Ten isofemale lines of Drosophila melanogaster, recently collected in a French vineyard, were submitted to 7 different developmental temperatures, from 12 to 31-degrees-C, encompassing the whole physiological range of the species. For each line and temperature, 10 flies of each sex were collected randomly and 2 size-related traits were measured: wing and thorax length. Both traits exhibited similar response curves: a maximum size, -at a low temperature and a decrease on both sides. ANOVA showed significant variations between lines and also significant line-temperature interactions, demonstrating different norms of reaction among the various lines. The shapes of the curves were further analysed by considering slope variations, ie by calculating empirical derivative curves. The most interesting observation is that the temperature of maximum size (TMS) is not the same for the wing (average 15.73 +/- 0.29-degrees-C) and the thorax (average 19.57 +/- 0.47-degrees-C). Genetic differences seem to exist between lines, and TMS for both traits are correlated. Sexual dimorphism was analysed by considering the female/male ratio for wing and thorax. Both traits provided the same information: sexual dimorphism increased, from 1.10 to 1.16, with increasing temperature, and significant differences were found between lines. Finally the wing/thorax ratio appeared as an original and most interesting trait. This ratio, which is less variable than wing or thorax, exhibited a monotonously decreasing sigmoid shape, from 2.80 to 2.40, with increasing temperature. It is suggested that this ratio, which may be related to flight capacity at various temperatures, could be the direct target of natural selection.
