The manner by which sexual selection acts on mate choice and the factors that influence choice are still not yet clear. Available evidence in gonochoristic animals (species with separate sex) indicates that the outcome of mate choice depends on complex interactions between females and males both before and after copulation. Sexual selection continues after copulation in the form of sperm competition and female manipulation of sperm. Sperm competition occurs when spermatozoa from different males compete in the reproductive tract of a female for the fertilization of her eggs. In a variety of species females have a physiologically and morphologically complex reproductive system, which may enable them to control or influence offspring paternity by postcopulatory sperm storage and selective sperm use. There is increasing evidence that females choose between sperm from different males after copulation and that this so-called cryptic female choice may also affect paternity.

In spite of considerable interest in postcopulatory sexual selection, separating the effects of sperm competition from cryptic female choice remains difficult because mechanisms underlying postcopulatory processes are poorly understood. This is particularly true for simultaneous hermaphrodites. Simultaneous hermaphrodites are functional female and male at the same time. This type of gender expression is widespread in the animal kingdom and among plants. In hermaphrodites, selection on male traits cannot be independent of selection on female traits within the same individual.

This proposal focuses on mechanisms of both selective sperm storage and use in a simultaneous hermaphrodite. The land snail Arianta arbustorum will be used as model organism because its reproductive biology is well-known, experimental manipulations and laboratory breeding can be performed with relative ease, and because techniques of determining paternity and staining sperm in the female reproductive tract are well established in this species. Four experiments are suggested in this proposal. The first two experiments (A and B) address a central question: ‘What determines which male’s sperm are used at fertilization?’ Under controlled situations the fertilization success of sperm from donors will be assessed  by determining the paternity of offspring from double-mated mother snails with respect to the genotype of the mother and the genetic complementarity and heterozygosity of the mates. Highly variable microsatellite markers will be used to assign paternity. In experiment  C and D, mechanisms of selective sperm storage, sperm use and digestion will be examined in snails mated with two partners. Sperm of the two donors can be distinguished in the female reproductive tract of the recipient by applying different sperm staining techniques (immunocytochemical staining  in one donor and a classical staining technique in the other). Understanding the mechanisms of postcopulatory sperm storage and use is a major challenge. Simultaneous hermaphroditism imposes evolutionary constraints on reproducing individuals that are drastically different from those in gonochoristic species. Understanding the mechanism of storage, use, and digestion of the sperm received from different mates is pivotal for explaining how sexual selection, via sperm competition and cryptic sperm choice, shapes the evolution of male and female morphology and physiology in hermaphrodites.  The planned research will contribute to an improved understanding of the underlying mechanisms of how sperm storage functions in different situations and to what extent it is controlled by the female role of the hermaphrodite. The research proposed will help to unravel still unknown aspects of postcopulatory sexual selection in hermaphrodites.