The salmonid class I MHC: limited diversity in a primitive teleost

Three MHC class I genes have been characterized in salmonids: A, B, and UA. Levels of polymorphism vary among the genes, but they all share one common feature: a lack of sequence diversity. Although individual species can car ry over 30 alleles at a given locus (A), intraspecific diversity is general ly less than 5% in Pacific salmon (genus Oncorhynchus), and less than 10% i n Atlantic salmon (genus Salmo). These levels of diversity suggest that few ancient allelic lineages have persisted within species, and that most of t he allelic radiation has occurred during or since speciation. Also apparent is the greater retention of allelic lineages in Atlantic salmon than Pacif ic salmon, which reflects historic differences of the two genera. Compariso n of the salmonid class I sequences with those of other teleosts reveals tw o well supported groups: one containing the Cypriniformes and the salmonid UA, and the other containing the neoteleosts and the salmonid A and B. Ther e is no homology between known Cypriniformes and neoteleostean sequences. I f this relationship is borne out, it offers strong support for the hypothes is that the higher teleosts diverged more recently from the Salmoniformes t han the Cypriniformes. The salmonid MHC may provide a snapshot of the neote leostean MHC prior to the extensive class I duplication that has taken plac e in at least some of die more advanced species.