Plastic and Genetic Variation in Wing Loading as a Function of Temperature Within and Among Parallel Clines in Drosophila subobscura

Drosophila subobscura is a European (EU) species that was introducedinto South America (SA) approximately 25 years ago. Previousstudies have found rapid clinal evolution in wing size and inchromosome inversion frequency in the SA colonists, and theseclines parallel those found among the ancestral EU populations.Here we examine thermoplastic changes in wing length in fliesreared at 15, 20, and 25°C from 10 populations on each continent.Wings are plastically largest in flies reared at 15°C (thecoldest temperature) and genetically largest from populationsthat experience cooler temperatures on both continents. We hypothesizethat flies living in cold temperatures benefit from reducedwing loading: ectotherms with cold muscles generate less powerper wing beat, and hence larger wings and/or a smaller masswould facilitate fight. We develop a simple null model, basedon isometric growth, to test our hypothesis. We find that bothEU and SA flies exhibit adaptive plasticity in wing loading:flies reared at 15°C generally have lower wing loadingsthan do flies reared at 20°C or 25°C. Clinal patterns,however, are strikingly different. The ancestral EU populationsshow adaptive clinal variation at rearing a temperature of 15°C:flies from cool climates have lower wing loadings. In the colonizingpopulations from SA, however, we cannot reject the null model:wing loading increases with decreasing clinal temperatures.Our data suggest that selective factors other than flight havefavored the rapid evolution of large overall size at low environmentaltemperatures. However, selection for increased flight abilityin such environments may secondarily favor reduced body mass.