Author's personal copy Mating rate influences female reproductive investment in a simultaneous hermaphrodite, Lymnaea stagnalis

Multiple mating often imposes direct fitness costs on females but can provide indirect benefits such as enhanced genetic diversity and offspring quality. The costs and benefits of multiple mating have been investigated extensively in separate-sex species but less so in simultaneous hermaphrodites, despite being highly relevant given their flexible resource allocation and ability to compensate for costs experienced in one sexual role by gains in the opposite role. At high mating rates, the promiscuous hermaphroditic pond snail Lymnaea stagnalis experiences depressed female fecundity mediated by seminal fluid compounds. By experimentally manipulating mating opportunities, we tested for effects on female reproductive investment over 10 weeks. As expected, continuous access to mating partners resulted in decreased female investment, in terms of both total number of eggs (fecundity) and egg mass dry weight. Total investment in offspring increased over time for all treatments but was significantly less pronounced in treatments with continuous access to partners, and this was irrespective of partner identity. Investment per offspring was positively correlated with higher mating rates across treatments. Thus, multiple mating resulted in higher investment in egg masses at low than at higher mating rates. In contrast, at higher mating rates the investment per egg was higher. We conclude that, in L. stagnalis, mating multiply can severely impact female reproductive success and, although we cannot entirely exclude reallocation of resources to the male function from this study, we argue that this is probably the result of sexual conflict caused by previously identified seminal fluid components.

Bateman gradients in hermaphrodites : an extendes approach to quantify sexual selection

Sexual selection is often quantified using Bateman gradients, which represent sex-specific regression slopes of reproductive success on mating success and thus describe the expected fitness returns from mating more often. Although the analytical framework for Bateman gradients aimed at covering all sexual systems, empirical studies are biased toward separate-sex organisms, probably because important characteristics of other systems remain incompletely treated. Our synthesis complements the existing Bateman gradient approach with three essential reproductive features of simultaneous hermaphrodites. First, mating in one sex may affect fitness via the opposite sex, for example, through energetic trade-offs. We integrate cross-sex selection effects and show how they help characterizing sexually mutualistic versus antagonistic selection. Second, male and female mating successes may be correlated, complicating the interpretation of Bateman gradients. We show how to quantify the impact of this correlation on sexual selection and propose a principal component analysis on male and female mating success to facilitate interpretation. Third, self-fertilization is accounted for by adding selfed progeny as a separate category of reproductive success to analyses of Bateman gradients. Finally, using a worked example from the snail Biomphalaria glabrata, we illustrate how the extended analytical framework can enhance our understanding of sexual selection in hermaphroditic animals and plants