LATITUDINAL VARIATION OF WING:THORAX SIZE RATIO AND WING-ASPECT RATIO IN DROSOPHILA MELANOGASTER

In dipterans, the wing-beat frequency, and, hence, the lift generated, increases linearly with ambient temperature. If flight performance is an important target of natural selection, higher wing:thorax size ratio and wing-aspect ratio should be favored at low temperatures because they increase the lift for a given body weight. We investigated this hypothesis by examining wing: thorax size ratio and wing-aspect ratio in Drosophila melanogaster collected from wild populations along a latitudinal gradient and in their descendants reared under standard laboratory conditions. In a subset of lines, we also studied the phenotypic plasticity of these traits in response to temperature. To examine whether the latitudinal trends in wing:thorax size ratio and wing-aspect ratio could have resulted from a correlated response to latitudinal selection on wing area, we investigated the correlated responses of these characters in lines artificially selected for wing area. In both the geographic and the artificially selected lines, wing:thorax size ratio and wing-aspect ratio decreased in response to increasing temperature during development. Phenotypic plasticity for either trait did not vary among latitudinal lines or selective regimes. Wing:thorax size ratio and wing-aspect ratio increased significantly with latitude in field-collected flies. The cline in wing:thorax size ratio had a genetic component, but the cline in wing-aspect ratio did not. Artificial selection for increased wing area led to a statistically insignificant correlated increase in wing:thorax size ratio and a decrease in wing-aspect ratio. Our observations are consistent with the hypotheses that high wing-thorax size ratio and wing aspect ratio are per se selectively advantageous at low temperatures.