Disparate patterns of thermal adaptation between life stages in temperate vs. tropical populations of Drosophila melanogaster

Ectotherms may be threatened by increasing global temperatures, particularly if thermal adaptation is slower than climate change, as recent work has suggested. But estimates of evolutionary rates depend on the strength of natural selection, which may vary across life stages. Thus, depending on the life stage examined, comparisons of thermal tolerance among populations and species may yield inaccurate estimates of the rate at which thermal tolerance can evolve. Here we compared acute thermal tolerance in two life stages, adults and embryos, among populations of Drosophila melanogaster that span a broad range of thermal habitats across temperate sites in eastern North America and tropical sites around the globe. We report no variation in adult thermal tolerance among populations across this broad geographic range. In contrast, embryonic thermal tolerance was significantly higher in individuals from tropical sites compared to temperate sites. Embryonic thermal tolerance was similar among temperate populations, but among tropical populations embryonic thermal tolerance was positively correlated with maximum habitat temperature. We further report that embryos live closer to their upper thermal limits than adults; i.e., thermal safety margins are smaller for embryos than adults. F1 hybrid embryos from crosses between temperate and tropical populations had embryonic thermal tolerance that matched that of tropical embryos, suggesting phenotypic dominance of heat-tolerant alleles. Together our findings demonstrate that thermal tolerance readily evolves in the earliest and most thermally sensitive life stage and suggest that thermal selection may be more limited in the adult life stage. Disparate patterns of embryonic thermal tolerance between temperate and tropical populations may be the result of regional differences in seasonality.