[Male sterility at high and low temperatures in Drosophila]

It is well known that in Mammals, spermatogenesis requires a temperature lower than that of the body. In Ectotherms, for example in Insects, male sterility/ fertility according to environmental conditions also remains a neglected field. In Drosophila melanogaster, a complete male sterility after development at 30 degrees C was described in 1971. A similar phenomenon, observed at low temperature, was described two years later. Recent comparative investigations have shown that what was found in D. melanogaster was also valid in other species. In each case, it is possible to define a range of temperatures compatible with a complete development. According to the investigated species, however, this range is very variable, for example 6-26 degrees C or 16-32 degrees C. In each case, the occurrence of sterile males is observed before the lethality threshold is reached. Such a phenomenon is probably important for understanding the geographic distributions of species. The cosmopolitan D. melanogaster lives under very different climates and exhibits corresponding adaptations. In countries with a very hot summer, such as India or the African Sahel, male sterility appears only at 31 degrees C. Crosses between a temperate population from France and a heat-resistant Indian population revealed that a large part of the genetic difference was carried by the Y chromosome. Such a result is surprising since the Y chromosome harbors only a very small number of genes. In conclusion, drosophilid species, during their evolution, were able to adapt to very different climates and the thermal sterility thresholds have changed, following these adaptations. But we still lack an evolutionary hypothesis for explaining why sterile males are, in all cases, produced at extreme, low or high temperatures.