Gonochorism, or the existence of separate sexes, is common among eukaryotic organisms. Yet, the underlying mechanisms of sex determination are poorly understood and highly variable across fish species. In hybridizing species, sex determination mechanisms may promote or impede reproductive isolation depending on whether mechanisms are similar between species. In Catostomus suckers, contemporary hybridization is variable and extensive, but prior to this study the genetic basis of sex determination was almost completely unknown. We aimed to describe the genetic basis of sex determination in Catostomus suckers to understand the impact of sex determination on reproductive isolation. We investigated sex determination in Catostomus fishes and how it varies between species. We used genotyping-by-sequencing genomic data from 260 individuals of three species (bluehead, white, and flannelmouth) and their hybrids. To identify sex-associated loci, those that are present in both sexes but with different genotypes, we used GEMMA to conduct a genome-wide association study. To identify sex-specific loci, loci that are present in one sex but not the other, we used Ms. SSLI, a custom program created for this research.
Results/Conclusions
We found a genetic basis of sex determination in Catostomus fishes, with a highly significant locus on chromosome 4. This chromosome was also sex-associated in just bluehead suckers, with two highly significant loci. These two loci are indicative of a master-sex determining region, especially as there was no indication of heteromorphic sex chromosomes in Catostomus fishes. Interestingly, chromosome 4 was not significant in white suckers or in bluehead x white hybrids, which suggests that there is a lack of a shared basis of sex determination across species investigated in this research. Variation in sex determination could potentially impact the level of reproductive isolation between species and therefore constrain hybridization outcomes. The underlying mechanisms of sex determination had not been documented in Catostomus fishes prior to this research, therefore this research adds to the body of work attempting to uncover the mechanisms promoting variation in hybridization outcomes. Future work should investigate chromosome 4 as a potential master sex-determining region as well as increase sample sizes to continue uncovering sex determination in white and bluehead x white hybrids.
