Longevity and resistance to cold stress in cold-stress selected lines and their controls in Drosophila melanogaster

Thermal environments can influence many fitness-related traits including life span. Here, we assess whether longevity in Drosophila melanogaster can experimentally evolve as a correlated response to cold-stress selection, and whether genotype-by-temperature and sex-by-temperature interactions are significant components of variation in life span. Three replicated S lines were cold-stress selected and compared with their respective unselected controls (Clines) in the 16(th) generation of thermal selection. Cold-stress resistance exhibited a substantial direct response to selection, and also showed a significant interaction between sex and type of line. Mean longevity exhibited a significant interaction between adult test temperature (14 and 25 degreesC) and line (with suggestive evidence for increased longevity of S lines when tested at 14 degreesC), but there was no evidence for increased longevity in S lines at normal temperatures (i.e. 25 degreesC). Another temperature-dependent effect was sex-specific, with males being the longer lived sex at 25 degreesC but the less long-lived sex at 14 degreesC. Additionally, we tested in an exploratory way the relationship between longevity and cold-stress resistance by also measuring resistance to a prefreezing temperature before and after one generation of longevity selection at 14 degreesC (selection intensity, i = 1.47 for S lines, and 1.42 for C lines). In this longevity selection, we found that cold-stress resistance increased by about 6% in S lines and 18% in C lines. However, taken together, the results indicate no simple relationship between longevity and cold-stress resistance, with genotype-by-sex interactions in both traits. Temperature dependent interaction in longevity is apparent between S and C lines, and sex-specific variation in mean longevity also depends on temperature.