Female-limited X-chromosome evolution effects on male pre- and post-copulatory success

Why is measuring and predicting fitness under genomic conflict so hard?

Genomic conflict between the sexes is caused by differences in the optimal male and female reproductive strategies, and is a major contributor to genetic, phenotypic, and life history variation. While early experimental work appeared to strongly support the sexual conflict paradigm, recent work has produced more ambiguous results. Recent advances in experimental evolution studies combined with theoretical arguments can shed light on why measuring fitness under a conflict is so challenging, including the incidental alteration of mating dynamics, demographic effects, and inherent complexity in what quantity selection maximizes. We stress that non-intuitive results do not necessarily mean the absence of conflict, and follow-up experiments to determine why a priori predictions failed can ultimately teach us more than if they had been confirmed.

Female-limited X chromosome evolution reveals that lifespan is mainly modulated by interlocus rather than intralocus sexual conflict

Abstract

Sexual dimorphism in somatic investment may be shaped by two distinct forms of sexual conflict; under intralocus sexual conflict (IASC), males and females have different optimal levels of somatic investment but are constrained from reaching their respective optima by their shared genome, while under interlocus sexual conflict (IRSC), males and females have different optimal sexual strategies, which could have direct or indirect effects on levels of somatic investment. We investigated effects of IASC and IRSC on two aspects of somatic investment, immune defence strategies and longevity, using previously established female-limited experimental evolution lines in Drosophila melanogaster. We found little evidence for any effect of either type of sexual conflict on investment in the immune defence resistance or tolerance. Nor did we find convincing evidence that longevity is subject to IASC in this species. However, we did find evidence that increased female control over mating rate had important and opposite effects on longevity between the sexes. Specifically, females that had adapted to high levels of female control over mating had a longer lifespan when kept in mixed-sex groups, while males had shorter longevity, perhaps due to increased investment in post-copulatory sexual selection. These novel results show that female control over mating rates may have important and unexpected effects on patterns of somatic investment.

Significance statement

Sexual conflict occurs between the two sexes over numerous life history traits, and it is complex to disentangle how these traits interact and affect each other. Here we use a long-term evolution experiment to investigate sexual dimorphism in somatic maintenance. We found no effect of feminising the X chromosome on female immune defence. However, we did find that increased female control over mating rate resulted in longer female lifespan, but reduced male lifespan, and that these effects were dependent on social context (isolated or in mixed-sex groups). Unlike previous studies on the effect of sexual conflict on longevity, our experiment did not manipulate environmental conditions nor the adult sex ratio, which is likely to reduce both pre- and post-copulatory sexual selection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00265-022-03231-4.