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Drosophila ananassae Doleschall is a cosmopolitan and domestic species. It occupies a unique status among Drosophila species due to certain peculiarities in its genetic behaviour and is of common occurrence in India. Quantitative genetics of sexual and non-sexual traits provided evidence for genetic control of these traits. D. ananassae exhibits high level of chromosomal polymorphism in its natural populations. Indian natural populations of D. ananassae show geographic differentiation of inversion polymorphism due to their adaptation to varying environments and natural selection operates to maintain three cosmopolitan inversions. Populations do not show divergence on temporal scale, an evidence for rigid polymorphism. D. ananassae populations show substantial degree of sub-structuring and exist as semi-isolated populations. Gene flow is low despite co-transportation with human goods. There is persistence of cosmopolitan inversions when populations are transferred to laboratory conditions, which suggests that heterotic buffering is associated with these inversions in D. ananassae. Populations collected from similar environmental conditions that initially show high degree of genetic similarity have diverged to different degrees in laboratory environment. This randomness could be due to genetic drift. Interracial hybridization does not lead to breakdown of heterosis associated with cosmopolitan inversions, which shows that there is lack of genetic co-adaptation in D. ananassae. Linkage disequilibrium between independent inversions in laboratory populations has often been observed, which is likely to be due to suppression of crossing-over and random genetic drift. No evidence for chromosomal interactions has been found in natural and laboratory populations of D. ananassae. This strengthens the previous suggestion that there is lack of genetic co-adaptation in D. ananassae.

The present study, which is one of the longest temporal (two decades) and largest spatial (different parts of India covered) investigations on inversion polymorphism in natural populations of D. ananassae, was undertaken to understand the dynamics of inversion polymorphism in a broad and comprehensive manner. Forty-five natural populations from different ecogeographic regions of the country (covering the regions from Kashmir to Kanniyakumari and Gujarat to Nagaland) were analysed for chromosomal inversions. All the populations show the presence of the three cosmopolitan inversions, frequencies of which vary among the populations analysed. Simple correlations between frequencies of different inversions and regression analysis of inversion frequencies with latitude, longitude and altitude were insignificant. This reinforces the concept of rigid polymorphism in D. ananassae. Genetic divergence (spatial and temporal) at the level of chromosomal polymorphism among natural populations was calculated. Results show spatial divergence but no temporal divergence. Rigid polymorphic systems of D. ananassae did not show long-term directional trends. On the basis of the present study, and after including comparisons with the studies conducted more than two decades ago, the most important conclusion to be drawn is that the three cosmopolitan inversions in D. ananassae segregate within populations at fairly similar frequencies, and the general geographic pattern has remained constant.

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