Percichthyids as a novel model for studying sex chromosome turnover
Sex-determining mechanisms are extremely diverse and may vary even in closely-related species or among populations of the same species (Kijas et al., 2018; Lubieniecki et al., 2015; Rodrigues, Merilä, Patrelle, & Perrin, 2014). In species with genotypic sex determination, an ancestral sex-determining locus can be replaced by another locus on the same or a different chromosome, resulting in chromosome turnover. Transitions to a different chromosome are more likely when sex chromosomes exhibit little differentiation, so that WW or YY genotypes are less likely to be lethal (Bachtrog et al., 2014). Therefore, species with homomorphic chromosomes and intraspecific variation of sex-determining loci provide a window into sex chromosome evolution in real time. Macquarie perch, with an order of magnitude fewer sex-linked loci than its sister-species golden perch, appears to have more recently-evolved sex chromosomes. Its XY-gametologous locus, putatively sex-determining in at least the two populations used in assay development, was less correlated with phenotypic sex in other populations and not sex-linked in other percichthyids. Whereas population-specific variation in genetic factors determining phenotypic sex has been observed in other fish species (Faggion, Vandeputte, Chatain, Gagnaire, & Allal, 2019; Ferraresso et al., 2021), it is also possible that genotypic sex determination in different Macquarie perch populations is variably influenced by environmental variation (Hill, Burridge, Ezaz, & Wapstra, 2018). In contrast to sex chromosomes of perches, heteromorphic chromosomes of the Murray cod (Shams et al., 2019) must be the oldest among the three percichthyids with sequenced genomes. The interspecific variation in candidate sex-determining mechanisms makes Percichthyidae a good system for studying sex chromosome turnover and dosage compensation. Comparative studies that demonstrate sex chromosome turnover within the same genus or families are still relatively rare (e.g. Gambusia(Kottler et al., 2020), Oreochromis (Tao et al., 2021),Oryzias (Tanaka, Takehana, Naruse, Hamaguchi, & Sakaizumi, 2007), Poeciliidae (Darolti et al., 2019; Schultheis, Böhne, Schartl, Volff, & Galiana-Arnoux, 2009), Gasterosteidae (Ross, Urton, Boland, Shapiro, & Peichel, 2009)).