Conservation implications
Most remaining Macquarie perch population are small and isolated and thus vulnerable to extinction through environmental stochasticity, loss of adaptive potential, and inbreeding. Accordingly, genetic admixture among populations has been recommended (Pavlova et al., 2017) and can have positive fitness effects during restoration of previously extinct populations (Lutz et al., 2021). Potential presence of intraspecific polymorphism for the putative sex-determining locus in Macquarie perch raises the question about the consequences of admixture between populations that have different sex-determination systems. Examples from other fish suggest that one of the systems will dominate over the other. In interspecific crosses between two sister species with different independently evolved sex chromosome systems, western mosquitofishGambusia affinis with ZW, and eastern mosquitofish G. holbrooki with XY, the Y chromosome is dominant over the W chromosome, and X is dominant over Z, such that YW fish develop as males (Kottler et al., 2020). In southern platyfish Xiphophorus maculatus , which has three well-defined sex chromosomes, YW fish develop as females, indicating that the W chromosome is dominant over the Y chromosome (Schultheis et al., 2009). Successful admixture of coastal and inland Macquarie perch lineages in the Cataract River for several generations (Pavlova et al., 2017) suggests that admixture will not cause severe issues for sex-ratios even if diverse sex-determination mechanisms exist.