Urban–rural diversification in response to chronic nighttime dim light stress in Drosophila suzukii

Human disturbance in cities leads to multiple environmental changes, such as habitat fragmentation, raised ambient temperature, night lighting, and high noise levels. However, the genetic and nongenetic impacts of these environmental changes on the reproductive performance of animals are not well elucidated. Here, we examined the plastic response of artificial light at night (ALAN) on reproductive traits in rural and urban populations of a fruit pest (Drosophila suzukii). The fly strains derived from urban and rural populations were reared under 12-h daylight (2500 Lx), followed by either 12-h darkness (0 Lx, control) or dim light (10 Lx, ALAN treatment), from an egg to an adult stage. Then, body sizes, male mating activity, and female fecundity were examined. Although body sizes decreased and wing-to-thorax ratios increased upon ALAN exposure irrespective of the origin of flies (urban/rural), the impact of ALAN stress on the morphological traits was more pronounced in rural populations than in urban populations. Male courtship activities during daytime were higher in urban populations than in rural populations and were enhanced by ALAN exposure in both populations. Meanwhile, the behavioral response against ALAN differed between the rural and urban populations. The eggs of individuals originating from urban populations were more elongated than those of individuals originating from rural populations. Both egg size and shape were slightly affected by ALAN, and the number of eggs laid was remarkably increased by ALAN stress. Our results suggested that 1) several ecologically and reproductively essential traits have evolved in urban populations, 2) chronic exposure to ALAN induces a plastic response in these traits, and 3) individuals in urban populations have evolved to mitigate the impacts of ALAN stress.