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 (Oncorhyn chus mykiss), does not conform to the mammalian paradigm. Ten of 12 ra ndomly selected beta(2)m cDNA clones from an individual fish have diff erent nucleotide sequences, A complex restriction fragment length poly morphism pattern is observed with rainbow trout, suggesting multiple b eta(2)m genes in the genome, in excess of the two genes expected from the ancestral salmonid tetraploidy. Additional duplication and diversi fication of the beta(2)m genes might have occurred subsequently. Varia tion in the beta(2)m cDNA sequences is mainly at sites that do not per turb the structure of the mature beta(2)m protein, showing that the ob served diversity of the trout beta(2)m genes is not primarily a result of pathogen selection.