Crossing over between linked inversions in Drosophila ananassae. B. N. SINGH and A. K. SINGH. Genetics Laboratory, Department of Zoology, Banaras Hindu ...
Heredifas IOY: 15-19 ( I Y X X l
Crossing over between linked inversions in Drosophila ananassae B. N. SINGH and A . K . SINGH Genetics Laboratory, Department of Zoology, Banaras Hindu University, India
SINGH, B. N. and SINGH, A . K . 1988. Crossing over between linked inversions in Drosophila ananassae. - H e r ~ d i r a s 109: 15-19. Lund. Sweden. ISSN 0018-0661. Received August 18, 1987
In Drosophila ananassae two linked inversions located in the opposite limbs of the third chromosome are of common occurrence in natural populations as well as in laboratory stocks. By utilising different stocks homozygous for the standard or inverted gene sequences in the second and third chromosomes, the rate of crossing over between linked inversions when heterozygous was studied under different karyotypic backgrounds in both sexes by means of the salivary gland smear technique. In spite of the long chromosome distance between them, the crossover rate is very low and it varies in different strains. Furthermore, there is a considerable increase of recombination between heterozygous inversions due to inversion heterozygosi t y in ZL, indicating interchromosomal effect. These findings provide evidence that recombination between linked inversions in D.ananassae is influenced by background karyotype and background genotype. R . N. Singh, Genetics Laboratory, Deparfment of Zoology, Banaras Hindu Universrry, Varanasi-221005, India
1985a). An imporChromosomal polymorphism due to inversions is peculiarities (see review by SINGH very common in the genus Drosophila and is an tant genetic peculiarity of this species is the presadaptive trait (DACUNHA 1960; DOBZHANSKY 1970; ence of spontaneous male crossing over which is 1970; SPERLICH 1973). The suppressive effects of inver- meiotic in origin (KALE 1969; HINTON et al. 1970). The two linked paracentric sions on recombination are not restricted to in- MORIWAKI verted section of the chromosome, they may inversions, namely delta (terminal) and eta (basal) strongly suppress recombination outside their limits occur in opposite arms of the third chromosome and 1970). The and RED- are cosmopolitan in distribution (SINGH (DOBZHANSKY and EPLING 1948; SCHULTZ FIELD 1951; CARSON 1953; LEVITAN 1958a). In certain present paper embodies the results of our investigaspecies of Drosophila, crossing over between linked tions on crossing over between these two linked ininversions when heterozygous has been studied versions when heterozygous under different karyocytologically. The crossover rate between hetero- typic backgrounds in various homozygous strains of zygous inversions of D. robusta varies considerably different geographic origins. in different chromosomes and is influenced by background karyotype and background genotype (CARSON 1953; MASSIE and LEVITAN 1956; LEVITAN 1958a, b). Crossing over between linked inversions Materials and methods has also been studied in D. willistoni (BATrAGLIAand et al. 1968; FRANCA and DA The delta (terminal) and eta (basal) are independBIRCH1956; FRANCA CUNHA 1968) and it has been suggested that the high ent inversions located in the opposite limbs of the rate of occurrence of crossing over between linked third chromosome of D . ananassae. The delta is loinversions is probably correlated with the high de- cated in 3L and eta in 3R. The distance between gree of polymorphism found in this species. SPER- these two inversions is nearly 25 percent of the LICH and FEUERBACH-MRAVLAG (1974) found com- chromosome length. The location of these two inplete suppression of recombination between two in- versions in the third chromosome and of the alpha versions of the 0 chromosome of D. subobscura (subterminal) inversion in 2L are depicted in Fig. 1. The alpha is a lengthy inversion in 2L and has been when studied cytologicall; Drosophila ananassue, a cosmopolitan and used to detect interchromosomal effect. During the present study $even homozygous domestic species, occupies unique status in the whole of genus Drosophila due to certain genetic stocks o f D . anunassae were used:
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Hereditas 109 (1988)
B N SlNCIH A N D A K \IN(,tl
MR-ST-derived from Madurai, Tamil Nadu; BH-ST and BH-AL-derived from Bhagalpur, Bihar; VN-ST and VN-AL-derived from Varanasi, Uttar Pradesh; GH-ST and GH-AL-derived from Ghazipur, Uttar Pradesh. The karyotypic constitutions o f all the stocks employed in the experiments dcscribed here are shown in Table 1. Since recombination occurs in D . ananassae males also, it was studied in both sexes. F, dihybrids heterozygous for delta and eta inversions were obtained by crossing females of MR-ST stock (DE/ DE. ET/ET) with males of standard stock (3L-ST/ ST. 3R-ST/ST). F, dihybrid males and females were then backcrossed to females and males of standard stocks. In total, six crosses were made as six stocks homozygous for S T gene order in the third chromosome were used. Fzlarvae were analysed for 3L and 3R karyotypes by usual acetocarmine method and data on the frequencies of parental and recombinant chromosomes among the gametes o f F, males and females were obtained. A comparison between heterozygotes and homozygotes among the F, larvae has also been made to test whether heterozygotes formed by chromosomes coming from distant localities exhibit heterosis.
Results The frequencies of parental and recombinant chromosomes in the offspring of F, females heterozygous for delta and eta inversions in the third chromosome derived from different stocks are presented in Table 2. The crossover rate between heterozygous inversions varies from 0.80 to 4.89 per cent. In all the three stocks (BH-AL, VN-AL, and G K A L ) , the frequency of crossing over is more than that in respective standard stocks. The F, females derived from A L stocks were karyotypi-
2L
12R
L JI
AL
3L
-
I-
DE
2L 3L
inversions cally heterozygous in 2L (STIAL). However, the F, females derived from S T stocks were structurally homozygous in 2L (ST/ST). There is some variation in the rate of crossing over when the stocks with similar karyotypes are compared. Thus the rate of crossing over between linked inversions of the third chromosome when heterozygous is strongly suppressed and is influcnced by strain (genic) factors as well as by karyotypic constitution in 2L. Table 3 shows the frequencies of non-recombinant and recombinant chromosomes in the progeny of F, males doubly heterozygous in the third chromosome under different karyotypic conditions in 2L. In all the stocks homozygous for STlST in 2L, recombinant chromosomes are completely absent. However, in two AL/AL stocks, recombinant chromosomes wcrc observed but their frequency is very low. It is interesting to note that recombinants were obserwd in males which were structurally heterozygous i n 2L, indicating interchromosomal effect. Table 4 gives the numbers of homo- and heterozygotes in .3L and 3K in F2 larvae resulting from backcross between dihyhrid flies and flies homozygous for standard gene order in the third chromosome. Following backcrossing, the larvae homozygous and heterozygous at both inversion loci should occur in equal ratios. It is apparent from the data in Table 4 that the invcrsion hcterozygotes are more frequent than the homozygotes in all the crosses, and deviations arc consistently in favour of heterozygotcs as differences are highly significant. Thus inversion heterozygotes formed by chromosomes derived from ecolngicully different and geo-
3R ST-\tand;ird:
Stocks
____
~-
MR-ST
BH-ST
BH-At,
VN-ST
VN-AL
