Frequent segmental sequence exchanges and rapid gene duplication characterize the MHC class I genes in lemurs

Major histocompatibility complex (MHC) class I genes have complicated and profound evolutionary histories. To reconstruct and better understand their histories, partial class I genes (exon 2-intron 2-exon 3) were sequenced in a sampling of prosimians (Strepsirhini, Primates). In total, we detected 117 different sequences from 36 Malagasy prosimians (lemurs) and 1 non-Malagasy prosimian (galago) representing 4 families, 7 genera, and 13 species. Unlike the MHC class II genes ( MHC-DRB), MHC class I genes show a generally genus-specific mode of evolution in lemurs. Additionally, no prosimian class I loci were found to be orthologous to HLA genes, even at highly conserved loci (such as HLA-E, HLA-F). Phylogenetic analysis indicates that nucleotide diversity among loci was very small and the persistence time of the polymorphisms was short, suggesting that the origin of the lemur MHC class I genes detected in this study was relatively recent. The evolutionary mode of these genes is similar to that of classic HLA genes, HLA-A, HLA-B, and HLA-C, in terms of their recent origin and rarity of pseudogenes, and differs from them with respect to the degree of gene duplications. From the viewpoint of MHC genes evolution, some interlocus sequence exchanges were apparently observed in the lemur lineage upon phylogenetic and amino acid motif analyses. This is also in contrast to the evolutionary mode of HLA genes, where intralocus exchanges have certainly occurred but few interlocus exchanges have taken place. Consequently, the gene conversion model for explaining the generation of the MHC diversity among different loci can be thought to play more important roles in the evolution of lemur MHC class I genes than in that of HLA genes.