Trans-species polymorphism of class II Mhc loci in danio fishes

Abstract

A characteristic feature of the major histocompatibility complex (Mhc) polymorphism in mammals is the existence of allelic lineages shared by related species. This trans-species polymorphism has thus far been documented only in primates, rodents, and artiodactyls. In this communication we provide evidence that it also exists in cyprinid (bony) fishes at the class IIA andB loci coding for the α and β polypeptide chains of the class II α: β heterodimers. The study has focused on three species of the family Cyprinidae, subfamily Rasborinae: the zebrafish (Danio rerio), the giant danio (D. malabaricus), and the pearl danio (D. albolineatus). The polymerase chain reaction was used to amplify and then sequence intron 1 and exon 2 of the class IIB loci and exon 2 of the class IIA loci in these species. Phylogenetic analysis of the sequences revealed the existence of allelic lineages whose divergence predates the divergence of the three species at both theA andB loci. The lineages at theB locus in particular are separated by large genetic distances. The polymorphism is concentrated in the peptide-binding region sites and is apparently maintained by balancing selection. Sharing of this uniqueMhc feature by both bony fishes and mammals suggests that the main function of theMhc (presentation of peptides to T lymphocytes) has not changed during the last 400 million years of its evolution.