MHC class IIB exon 2 polymorphism in the Grey partridge (Perdix perdix) is shaped by selection, recombination and gene conversion.

Marta Promerová,Anna Bryjová,Josef Bryja,Tereza Králová,Tomáš Albrecht

doi:10.1371/journal.pone.0069135

Abstract

Among bird species, the most studied major histocompatibility complex (MHC) is the chicken MHC. Although the number of studies on MHC in free-ranging species is increasing, the knowledge on MHC variation in species closely related to chicken is required to understand the peculiarities of bird MHC evolution. Here we describe the variation of MHC class IIB (MHCIIB) exon 2 in a population of the Grey partridge (Perdix perdix), a species of high conservation concern throughout Europe and an emerging galliform model in studies of sexual selection. We found 12 alleles in 108 individuals, but in comparison to other birds surprisingly many sites show signatures of historical positive selection. Individuals displayed between two to four alleles both on genomic and complementary DNA, suggesting the presence of two functional MHCIIB loci. Recombination and gene conversion appear to be involved in generating MHCIIB diversity in the Grey partridge; two recombination breakpoints and several gene conversion events were detected. In phylogenetic analysis of galliform MHCIIB, the Grey partridge alleles do not cluster together, but are scattered through the tree instead. Thus, our results indicate that the Grey partridge MHCIIB is comparable to most other galliforms in terms of copy number and population polymorphism.

Highlights

Genes of the major histocompatibility complex (MHC) encode for molecules which bind small fragments of peptides and present these on cell surfaces for recognition by immune cells
The evolutionary pressure of parasite burden has been suggested to contribute to the extraordinary polymorphism of MHC genes, which belong among the most variable genes of vertebrate genomes known to date [1,3,4]
We studied the structure, variation and evolutionary mechanisms acting on MHC class IIB (MHCIIB) in the Grey partridge (Perdix perdix), a galliform bird that is diverged from the chicken by 33.7 million years

Summary

Introduction

Genes of the major histocompatibility complex (MHC) encode for molecules which bind small fragments of peptides and present these on cell surfaces for recognition by immune cells. Apart from pathogen pressure, sexual selection has been shown to influence MHC class I and class II polymorphism by a number of studies in mammals The most studied and best described MHC is that of the chicken It consists of two unlinked clusters of genes: the Blocus and the Y-locus ( called Rfp-Y). Other characteristic features of the chicken MHC include its small physical size, dense organisation with short introns, lack of redundancy, and low number of other class I and class II genes with very few pseudogenes [34–36]. In avian species outside the galliforms, the genetic complexity of MHC class I and class II might vary from low copy numbers, e.g. in owls [42,43] or great snipe (Gallinago media) [44], to passerines displaying the most extreme patterns of polymorphism, typically with high levels of gene duplication Many of the duplicated genes have been shown to transcribe to cDNA (e.g. [49]), little is known about the levels of expression and true functional diversity of class I and class II genes in passerines

Methods

Results

Conclusion