Comparative genomic analysis of the major histocompatibility complex class I region in the teleost genus Oryzias

The major histocompatibility complex (MHC) class I region of teleosts harbors a tight cluster of the class IA genes and several other genes directly involved in class I antigen presentation. Moreover, the dichotomous haplotypic lineages (termed d- and N- lineages) of the proteasome subunit beta genes, PSMB8 and PSMB10, are present in this region of the medaka, Oryzias latipes. To understand the evolution of the Oryzias MHC class I region at the nucleotide sequence level, we analyzed bacterial artificial chromosome clones covering the MHC class I region containing the d- lineage of Oryzias luzonensis and the d- and N- lineages of Oryzias dancena. Comparison among these three elucidated sequences and the published sequences of the d- and N- lineages of O. latipes indicated that the order and orientation of the encoded genes were completely conserved among these five genomic regions, except for the class IA genes, which showed species-specific variation in copy number. The PSMB8 and PSMB10 genes showed trans-species dimorphism. The remaining regions flanking the PSMB10, PSMB8, and class IA genes showed high degrees of sequence conservation at interspecies as well as intraspecies levels. Thus, the three independent evolutionary patterns under apparently distinctive selective pressures are recognized in the Oryzias MHC class I region.