Sperm competition, but not major histocompatibility divergence, drive differential fertilization success between alternative reproductive tactics in Chinook salmon

Post-copulatory sexual selection processes, including sperm competition and cryptic female choice (CFC), can operate based on major histocompatibility (MH) genes. We investigated sperm competition between male alternative reproductive tactics (jack (sneaker) and hooknose (guard)) of Chinook salmon (Oncorhynchus tshawytscha). Using a full factorial design, we examined in vitro competitive fertilization success of paired jack and hooknose males at three time points after sperm activation (0, 15 and 60 seconds) to test for male competition, CFC and time effects on male fertilization success. We also examined egg-mediated CFC at two MH genes by examining both the relationship between competitive fertilization success and MH divergence as well as inheritance patterns of MH alleles in resulting offspring. We found that jacks sired more offspring than hooknose males at 0 seconds post-activation; however, jack fertilization success declined over time post-activation, suggesting a trade-off between sperm speed and longevity. Enhanced fertilization success of jacks (presumably via higher sperm quality) may serve to increase sneaker tactic competitiveness relative to dominant hooknose males. We also found evidence of egg-mediated CFC (i.e., female x male interaction) influencing competitive fertilization success, however CFC was not acting on the MH genes as we found no relationship between fertilization success and MH II β1 or MH I α1 divergence and we found no deviations from Mendelian inheritance of MH alleles in the offspring. Our study provides insight into evolutionary mechanisms influencing variation in male mating success within alternative reproductive tactics, thus underscoring different strategies that males can adopt to attain success