Abstract: Ectothermic drosophilids are profoundly affected by thermal selection (i.e., genetic effects) or through induced effects on phenotype (i.e., plasticity). Phenotypic plasticity is a powerful means of adaptation in diverse organisms but has received less attention for different drosophilids. We analyzed reaction norms of melanisation in Drosophila species which differ in developmental thermal range and geographical distribution. D. ananassae and D. jambulina are cold sensitive, and these species can be cultured between 18 to 32℃. By contrast, D. nepalensis is cold-tolerant and heat-sensitive species which can be raised between 12 and 25℃. The cosmopolitan species D. melanogaster has a broader thermal range (13-31℃). Significant differences were observed between reaction norms of melanisation in three anterior vs. three posterior abdominal segments in these species. In D. nepalensis, all the six abdominal segments (2nd-7th) are highly plastic. However, only the last three abdominal segments are plastic in D. melanogaster. In contrast, D. ananassae (a tropical species) lacks plasticity for all abdominal segments. Cosmopolitan species (D. melanogaster), even from much colder climates, does not show darker phenotypes similar to that observed in D. nepalensis. The aim of this study is to understand the processes involved in generating the morphological diversity of color patterns and adaptation of Drosophila species to different geographical regions. Finally, comparing body melanisation patterns to phylogeny suggests recurrent adaptations for genetic polymorphism vs. phenotypic plasticity in different evolutionary lineages.

Key words: Drosophila, phenotype, climate adaptation, body melanisation, developmental plasticity, genetic polymorphism, species distribution