Genome-wide selection signatures reveal widespread synergistic effects of two different stressors in Drosophila melanogaster
Experimental evolution combined with whole-genome sequencing (E&R) is a powerful approach to study the adaptive architecture        of selected traits. Nevertheless, so far the focus has been on the selective response triggered by a single stressor. Building on the        highly parallel selection response of founder populations with reduced variation, we evaluated how the presence of a second stressor        affects the genomic selection response. After 20 generations of adaptation to laboratory conditions at either 18°C or 29°C,        strong genome-wide selection signatures were observed and almost the entire genome was affected by selection. Only about one third of        the selection signatures can be attributed to laboratory adaptation (no difference between temperature regimes). The remaining selection        responses are either caused by temperature specific effects, or reflect the joint effects of temperature and laboratory adaptation (same        direction, but the magnitude differs between temperatures). The allele frequency changes resulting from the combined effects of temperature        and laboratory adaptation were more extreme in the hot environment for 83% of the affected genomic regions – indicating widespread        synergistic effects of the two stressors. We conclude that E&R with reduced genetic variation is a powerful approach to study        genome-wide fitness consequences driven by the interaction of multiple environmental factors. Furthermore, laboratory adaptation can have        a major impact on the selection response and E&R studies contrasting ancestral and evolved populations can easily confound laboratory        adaptation with a response to the focal trait.