Nucleotide sequence of the MHC class I genomic region of a teleost, the medaka (Oryzias latipes)
Teleost orthologs of the human major histocompatibility complex (MHC)-encoded genes show a dispersed distribution over several chromosomal loci. However, some of them, including the MHC class I alpha chain genes, are tightly linked to each other, forming the teleost MHC class I region. To elucidate the evolution of vertebrate MHC, we have determined the complete nucleotide sequences of two bacterial artificial chromosome (BAC) clones, encompassing the MHC class I region of the inbred Hd-rR strain of the medaka (Oryzias latipes). The 425862 bp nucleotide sequence predicted 20 possibly expressed genes and three pseudogenes. Except for one gene, CIZ, whose human ortholog is located at 12p13.31, all identified genes were orthologs or closely related relatives of the human MHC-encoded genes. Two classical class I alpha chain genes and the six other genes directly involved in class I antigen presentation formed an uninterrupted cluster. Comparison of the MHC class I region genes among three teleost species, the medaka, zebrafish and pufferfish, indicated that the content, but not the order or transcriptional orientation, of the genes is highly conserved. These results suggest that a strong selective pressure has conserved the linkage of certain MHC genes during vertebrate evolution, despite recurrent genetic rearrangements.