradiation time of fixation). Larvae in the tions, chromatid gaps, chromosome deletions, chromo- second group were dissected in Poels' salt tissue culture some gaps, isochromatid deletions, chromosome medium after 2 h of irradiation and processed as exchanges and discentrics. Some of the examples are described above (4 h of the post-irradiation time of shown in Fig. 1. Data on chromatid deletions and chro- fixation). The air-dried slides were stained in a 5 per matid gaps were pooled together. Similarly, chromo- cent Giemsa and examined for different types of some deletions, chromosome gaps and isochromatid aberration. The final data concerning the type and fre- deletions were pooled together. Frequencies of the quency of chromosome aberrations were obtained by different types of aberration are not presented here but adding together many partial data, and not more than may be obtained upon request. Table 1 gives the total 10 metaphase plates were scored from each larva. number of breaks among different chromosomes. The observations clearly show that the sexes had different Results radiosensitivity. The males show a higher number of breaks than the females at each dose of X-rays and Athorough analysis of a large number of irradiated post-irradiation time of fixation. The Y chromosome cells revealed various types of induced chromosome was found to be more resistant to breakage, and the CHROMOSOMAL ABERRATIONS IN 0. NASUTA 265 Table 1 Total number of X-ray-induced breaksf on the others with closely similar break points. Although this different chromosomes in the neural cellst of Drosophila hypothesis obtained some support from an irradiation nasuta study carried out by Bernstein & Goldstein (1961), it has been rejected by several others on the grounds that Post-irradiation Chromosomes spontaneous aberrations are far too common (White, Dose time of fixation 1973). The mechanical hypothesis would seem Sex XY2 3 (rad) (h) T possible if the frequencies of spontaneous aberrations Control0 99 00 0 0 0 (rearrangements) were very low, so that non-random- dd 00 0 0 0 ness in their occurrence would play a significant role in 625 2 9911 100 512 27 determining the concentration of break points on a ddlt147 3626 83** particular chromosome (White, 1973). On the other 4 9911 120 8 8 28 hand, if the spontaneous rearrangements are common ddII1302216 51* (of the order of 1 per 1000 gametes) then it appears 1250 2 99114001724 81 more likely that the final picture would be determined ddlI656 4147159** by natural selection (White, 1973). Studies in several 4 99112503216 73 animals have shown that spontaneous rearrangements ddlI441 7056171** are indeed far more common (White, 1973). Spon- taneous rearrangements are also quite common in D. fTotal number of breaks included all types of aberrations: chromatid and chromosome breaks were considered as a nasuta (Kumar & Gupta, 1988). The evidence thus single event; exchanges and dicentrics were considered as appears to favour the co-adaptation hypothesis of two events. Wasserman (1963). According to this hypothesis, a A total of 200 mitotic cells were scored for each sex at each marked concentration of inversions on a specific time.