RESUMO
BACKGROUND: Sexual conflicts between mating partners can strongly impact the evolutionary trajectories of species. This impact is determined by the balance between the costs and benefits of mating. However, due to sex-specific costs it is unclear how costs compare between males and females. Simultaneous hermaphrodites offer a unique opportunity to determine such costs, since both genders are expressed concurrently. By limiting copulation of focal individuals in pairs of pond snails (Lymnaea stagnalis) to either the male role or the female role, we were able to compare the fecundity of single sex individuals with paired hermaphrodites and non-copulants. Additionally, we examined the investment in sperm and seminal fluid of donors towards feminized snails and hermaphrodites. RESULTS: Compared to non-mating focal snails, reciprocating individuals as well as male and female copulants experienced a significant fecundity reduction (~40%) after, on average, 3.07 ± 0.12 copulations in their allowed roles (for donors 2.98 ± 0.16 copulations and for recipients 3.14 ± 0.12 copulations). In a single copulation, significantly more sperm was donated to partners that were restricted to mating in the female role than to hermaphrodites, while seminal fluid transfer was unaffected by recipient type. CONCLUSIONS: Our data indicate that the costs of mating in both sex functions are high in L. stagnalis. This conclusion is based on fecundity data collected separately for male and female copulants. Male mating costs result from investment in expensive ejaculates, composed of sperm and seminal fluid. For female copulants, fecundity reduction correlated with transferred sperm numbers in the first copulation, while differences in transferred quantities of seminal fluid were not detected. These findings may point toward a "sperm effect" as a novel feature of pond snail reproductive ecology. In conclusion, sex allocation and sexual conflict both contribute to decreased female fecundity in pond snails.
Assuntos
Lymnaea/fisiologia , Animais , Copulação/fisiologia , Feminino , Fertilidade , MasculinoRESUMO
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.
Assuntos
Interpretação Estatística de Dados , Preferência de Acasalamento Animal/fisiologia , Animais , Biomphalaria/fisiologia , Transtornos do Desenvolvimento Sexual , Feminino , MasculinoRESUMO
Mating partners often have conflicting interests when copulating. One of the major agents affecting female mating partners is seminal fluid, transferred along with sperm. The role of seminal fluid proteins (SFPs) in reproductive success is well studied in separate-sexed animals but is much less so in simultaneous hermaphrodites. The latter potentially have a unique target to exploit for the sperm donor's own benefit: the male function of their mating partners. Here we show that, in the great pond snail Lymnaea stagnalis, receipt of specific SFPs reduces both sperm transfer and paternity success in a subsequent insemination event. Lowering investment in the mating partner's male function constitutes a novel role for SFPs. This demonstrates for the first time that hermaphrodites alter their mates' male as well as female reproductive output. Although it remains to be tested whether this represents mate manipulation or an adaptive response of recipients, our findings identify male investment as a new target for postcopulatory sexual selection.