D3359: How does the male secreted short (MSS) glycoprotein provide a competitive advantage to Caenorhabditis sperm?

Sperm competition has been observed in numerous organismal clades, yet our understanding of the molecular mechanisms that mediate it in vivo remains extremely limited. MSS (Male Secreted Short) proteins are small sperm surface glycoproteins essential for successful competition in outcrossing Caenorhabditis; however, the molecular basis for this advantage is currently unknown. MSS-decorated sperm may reach oocytes faster and/or spatially exclude sperm lacking these proteins. Alternatively, MSS-containing sperm may possess an enhanced fertilization ability compared to wild-type sperm. To test these hypotheses in real time, we mated feminized C. briggsae with vital dye-stained males that either possessed or lacked a mss(+) transgene. After mating, the distribution of sperm in sections of the hermaphroditic gonad (spermatheca, uterus, vulva) were quantified. While there were no significant differences between the number of mss(+/-) sperm in each gonad area, mss(+) sperm were more commonly observed proximal to the spermathecal valve regardless of mating order. More intriguingly, ectopic mss(+) sperm were frequently observed beyond the spermathecal valve. This suggests that mss(+) sperm reach receptive oocytes more efficiently than mss(-) counterparts. Time-lapse imaging revealed that invasivity occurs during ovulation when the valve is briefly open. Ectopic sperm cannot fertilize pre-ovulatory oocytes, but may be in position to be the first to do so upon ovulation. To clarify the structural determinants of MSS action, we used microparticle bombardment to overexpress two endogenous MSS-related proteins (MSRPs) in C. briggsae. Phenotyping experiments indicate that neither confers competitive advantages to the sperm bearing them, suggesting some alternative mechanism underlying MSS success.