Sperm traits of masculinized fish relative to wild-type males: a systematic review and meta-analyses

Environmental sex reversal (ESR), whereby environmental effects (e.g. exogenous chemicals) override genetic sex determination, is a commonly used technique in aquaculture and physiology research. We performed a systematic review and meta-analyses of the literature that compares the sperm characteristics of masculinized genotypic females to wild-type males. We detected no mean differences between the ejaculate volume, sperm motility, duration or linearity of each type of male. We found some large mean differences in sperm concentration (d=2.541, CI=-0.004 to 5.086), reproductive success (d=-1.400, CI=-2.943 to 0.142), semen osmolality (d=1.850, CI=0.622 to 3.077) and sperm velocity (d=-0.933, CI=-1.426 to -0.441); in the case of the latter two traits, the mean effect was statistically significant. However, any significance did not stand up to a more conservative analysis. Additionally, heterogeneity was high and we found that where large differences between the sperm of sex-reversed and wild-type males are reported, these effects are attributable to sperm sampling methodology. Overall, we found little evidence for large systematic differences between the sperm produced by masculinized and wild-type male fish. Thus, masculinized genotypic females may enjoy reproductive success comparable to genotypic males. This conclusion leads to two potential implications: (i) sex-reversed fish may influence the dynamics of wild populations and (ii) aquaculture practices may use ESR to produce males with sperm quality similar to that of genotypic males. Most studies appear to have been performed in aquaculture species (i.e. Salmonidae); thus future experiments in non-model organisms may provide important insights in to the uniformity of the effects described.