MHC Class I Exon 4 in the Multiocellated Racerunners (Eremias multiocellata): Polymorphism, Duplication and Selection

Abstract

The major histocompatibility complex (MHC) is a dynamic genetic region with an essential role in the adaptive immunity of jawed vertebrates. The MHC polymorphism is affected by many processes such as birth-anddeath evolution, gene conversion, and concerted evolution. Studies investigating the evolution of MHC class I genes have been biased toward a few particular taxa and model species. However, the investigation of this region in nonavian reptiles is still in its infancy. We present the first characterization of MHC class I genes in a species from the family Lacertidae. We assessed genetic diversity and a role of selection in shaping the diversity of MHC class I exon 4 among 37 individuals of Eremias multiocellata from a population in Lanzhou, China. We generated 67 distinct DNA sequences using cloning and sequencing methods, and identified 36 putative functional variants as well as two putative pseudogene-variants. We found the number of variants within an individual varying between two and seven, indicating that there are at least four MHC class I loci in this species. Gene duplication plays a role in increasing copy numbers of MHC genes and allelic diversity in this species. The class I exon 4 sequences are characteristic of low nucleotide diversity. No signal of recombination is detected, but purifying selection is detected in β2-microglobulin interaction sites and some other silent sites outside of the function-constraint regions. Certain identical alleles are shared by Eremias multiocellata and E. przewalskii and E. brenchleyi, suggesting trans-species polymorphism. The data are compatible with a birth-and-death model of evolution.class II genes, have been the subjects of the majority of research in the fields of ecology and evolution (reviewed in Sommer, 2005; Milinski, 2006;Piertney and Oliver, 2006;Ujvari and Belov, 2011). This is because MHC includes the most polymorphic genes in vertebrate populations. Given the extraordinary richness and diversity of continuously evolving pathogens in the environment, it is not surprising that the MHC harbors the most polymorphic genes described thus far, with some loci, such as the human HLA-B locus, possessing more than 2000 alleles (de Bakker and Raychaudhuri, 2012). The number of genes in the MHC varies enormously between species, even between individuals of the same species (Malaga-Trillo et al., 1998;Figueroa et al., 2001). It has been noted that balancing selection is an evolutionary force that can maintain many haplotypes over long periods of evolutionary time. Genes that