Evolution of the sex chromosomes in salmonid fishes

Most of the information on sex chromosomes in salmonid fishes is for species in the 3 genera of the subfamily Salmoninae found in North America: Salvelinus, Salmo and Oncorhynchus. All of the species are male heterogametic with XY sex determination. Morphologically distinguishable sex chromosomes are found only in Salvelinus namaycush,S. fontinalis and Oncorhynchus mykiss. Sex chromosomes have been identified in the other species using a combination of chromosome mapping and fluorescence in situ hybridization with probes containing sex-linked markers. Although all species share conserved linkage groups, the major sex-determining locus (SD) is found at the telomere of a different linkage group in almost every species, suggesting that the SD often transposes to a new location at the time of speciation. In a couple of species, intraspecific variation has been found in the chromosomal location of the SD. Recently, sdY has been identified as the major sex-determining gene in rainbow trout, and it maps to the sex linkage group in all of these species. BACs containing sdY have been isolated and sequenced in O.mykiss, and the genetic markers adjacent to sdY are not sex-linked in the other Oncorhynchus species, suggesting that the transposed region is very small. Possible explanations for the frequent occurrence of transposition of the SD are discussed.