Population genetics of inversion polymorphism in Drosophila ananassae.

Drosophila ananassae, although a cosmopolitan and domestic species, is largely circumtropical in distribution. It occupies unique status in the whole of genus Drosophila due to certain peculiarities in its genetical behaviour. The most unusual feature of this species is spontaneous male recombination in appreciable frequency. This species is of common occurrence in India. Population genetics of inversion polymorphism has been extensively studied in Indian D. ananassae in our laboratory. The main findings are: (i) Three cosmopolitan inversions namely AL in 2L, DE in 3L and ET in 3R are of frequent occurrence in Indian populations and have become coextensive with the species. (ii) Quantitative data on the frequency of three cosmopolitan inversions in 29 natural populations of D. ananassae suggest that there is a considerable degree of genetic divergence at the level of inversion polymorphism. In general, the populations from south India show more differentiation than those from the north. (iii) These three cosmopolitan inversions exhibit heterosis. Interracial hybridization does not lead to breakdown of heterosis, which suggests that evidence for coadaptation is lacking in geographic populations of D. ananassae. Heterosis appears to be simple luxuriance rather than populational heterosis (coadaptation). (iv) Data on intra- and interchromosomal associations in natural and laboratory populations of D. ananassae show that interchromosomal associations occur randomly indicating no interchromosomal interactions but two inversions of the third chromosome often show strong linkage disequilibria in laboratory populations which is due to epistatic interaction and suppression of crossing over. The role of genetic drift causing linkage disequilibria in isofemale strains of D. ananassae has also been suggested. (v) Mating ability of homo- and hetero-karyotypes due to subterminal (2L) inversion derived from natural populations of D. ananassae in which the frequency of different chromosome arrangements were known, was tested and the results indicate that the chromosome occurring in high frequency is associated with higher mating activity in all the populations analysed and heterokaryotypic males are superior in mating propensity to the corresponding homokaryotypes, indicating the existence of heterosis associated with the AL inversion with respect to male mating activity. Thus inversion polymorphism in D. ananassae may have a partial behavioural basis as has been demonstrated in other species of Drosophila.