Sibling Drosophila species (Drosophila leontia and Drosophila kikkawai) show divergence for thermotolerance along a latitudinal gradient

Understanding adaptations to climatic stresses has been a longstanding issue in evolutionary biology. Clinal patterns for stress tolerance traits at interspecific as well as intraspecific levels provide an opportunity to comprehend the adaptive divergence acquired in the closely related species among different geographical populations. In the present study, we investigated the geographical differences for basal as well as induced thermal tolerance (hardening) in two sympatric sibling Drosophila species (D. leontia and D. kikkawai) along a latitudinal transect (South: 8A degrees 06'N to North: 32A degrees 40'N) across India. A higher relative abundance of D. leontia was observed in the southern localities (tropical) while D. kikkawai was more abundant in the northern localities (sub-tropical). Both the species showed a negative cline for heat tolerance along the latitude i.e. following a trend of increased tolerance level in the southern populations compared to northern ones. Contrarily, a positive cline for cold tolerance was evident in D. kikkawai, while such intra-specific population differences were non-significant in D. leontia. Further, we found the evidence for heat and cold hardening effects only in D. kikkawai. In D. kikkawai, higher hardening capacity for heat as well as cold tolerance was observed in northern populations compared to southern populations, despite lower basal heat tolerance in northern populations. In conclusion, clinal patterns for hardening capacity for heat and cold tolerance in D. kikkawai while absence of the same in D. leontia suggest divergent evolutionary history of these two sympatric sibling species.