Divergent strategies in low temperature environment for the sibling species Drosophila melanogaster and D-simulans:: overwintering in extension border areas of France and comparison with African populations

The two sibling species D. /melanogaster and D. simulans adopt different overwintering strategies in northern border areas situated in France. If the winter is mild, both species reappear in early spring to refound the population. If the winter is cold, with several weeks of temperatures below 0degreesC, D. melanogaster leave their shelters in April whereas D. simulans, which do not use shelters, reappear in late June, probably after returning from further south. Here, we tried to identify life-history characteristics responsible for this difference. For this, we studied developmental duration, viability, fecundity, fertility and longevity, and compared the abilities of French and African populations to survive when food supplies were inadequate, at different temperatures (14, 11, 7degreesC). These temperatures are lower than those commonly used in the laboratory but closer to real conditions encountered in the wild. When the temperature was mild (14 or 11degreesC) and the food supply was adequate, D. simulans performed better than D. melanogaster: it had a higher fecundity, a longer life expectancy and the males remained fertile, allowing outdoor reproduction late in winter. However, D. simulans was less resistant in more extreme conditions. At 7degreesC D. simulans survived shorter on normal medium and its ability to survive when food supplies were inadequate was insufficient to allow outdoor overwintering. In contrast, D. melanogaster could not reproduce during winter: its fecundity was low and males were sterile at 11degreesC. Nevertheless, if only protein-deficient resources were available, temperate D. melanogaster could survive for longer than D. simulans at all the temperatures tested. This greater resistance to underfeeding allows the species to survive until spring, in shelters for several months. A comparison of French and African population performances showed differences in the evolution of the two species during the colonization of more northern areas. African D. simulans, which are efficient at mild temperatures, underwent few modi. cations. In contrast, the viability of D. melanogaster improved at low developmental temperatures. This species also displayed higher fecundity, longer survival and higher underfeeding resistance at low temperatures. The relationship between the long retention genotype and underfeeding resistance or survival ability observed in French D. melanogaster populations may not exist in African populations.