Divergence of reaction norms of size characters between tropical and temperate populations of Drosophila melanogaster and D. simulans

Reaction norms to growth temperature of two size-related traits, wing and thorax length, were compared in tropical (West Indies) and temperate (France) populations of the two sibling species, Drosophila melanogaster and D. simulans. A major body size difference was found in D. melanogaster, with much smaller Caribbean flies, while D. simulans exhibited little size variation between geographical populations. The concave norms of reaction were adjusted to second- or third-degree polynomials, and characteristic points calculated i.e. maximum value (MV) and temperature of maximum value (TMV). TMVs were confirmed to be higher for thorax than for wing length, higher in D. melanogaster than in D. simulans, and higher in females than in males. For both traits Caribbean populations exhibited higher TMVs in the two species, strongly suggesting an adaptive shift of the reaction norms toward higher temperature in warm-adapted populations. The wing/thorax ratio was also analysed, and found to be significantly lower in tropical populations of both species. This ratio, which is related to wing loading and flight capacity, might evolve independently of body weight itself.