Evolution of Sex Chromosomes in Salmonid Fishes

Sex chromosomes have been described for most of the species in the subfamily Salmoninae which include salmon, trout and char.  They are male heterogametic with an XX/XY system.  The sex chromosome pair was identified morphologically in rainbow trout (Oncorhynchus mykiss) and lake trout (Salvelinus namaycush), and later genetic mapping has shown that other members of the Salmoninae share linkage groups with rainbow trout, but that the sex determining gene (SD) is found on a different linkage group in each species. Using either male-specific markers or BACs containing closely linked genetic markers as probes in fluorescence in situ hybridization (FISH) experiments, the sex chromosomes have been identified in all of the species in Oncorhynchus and Salmo, and several species in Salvelinus. Recently a candidate sex determining gene, sdY has been identified in rainbow trout and several other species of the Salmoninae. In rainbow trout sdY is very closely linked to a male-specific marker, OmyY1 which is conserved in all members of Oncorhynchus, although other nearby markers on the rainbow trout Y chromosome are not sex-linked in the other species. Apparently a very small segment containing sdY is transposing to new chromosomal locations and producing “new” sex chromosomes in each species. BAC clones that contain both sdY and OmyY1 also contain large amounts of rDNA and transposons, which may be involved in this process. We have found sdY negative males in several populations of Oncorhynchus species, and in at least one case these appear to be XX females that were sex-reversed. Implications of these experiments will be discussed.