How to overwinter and be a founder: egg-retention phenotypes and mating status in Drosophila melanogaster

In temperate regions, Drosophila melanogaster has perennial overwintering populations. These populations present seasonal variations, under the influence of developmental temperature, for several genetically determined physiological traits. The capacity of virgin females to control ovulation is one of these characteristics. Phenotypes able to postpone egg-laying in the absence of insemination are favored under low temperature development and are numerous in Autumn generations. Moreover, a shift between Autumn and the following Spring has often been observed in favor of these phenotypes. The aim of the present study is to determine the characteristics and situations which confer an advantage on long as compared with short-retention phenotypes, during this non-reproductive period. Several traits were studied: resistance to cold shocks, resistance to long cold periods, developmental duration and viability, longevity and starvation resistance. Long-retention phenotypes (LL) had a longer life expectancy than short-retention phenotypes (ss) under virgin or mated status and greater resistance to starvation, by avoiding waste material (eggs). At 14 °C, flies that had mated once survived for several months on normal substrate with live spermatozoa, and flies on deficient medium (without proteins) survived for more than 3 months varying with phenotype. Flies with the best chance of overwintering are the long-retention phenotypes and some hybrids. The most favorable situation for population restoration is when flies are inseminated once in Autumn rather than when they remain virgin until Spring, because males die sooner than females.