Thermal plasticity of wing size and shape in Drosophila melanogaster, D. simulans and their hybrids

Populations of Drosophila melanogaster, as well as of other Drosophila species, show body size differences according to their geographic origin. Thermal selection is considered to be the most likely cause explaining these differences. We investigated wing size, wing shape and their rela- tionship in 3 different geographic populations of D. melanogaster, 1 population of D. simulans, and their interspecific hybrids grown at 18, 21, 25 and 28°C. The aim was to explore how the past adap- tive history of 2 related species, or populations of the same species, modulates the plastic response to the environment. The wing size plasticity of hybrids between the temperate D. simulans (Bologna) and the 2 tropical D. melanogaster populations (Belem and Rio de Janeiro) was higher than that of parental species, probably as a side effect of a decanalized and compromised development at higher temperatures. The wing size plasticity of Bologna hybrids was the same as the parents, suggesting that the 2 species are subjected to the same plasticity selection. Wing shape was typical of each spe- cies, population and temperature. Shape differences increased in hybrids at higher temperatures as a consequence of developmental perturbation. The allometric relationship between size and shape changed among temperatures and among species, suggesting that the wing development is differ- ently regulated in the 2 species and can be altered by natural selection. The allometry changed between populations of different geographic origin of D. melanogaster, but was similar in the 2 spe- cies that shared the same selective environment. Two species would have been subjected to the same plasticity selection, so the breakdown of the plastic response is avoided in hybrids. As a whole, these data suggest that thermal plasticity is a trait under selection and that similar mechanisms are at work in different species.