Abstract
Seminal fluid proteins (SFPs) are components of the ejaculate transferred to females together with sperm. SFPs trigger drastic changes in females. Some of the SFP effects are essential and beneficial for both parents to obtain reproductive success, and others are sexually antagonistic, meaning males try to exploit females for their own benefits. Another remarkable aspect is that SFPs are one of the most rapidly evolving classes of proteins in sexually reproducing animals. The current challenge of SFP research is to establish why SFPs evolve rapidly. To date, several non-mutually exclusive hypotheses have been proposed, and they can be broadly classified into two categories: selection or sex-specific expression. In the selection scenario, natural- or sexual selection is proposed to drive SFP evolution. Given the sexually-antagonistic functions of SFPs, it is plausible that their evolution is influenced by sexual selection. However, this scenario is based on extrapolation between very different types of data (highly-controlled laboratory studies and cross-species comparisons), and it remains to be investigated whether SFPs are under on-going selection at a population level. Furthermore, the sex-specific expression scenario proposes that sex-specific expression of SFPs leads to their rapid evolution. Since only males express SFPs, these traits have reduced opportunity for selection, which can lead to rapid evolution even in the absence of directional selection. Recent transcriptomic studies confirm that male-specific genes evolve rapidly, but the impact of sex-specific expression per se on evolutionary processes remains to be evaluated. Given this background, we propose to investigate whether SFPs evolve rapidly in simultaneous hermaphrodites (i.e. being male and female at the same time) by quantifying SFP variation at individual, population and species levels. The unique advantage of using hermaphrodites is that they do not have sex-specific expression. If sex-specific expression alone drives rapid SFP evolution, SFPs in hermaphrodites should not evolve rapidly. To examine SFP evolution in hermaphrodites, We will use the great pond snail Lymnaea stagnalis and related freshwater snail species, because of the advantages they offer to investigate SFP evolution (e.g., established SFP bioassay, distinct reproductive modes). Specifically, we will (1) extensively screen SFPs of L. stagnalis using transcriptomic and proteomics approaches, (2) test whether SFPs are diversified between populations of L. stagnalis, in terms of the sequence, gene expression and function of SFPs, and (3) test whether SFPs are under positive selection in a wide range of freshwater snail species, examining their SFP transcripts. This project’s outcomes are expected to expand the understanding of reproductive protein evolution in sexually reproducing animals.
Netherlands