Identification of sex-determining loci in hybridizing Catostomus fish species

1Despite the near-universality of gonochorism (separate sexes) in eukaryotic organisms, the underlying mechanisms of sex determination are poorly understood and highly variable in some taxa. Sex determination mechanisms may promote or impede reproductive isolation depending on whether mechanisms are similar between related species, so identifying genomic regions associated with sex determination is especially relevant for understanding evolutionary diversification and hybridization between closely related species. In Catostomus fishes, contemporary hybridization is variable and extensive, but factors influencing hybridization dynamics are not fully understood. In the present study, we aim to describe the genetic basis of sex determination in bluehead (Catostomus discobolus) and white suckers (Catostomus commersonii ) to understand the potential impact of sex determination on reproductive isolation. We used genotyping-by-sequencing data from Catostomus species and their hybrids to identify regions of the genome associated with sex using a genome-wide association study and the identification of sex-specific loci. We identified a genetic basis of sex determination in Catostomus fishes, with a region of the genome significantly associating with sex in bluehead suckers. This region is suggestive of a master sex-determining region in bluehead suckers but is not significant in white suckers, implying that either the sex-determining region of the genome differs in these two species that hybridize, or that sample size was insufficient to identify this genomic region in white suckers. By describing and comparing sex-determination systems across Catostomus fish species, we highlight the relationship between sex determining systems and hybridization in closely related fish species.