ABSTRACT. Major histocompatibility complex (MHC) genes play an important role in the immune response of vertebrates. Allelic polymorphism and evolutionary mechanism of MHC genes have been investigated in many mammals, but much less is known in teleosts. We examined the polymorphism, gene duplication and balancing selection of the MHC class II DAB gene of the half-smooth tongue sole (Cynoglossus semilaevis); 23 alleles were found in this species. Gene duplication manifested as three to six distinct sequences at each domain in the same individuals. Non-synonymous substitutions occurred at a significantly higher frequency than synonymous substitutions in the PBR domain, suggesting balancing selection for maintaining polymorphisms at the MHC II DAB locus. Many positive selection sites were found to act very intensely on antigen-binding sites of MHC class II DAB gene.

ABSTRACT. b 2 -Microglobulin (β 2 m) is related to major histocompatibility complex class I alpha chains, and forms cellsurface glycoproteins that mediate a variety of functions in immune defense. In general, β 2 m has no isoforms and is not polymorphic in higher vertebrates, but polymorphisms between different alleles have been found in some fish species. In this study, full-length β 2 m cDNA and genomic sequences were cloned from the miiuy croaker (Miichthys miiuy). The miiuy croaker β 2 m gene shares many of the same characteristics as other fish species. Three exons and two introns were identified in the miiuy croaker β 2 m gene; these genomic structural features are similar to those present in other fish. The deduced β 2 m amino acid sequence exhibited 34.7-90.1% identity with mammal and teleost β 2 m amino acid sequences. Sequence polymorphism analysis in six individuals identified three alleles that encoded two proteins, confirming that β 2 m polymorphisms exist in this species. Phylogenetic analysis elucidated the evolutionary history of the β 2 m protein among warm-blooded vertebrates and bony fish.