Evolution of the Major Histocompatibility Complex: A Lesson from the Oryzias Species
The genomic organization of the teleost major histocompatibility complex (MHC) shows a significant deviation from that of the “standard” MHC of the eutherian mammals in that the class IA genes are not linked to the class IIA and B genes. However, progress in the phylogenetic analysis of the jawed vertebrate MHC reveals that the “standard” mammalian MHC is also highly derived because the tight linkage between the class IA genes and the genes directly involved in the class I antigen processing/presentation process is disrupted. In the medaka MHC class I region, these genes form a tight and uninterrupted cluster, probably reflecting the ancestral genomic organization of the MHC. One of these genes, PSMB8 (proteasome subunit beta type 8), which is responsible for the generation of the peptides presented by the MHC class I molecules, shows a marked dimorphism in medaka. The same dimorphic alleles are present in other Oryzias species, indicating that they are under balancing selection and have been transmitted from species to species. Although the physiological or evolutionary meaning of this balancing selection is still to be clarified, similar dimorphisms of PSMB8 are widely recognized among non-eutherian vertebrates, suggesting that the presence of the PSMB8 dimorphism is associated with the tight linkage between the class IA genes and the genes directly involved in the class I antigen processing/presentation process.
KeywordsMajor Histocompatibility Complex Major Histocompatibility Complex Class Bacterial Artificial Chromosome Clone Major Histocompatibility Complex Region Human Major Histocompatibility Complex
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