Sexual Conflict and Protein Polymorphism

Abstract

Abstract. Sexual conflict, where male and female reproductive interests differ, is probably widespread and often mediated by male or sperm proteins and female or egg proteins that bind to each other during mating or fertilization. One potential consequence is maintenance of polymorphism in these proteins, which might result in reproductive isolation between sympatric subpopulations. I investigate the conditions for polymorphism maintenance in a series of mathematical models of sexual conflict over mating or fertilization frequency. The models represent a male or sperm ligand and a female or egg receptor, and they differ in whether expression of either protein is haploid or diploid. For diploid expression, the conditions imply that patterns of dominance, which involve neither overdominance nor underdominance, can determine whether polymorphism is maintained. For example, suppose ligand expression is diploid, and consider ligand alleles L 1 and L 2 in interactions with a given receptor genotype; if L 1 /L 1 males are fitter than L 2 /L 2 males in these interactions, then polymorphism is more likely to be maintained when L 1 /L 2 males more closely resemble L 1 /L 1 males in these interactions. Such fitter-allele dominance might be typical of a ligand or its receptor due to their biochemistry, in which case polymorphism might be typical of the pair.

Extraordinary intraspecific diversity in oyster sperm bindin

“…In contrast, the maintenance and elimination of variation within species does not follow any simple pattern in the sperm GRP that have been studied (5). The spectrum of possible polymorphism syndromes exist: from coding monomorphism in abalone and some sea urchins, through moderate polymorphism in blue mussels and many sea urchin species, to the extreme polymorphism in oyster bindin reported here.…”

“…Because GRP divergence could initiate reproductive isolation between incipient species, studying mechanisms that create intraspecific variation is key to understanding how GRPs and the selective pressures that cause their divergence contribute to speciation. The amount of polymorphism observed in GRPs varies considerably (5). Abalone sperm lysin monomorphism (6) is contrasted by considerable polymorphism within and between populations in mussel sperm lysin-M7 (7,8) and sea urchin sperm bindin (9,10), yet these taxa are all free-spawning invertebrates.…”

Selection in the Rapid Evolution of Gamete Recognition Proteins in Marine Invertebrates

“…Conversely, when sperm densities were low, the common su-bindin alleles were found in the surviving embryos. These results can be explained by sexual conflict, the balance between sperm competition and egg avoidance of polyspermy (Rice and Holland 1997;Frank 2000;Haygood 2004). Theoretically, at high sperm densities, eggs with altered EBR1s survive to become embryos because rare su-bindin alleles fit altered EBR1s poorly, the result being the slowing of sperm fusion and monospermy.…”

Sexual selection in hermaphrodites, sperm and broadcast spawners, plants and fungi

“…One central theoretical prediction is that under high sperm concentrations (and therefore high sperm competition) males will face selection to release even more sperm despite the increased risks of polyspermy to females [63]. For their part, females will be under increased selection to block polyspermy under conditions of sperm excess, for example, by evolving highly polymorphic sperm-egg receptor proteins that reduce fertilization rates [67,68] or through the facultative adjustment of egg size so that eggs are smaller targets when sperm concentrations are high [69]. Sperm competition can therefore be an important driver of sexual conflict (i.e.…”