THERMAL EVOLUTION OF EGG SIZE IN DROSOPHILA-MELANOGASTER

We measured the size of eggs produced by populations of Drosophila mel anogaster, that had been collected along latitudinal gradients in diff erent continents or that had undergone several years of culture at dif ferent temperatures in the laboratory. Australian and South American p opulations from higher latitudes produced larger eggs when ail were co mpared at a standard temperature. Laboratory populations that had been evolving at 16.5 degrees C produced larger eggs than populations that had evolved at 25 degrees C or 29 degrees C, suggesting that temperat ure may be an important selective agent in producing the latitudinal d ines. Flies from laboratory populations produced larger eggs at an exp erimental temperature of 16.5 degrees C than at 25 degrees C, and ther e was no indication of genotype-environment interaction for egg size. Evolution of egg size in response to temperature cannot be accounted f or by differences in adult body size between populations. It is not cl ear which life-history traits are direct targets of thermal selection and which are showing correlated responses, and disentangling these is a task for the future.