Postponed reproduction as an adaptation to winter conditions in Drosophila melanogaster:: evidence for clinal variation under semi-natural conditions

Patterns of climatic adaptation in drosophila and other insects have largely been inferred from laboratory comparisons of traits that vary clinally. Here, we extend this research to comparisons under seminatural conditions. To test for clinal variation in reproductive patterns and survival over winter, Drosophila melanogaster populations were initiated from seven collection sites along the eastern coast of Australia, ranging front tropical to temperate regions. The fecundity and survival of these populations were monitored in field cages at a temperate location until all adults had died more than 5 months later. Total fecundity showed a curvilinear relationship vith latitude, due to higher egg production by high- and low-latitude populations. Adults front temperate locations survived winter conditions better than those from subtropical populations but not tropical ones. There vas a linear cline in the timing of egg production: temperate populations produced eggs later than populations front lower latitudes. This cline is likely to be adaptive because egg-to-adult viability experiments indicated that only eggs laid in spring developed successfully to the adult stage. There vas no evidence for climatic adaptation in the immature stages. The adult mortality rate increased gradually over winter, and in some populations vas also correlated with the minimum ambient temperature. These results indicate that adaptation to winter conditions in D. melanogaster has involved shifts in reproductive patterns.