Comparative genomics of medaka: the major histocompatibility complex (MHC).

The major histocompatibility complex (MHC) is one of the best characterized regions of the vertebrate genome. The human MHC has three subregions, classes I, II, and III. The MHC of chicken and Xenopus contain all three subregions. In contrast, in all teleost species so far analyzed, the class I and II genes are not linked to each other, suggesting that there was extensive genomic reorganization of the MHC region during an early stage of vertebrate evolution. To elucidate the details of this reorganization, we carried out genetic and physical analyses of the medaka MHC genes. We isolated the medaka counterparts of human MHC genes, class I A, class II B, LMP2, LMP7, TAP2, complement Bf and C4, and subjected them to linkage analysis. Except for the linkage of class IA, LMP2, LMP7, and TAP2 on linkage group (LG)11, all other genes were assigned to separate linkage groups. Thus, the class IA gene and the genes involved in class I antigen presentation seem to form the evolutionary stable core of the MHC. A physical analysis of medaka MHC class I region is in progress.