Abstract: In subtropical parts of Indian subcontinent, autumn is cold and dry while spring is wet and humid and ectothermic drosophilids are expected to evolve desiccation resistance to cope with drier climatic conditions. Drosophila jambulina exhibits color dimorphism. Previous studies have described that in tropical populations of D. jambulina, body color polymorphism is maintained through humidity changes as opposed to thermal melanism and seasonal changes in the frequency of body color morphs in this tropical species supports melanism-desiccation hypothesis. But the mechanistic bases of such climatic adaptations in two body color morphs of D. jambulina are largely unknown. We tested the hypothesis that divergence in the physiological basis of desiccation-related traits is consistent with body color morph-specific adaptations to climatic conditions, for which we examined the response of water balance to relative humidity (RH), temperature, and their interaction in D. jambulina, using two body color morphs that had been allowed to rear at low (40%) or high humidity (80%) at 17℃ and 25℃. We found that, at low RH, dark body color morph had significantly greater physiological and desiccation trait values than light body color morph at both the temperatures. A comparative analysis of water budget of the two body color morphs showed that higher water content, reduced rate of water loss and greater dehydration tolerance confer higher desiccation resistance in dark morph of D. jambulina at low RH. We found that carbohydrates act as metabolic fuel during desiccation stress in both the morphs, but a higher level of stored carbohydrates was evident in dark morph at low humidity. Further, total energy budget differ significantly between these two body color morph at two humidity. Thus, body color morph-specific divergence in water-balance-related traits in D. jambulina is consistent with their adaptations to wet and dry habitats.

Key words: Drosophila jambulina, water balance, body color morph, polymorphism, relative humidity, energy budget, climatic adaptation