Sexual conflict over remating interval is modulated by the sex peptide pathway.

Sexual conflict, in which the evolutionary interests of males and females diverge, shapes the evolution of reproductive systems across diverse taxa. Here, we used the fruit fly to study sexual conflict in natural, three-way interactions comprising a female, her current and previous mates. We manipulated the potential for sexual conflict by using sex peptide receptor (SPR) null females and by varying remating from 3 to 48 h, a period during which natural rematings frequently occur. SPR-lacking females do not respond to sex peptide (SP) transferred during mating and maintain virgin levels of high receptivity and low fecundity. In the absence of SPR, there was a convergence of fitness interests, with all individuals gaining highest productivity at 5 h remating. This suggests that the expression of sexual conflict was reduced. We observed an unexpected second male-specific advantage to early remating, resulting from an increase in the efficiency of second male sperm use. This early window of opportunity for exploitation by second males depended on the presence of SPR The results suggest that the SP pathway can modulate the expression of sexual conflict in this system, and show how variation in the selective forces that shape conflict and cooperation can be maintained.

Feeding, fecundity and lifespan in female Drosophila melanogaster.

Male seminal fluid proteins induce a profound remodelling of behavioural, physiological and gene signalling pathways in females of many taxa, and typically cause elevated egg production and decreased sexual receptivity. In Drosophila melanogaster, these effects can be mediated by an ejaculate 'sex peptide' (SP), which, in addition, contributes significantly to the cost of mating in females. Recent research has revealed that SP can stimulate female post-copulatory feeding, raising the possibility that the widespread female cost of mating could be due to over-feeding. In this study, we used D. melanogaster as a model to test this hypothesis. We first show that elevated post-mating feeding is dependent upon egg production and does not occur in sterile ovoD1 mutant females. This conclusion was also supported by the increase in feeding of virgin females whose egg production was experimentally elevated. We then demonstrated that sterile ovoD1 and fertile females experienced identical survival costs of mating, related to their frequency of mating and not to female feeding rate or to egg production. We conclude that female mating costs are not the result of over-feeding, but may be due to other, potentially more direct, effects of ejaculate molecules.

A functioning ovary is not required for sex peptide to reduce receptivity to mating in D. melanogaster.

In many species of invertebrates that mate multiply, mating induces a temporary reduction in sexual receptivity and an increase in the rate of egg laying. These processes often appear to be co-ordinately regulated, and triggered by the passage of seminal fluid components. However, little is known about the mechanisms of the links between these processes. In Drosophila melanogaster females, post-mating sexual receptivity is decreased and egg laying increased by the actions of the male ejaculate-derived sex peptide (SP). Effects of SP on egg laying and receptivity have not been observed separately, which has led to the suggestion that the reduction in receptivity is at least partially dependent on the status of egg development or egg laying, with the presence of an egg in the uterus being a strong predictor of receptivity state. Here, we examine the response to SP of females in which egg development is arrested at an early, pre-vitellogenic stage. We find that females in which egg development is arrested mate normally and that normal receptivity responses to SP are independent of early egg arrest. Among fertile control females that laid eggs, a significant effect of SP on receptivity was also observed, independent of whether an egg was present in the uterus. The results show that the effects of SP on receptivity are not dependent upon a fully functional ovary, and hence that egg development or laying is not causal in the SP receptivity response.

No extension of lifespan by ablation of germ line in Drosophila.

Increased reproduction is frequently associated with a reduction in longevity in a variety of organisms. Traditional explanations of this 'cost of reproduction' suggest that trade-offs between reproduction and longevity should be obligate. However, it is possible to uncouple the two traits in model organisms. Recently, it has been suggested that reproduction and longevity are linked by molecular signals produced by specific reproductive tissues. For example, in Caenorhabditis elegans, lifespan is extended in worms that lack a proliferating germ line, but which possess somatic gonad tissue, suggesting that these tissues are the sources of signals that mediate lifespan. In this study, we tested for evidence of such gonadal signals in Drosophila melanogaster. We ablated the germ line using two maternal effect mutations: germ cell-less and tudor. Both mutations result in flies that lack a proliferating germ line but that possess a somatic gonad. In contrast to the findings from C. elegans, we found that germ line ablated females had reduced longevity relative to controls and that the removal of the germ line led to an over-proliferation of the somatic stem cells in the germarium. Our results contrast with the widely held view that it is downstream reproductive processes such as the production and/or laying of eggs that are costly to females. In males, germ line ablation caused either no difference, or a slight extension, in longevity relative to controls. Our results indicate that early acting, upstream reproductive enabling processes are likely to be important in determining reproductive costs. In addition, we suggest that the specific roles and putative patterns of molecular signalling in the germ line and somatic tissues are not conserved between flies and worms.

Mating and immunity in invertebrates.

Mating and immunity are intimately linked to fitness. In both vertebrates and invertebrates, recent investigations into mate choice for immunity, tradeoffs between reproduction and immunity, and the relationships between post-mating processes and immune function have revealed that mating and immunity are also intimately linked to each other. Here, we focus on invertebrates and critically examine the evidence that immunity is under sexual selection, both pre- and post-mating, and explore other hypotheses linking mating and immunity. We find little evidence for a consensus regarding which theories best account for the accumulating empirical data. However, we suggest that progress can quickly be made by exploiting the intrinsic strengths of invertebrate model systems.