Different modes of Mhc evolution in primates.

The human major histocompatibility complex (Mhc) is a chromosomal segment approximately 4 million bp long that contains > or = 84 genes. Some of these genes code for the class I and class II molecules, while the remaining genes code for complement components, cytochrome P450, tumor necrosis factor, and many other, unrelated proteins. We demonstrate on three examples (DP, C4-CYP21, and DRB) that different regions of the Mhc have different evolutionary histories. The organization of the DP region, which in humans contains four genes, was established in the ancestral Anthropoidea or earlier and has not changed since. The duplication that generated the two C4-CYP21 modules occurred in the ancestral Catarrhini or earlier, but the region has been undergoing periodic homogenizations via unequal crossing-over, which make paralogous genes in the same species more similar to each other than to orthologous genes of different species. The eight or nine genes of the DRB region were also generated in the ancestral Catarrhini, but the region has since been subject to frequent rearrangements, which generated various DRB haplotypes. Not only the alleles but, in part, also the haplotype polymorphism is evolving transspecifically. The DRB region of the Platyrrhini has an origin different from that of the Catarrhini. The picture emerging from these studies is that of both stability in some regions of the Mhc and tremendous evolutionary instability in other regions.