Unexpected beta(2)-microglobulin sequence diversity in individual rainbow trout

For mammals beta(2)-microglobulin (beta(2)m), the light chain of major histocompatibility complex (MHC) class I molecules, is invariant (or highly conserved) and is encoded by a single gene unlinked to the MHC. We find that beta(2)m of a salmonid fish, the rainbow trout (Oncorhynchus mykiss), does not conform to the mammalian paradigm. Ten of 12 randomly selected beta(2)m cDNA clones from an individual fish have different nucleotide sequences, A complex restriction fragment length polymorphism pattern is observed with rainbow trout, suggesting multiple beta(2)m genes in the genome, in excess of the two genes expected from the ancestral salmonid tetraploidy. Additional duplication and diversification of the beta(2)m genes might have occurred subsequently. Variation in the beta(2)m cDNA sequences is mainly at sites that do not perturb the structure of the mature beta(2)m protein, showing that the observed diversity of the trout beta(2)m genes is not primarily a result of pathogen selection.