Increased body size confers greater fitness at lower experimental temperature in male Drosophila melanogaster

Genetic variation of body size along latitudinal clines is found globally i n Drosophila melanogaster, with larger individuals encountered at higher la titudes. Temperature has been implicated as a selective agent for these cli nes, because the body size of laboratory populations allowed to evolve in c ulture at lower temperatures is larger. In this study, we investigated the hypothesis that larger size is favoured at lower temperature through natura l selection on adult males. We measured life-span and age-specific fertilit y of males from lines of flies artificially selected for body size at two d ifferent experimental temperatures. There was an interaction between experi mental temperature and body size selection for male fitness; large-line mal es were fitter than controls at both temperatures, but the difference in fi tness was greater at the lower experimental temperature. Smaller males did not perform significantly differently from control males at either experime ntal temperature. The results imply that thermal selection for larger adult males is at least in part responsible for the evolution of larger body siz e at lower temperatures in this species. The responsible mechanisms require further investigation.