A proteomic study of sperm competition in mammals

Sperm competition, when sperm from more than one male compete to fertilise the same ova, has driven a diversity of adaptations. Increasingly, molecular techniques have been used to study the effect of post-copulatory sexual selection, including sperm competition, on proteins that are essential to reproduction. Genomic studies have revealed the rapid evolution of ejaculate proteins in polyandrous species. Additionally, there is evidence that gene expression can be altered plastically, in response to cues of sperm competition. Such studies are limited however, as the transcriptome does not always equal the proteome. Here, quantitative proteomics techniques are utilised to explore plasticity in reproduction, at a molecular level, in the house mouse (Mus musculus domesticus) (Chapters 2 – 4). In addition, adaptations to sperm competition are considered within mammalian testes and sperm proteins (Chapters 5 and 6). Contrary to predictions of sperm competition theory, within rodents, sperm production is suppressed in subordinate males. In addition, dominant males develop significantly larger seminal vesicles. Here, quantitative proteomic analysis reveals that the composition of the secretion within the major accessory sex gland in house mice differs according to social status. Subordinate males produce a more concentrated protein secretion, which contains a greater proportion of the protein SVS2. This protein is essential in copulatory plug formation, and increasing the proportion of SVS2 within their seminal fluid may allow subordinate males to produce copulatory plugs of an equivalent size to those produced by dominant. Within mammals, the oviduct is the site of fertilisation and can exert control over sperm storage and movement. As male house mice can plastically alter their ejaculate production and allocation according to local sperm competition risk, it is feasible that females may alter the environment within the oviduct in response to similar cues of multiple mating, to maintain control of paternity and reduce the risk of polyspermy. I found no evidence for plasticity in the oviduct proteins of female house mice following either a high or low level of interactions with male house mice. Indeed, the oviduct proteome was more similar within siblings, balanced across treatments, than within treatment groups. I did find evidence for plasticity in female behaviour, when interacting with a novel male, and in female ano-genital distance. Mothers in populations with high levels of sperm competition may benefit from producing more competitive male offspring. I analysed male offspring produced by female house mice that had encountered either a high or low level of male odour and interactions prior to pregnancy. Quantitative proteomic investigation revealed that ejaculate composition differed according to the level of male interaction their mother had encountered before pregnancy. In contrast, I found no evidence for variation in male mating behaviour, reproductive morphology or ejaculate size. This study reveals the potential for maternally driven, subtle alterations in ejaculate composition. Sperm competition has driven the evolution of testicular architecture. Comparative analyses reveal that relatively larger testes also produce sperm more efficiently. Improved spermatogenesis efficiency is primarily due to a faster rate of each seminiferous epithelial cycle, thereby increasing sperm production rate. This trait is linked to sperm competition, and so sperm competition is selecting for a greater sperm output, primarily by increasing the rate of production. Sexual conflict and post-copulatory sexual selection drive the rapid evolution of genes involved in reproduction. The reproductive proteomes of relatively closely related species may therefore be very different from each other. Comparative proteomic analysis of cauda epididymal samples from two groups of mammals, rodents and ungulates, reveals broad similarities in the sperm proteome. Closer analysis of proteins known to be involved in sperm – egg interactions suggests these proteins are very divergent, and exhibit a low level of sequence homology.