Effect of environmental stress in Drosophila melanogaster evolved under differential levels of sexual selection

Abstract

Evolving under biased sex ratios, male- or female-biased, introduces differential levels intersexual conflict between the sexes. While male-biased (M) populations undergo an antagonistic coevolution between the sexes that selects for exaggerated reproductive traits in the males and resistance traits in the females, female-biased (F) populations face a relaxed selection on these traits. Also, variation in the expression of reproductive traits is the raw material for selection to act on, and has an environmental component. It is therefore important to study how a key factor such as environmental variation influences these reproductive traits. In this study, we address this issue in a systematic manner. We investigated the effect of a key environmental factor, Larval density during development, on the male and female D. melanogaster from populations evolving under altered operational sex ratios. To do this, we reared flies from the male- and female- biased selection regimes in the control density and a 4-fold high density, and compared various life-history and reproductive trait values. High larval density had a detrimental effect on all the life-history traits studied in both sexes. And, both male-biased and female-biased selection regime flies were affected to similar levels. However, a sex-specific response to larval crowding was observed in the reproductive fitnesses. The female competitive fitness was robust to larval crowding, whereas the male competitive fitness was detrimentally affected by larval crowding. This study concludes that the evolutionary history of sexual selection does not play a crucial role in modulating the responses to developmental environment variation in D. melanogaster.