Sex determining gene transposition as an evolutionary platform for chromosome turnover

Despite the key role that sex-determination plays in evolutionary processes, it is still poorly understood in many species. In salmonids, which are the best studied family of fishes, the master sex-determining gene sexually dimorphic on the Y-chromosome (sdY) has been identified. However, sdY displays unexplained discordance to the phenotypic sex, with a variable frequency of phenotypic females being reported as genetic males. Multiple sex determining loci in Atlantic salmon have also been reported, possibly as a result of transposition, suggesting a recent and non-random sex chromosome turnover in this species. We hypothesized the existence of an autosomic pseudocopy of sdY that is transmitted in accordance with Mendelian inheritance. To test this we developed a qPCR methodology to detect the number of sdY copies present in the genome. Based on the observed phenotype/genotype frequencies and linkage analysis among 2025 offspring from 64 pedigree-controlled families of accurately phenotyped Atlantic salmon, we identified both males and females carrying one or two autosomic copies in addition to the Y-specific copy present in males. Copy number frequencies were consistent with Mendelian inheritance. Pseudocopy loci were mapped to different chromosomes evidencing non-random transitions of the sex determining gene in Atlantic salmon and the existence of functional constraints for chromosome turnover.