TRANSSPECIES POLYMORPHISM OF CLASS-II MHC LOCI IN DANIO FISHES

A characteristic feature of the major histocompatibility complex (Mhc) polymorphism in mammals is the existence of allelic lineages shared b y related species. This trans-species polymorphism has thus far been d ocumented only in primates, rodents, and artiodactyls. In this communi cation we provide evidence that it also exists in cyprinid (bony) fish es at the class II A and B foci coding for the alpha and beta polypept ide chains of the class II alpha:beta heterodimers. The study has focu sed on three species of the family Cyprinidae, subfamily Rasborinae: t he 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 II B loc i and exon 2 of the class II A loci in these species. Phylogenetic ana lysis of the sequences revealed the existence of allelic lineages whos e divergence predates the divergence of the three species at both the A and B loci. The lineages at the B locus in particular are separated by large genetic distances. The polymorphism is concentrated in the pe ptide-binding region sites and is apparently maintained by balancing s election. Sharing of this unique Mhc feature by both bony fishes and m ammals suggests that the main function of the Mhc (presentation of pep tides to T lymphocytes) has not changed during the last 400 million ye ars of its evolution.