Divergence of desiccation-related traits in two Drosophila species of the takahashii subgroup from the western Himalayas.

Drosophila nepalensis is more abundant under colder and drier montane habitats in the western Himalayas compared with Drosophila takahashii, but the mechanistic basis of such a climatic adaptation is largely unknown. We tested the hypothesis that divergence in the physiological basis of desiccation-related traits is consistent with species-specific adaptations to climatic conditions. Drosophila nepalensis showed approximately twofold higher desiccation resistance, hemolymph content as well as carbohydrate content than D. takahashii despite a modest difference in rate of water loss (0.3% h(-1)). Water loss before succumbing to death (dehydration tolerance) was much higher in D. nepalensis (82.32%) than in D. takahashii (∼50%). A greater loss of hemolymph water under desiccation stress until death is associated with higher desiccation resistance in D. nepalensis. In both species, carbohydrates were utilized under desiccation stress, but a higher level of stored carbohydrates was evident in D. nepalensis. Further, we found increased desiccation resistance in D. nepalensis through acclimation whereas D. takahashii lacked such a response. Thus, species-specific divergence in water-balance-related traits in these species is consistent with their adaptations to wet and dry habitats.