Abstract
Polyandry and post-copulatory sexual selection provide opportunities for the evolution of female differential sperm selection. Here, we examined the influence of variation in major histocompatibility (MH) class I allelic composition upon sperm competition dynamics in Atlantic salmon. We ran in vitro fertilization competitions that mimicked the gametic microenvironment, and replicated a paired-male experimental design that allowed us to compare differences in sperm competition success among males when their sperm compete for eggs from females that were genetically either similar or dissimilar at the MH class I locus. Concurrently, we measured variation in spermatozoal traits that are known to influence relative fertilization success under these conditions. Contrary to the findings demonstrating mechanisms that promote MH complex heterozygosity, our results showed that males won significantly greater relative fertilization success when competing for eggs from genetically similar females at the MH class I. This result also showed covariation with the known influences of sperm velocity on relative fertilization success. We discuss these unexpected findings in relation to sperm–egg recognition and hybridization avoidance mechanisms based upon immunogenetic variation.
Highlights
Biologists know that sexual selection operates after mating to the level of the gamete (Parker 1970), where forces and adaptations recognized at the whole organism level for male–male competition and female choice can be paralleled by adaptations for sperm competition and cryptic female choice (Eberhard 1996; Birkhead & Moller 1998)
Our results revealed a significant difference between individual male sperm competition success dependent on major histocompatibility (MH) genetic similarity: males won approximately 15 per cent more fertilizations when competing for MH-similar females
Monte Carlo iterations of all possible relative fertilization success differences showed that when males competed for MH-dissimilar females, they consistently achieved an average 13 per cent reduced relative fertilization success where the lower confidence limit did not breach zero (999 iterations: mean marginal difference in % fertilization successZK13.13%, lower confidence limitZK22.06%, upper confidence limitZK3.9%)
Summary
Biologists know that sexual selection operates after mating to the level of the gamete (Parker 1970), where forces and adaptations recognized at the whole organism level for male–male competition and female choice can be paralleled by adaptations for sperm competition and cryptic female choice (Eberhard 1996; Birkhead & Moller 1998). We build on previous work (Gage et al 2004) to explore cryptic female choice within the external fertilization system of the Atlantic salmon Salmo salar by conducting controlled in vitro sperm competition experiments, while measuring spermatozoal traits known to explain differential fertilization success. By concurrently assessing spermatozoal traits in the environment to which gametes are naturally adapted (Gage et al 2004), and assigning paternity using microsatellite markers, we can determine the variance in sperm competition success that is attributable to MHC dissimilarity, and cryptic female choice. Measurement of spermatozoal characteristics were repeated for each different fertilization trial, so that any within-male changes in sperm traits between
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