“Genetic quality affects the rate of male and female reproductive ageing differently in Drosophila melanogaster”
Martin Brengdahl, Christopher M. Kimber, Jack Maguire-Baxter, Antonino Malacrinò, and Urban Friberg (Dec 2018)
Genetic quality affects the rate of male and female reproductive ageing differently in Drosophila melanogaster
Why do males and females age at different rates?
Why males and females of many species display different rates of ageing is puzzling and may hold the key to explain the enormous variation in ageing found among species. A team of researchers from Linköping University in Sweden has investigated the hypothesis that sex differences in reproductive ageing result from the sexes investing different amounts of their energy budgets into current reproduction and maintaining their bodies. To test this hypothesis, they varied the genetic quality of male and female fruit flies through the number of deleterious mutations they expressed, with consequences for how efficiently they could compete over food resources and convert these into usable energy. In response to the manipulation, the authors find that the pace of male reproductive ageing is unaltered by genetic quality, but that high-quality females show considerably slower reproductive ageing compared to low-quality females. These results suggest that high-quality females invest relatively more energy than low-quality females into maintaining their bodies and the opportunity to reproduce also later in life, while males, independently of their quality, focus on current reproduction. These results fit with the general view that males and females are selected to peruse different strategies to maximize reproductive success; as a consequence, males face fierce competition over access to females, and female production of eggs and offspring is primarily limited by access to resources.
Males and females often maximize fitness by pursuing different reproductive strategies, with males commonly assumed to benefit more from increased resource allocation into current reproduction. Such investment should trade-off with somatic maintenance and may explain why males frequently live shorter than females. It also predicts that males should experience faster reproductive ageing. Here we investigate if reproductive ageing and lifespan respond to condition differently in male and female Drosophila melanogaster, as predicted if sexual selection has shaped male and female resource allocation patterns. We manipulate condition through genetic quality, by comparing individuals inbred or outbred for a major autosome. While genetic quality had a similar effect on condition in both sexes, condition had a much larger general effect on male than female reproductive output, as expected when sexual selection on vigor acts more strongly on males. We find no differences in reproductive ageing between the sexes in low condition, but in high condition reproductive ageing is relatively faster in males. No corresponding sex-specific change was found for lifespan. The sex difference in reproductive ageing appearing in high condition was due specifically to a decreased ageing rate in females, rather than any change in males. Our results suggest that females age slower than males in high condition primarily because sexual selection has favored sex differences in resource allocation under high condition, with females allocating relatively more towards somatic maintenance than males.