Modes of salmonid MHC class I and II evolution differ from the primate paradigm

Rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) represen t two salmonid genera separated for 15-20 million years. cDNA sequences wer e determined for the classical MHC class I heavy chain gene UBA and the MHC class LT beta -chain gene DAB from 15 rainbow and 10 brown trout. Both gen es are highly polymorphic in both species and diploid in expression. The MH C class I alleles comprise several highly divergent lineages that are repre sented in both species and predate genera separation. The class II alleles are less divergent, highly species specific, and probably arose after gener a separation, The striking difference in salmonid MHC class I and class LI evolution contrasts with the situation in primates, where lineages of class II alleles have been sustained over longer periods of time relative to cla ss I lineages, The difference may arise because salmonid MHC class I and II genes are not linked, whereas in mammals they are closely linked, A preval ent mechanism for evolving new MHC class I alleles in salmonids is recombin ation in intron II that shuffles alpha1 and alpha2 domains into different c ombinations.