Gupta, S.

In this study, we collected four geographical populations of Drosophila melanogaster and Drosophila ananassae from the southern and northern axes of India, spanning latitudes ranging from 13.04 degrees N to 29.58 degrees N. We observed significant clinal variation at the alcohol dehydrogenase (Adh) locus, with the frequency of the Adh F allele increasing by 3% and 1.5% per degree latitude in D. melanogaster and D. ananassae, respectively. Adult populations of D. melanogaster were found to tolerate ethanol concentrations ranging from 10% to 14.75%, while D. ananassae exhibited tolerance levels between 2.4% and 4.0%. These patterns of interspecific divergence in ethanol tolerance levels corresponded with their natural habitats: D. ananassae primarily utilizes fermented organic matter and fruits in the wild, while D. melanogaster exploits ethanol-rich resources found indoors. Ethanol serves as a resource at low concentrations but becomes a stressor at higher concentrations for D. ananassae. Both traits exhibited a clinal pattern that is increase in ethanol tolerance and of the frequency of Adh-F allele with latitude. Our observations suggest that D. ananassae exhibits adaptive characteristics for ethanol utilization, while D. melanogaster populations demonstrate both ethanol utilization and detoxification abilities. The observed latitudinal variation in Adh F frequency and ethanol tolerance in these two Drosophilids may be maintained by the interplay of balancing natural selection and climatic factors that vary spatially along the south-north axis of the Indian subcontinent.