Characterization of a non-classical MHC class II gene in the vulnerable Chinese egret (Egretta eulophotes)

Not all birds have a single dominantly expressed MHC-I gene: Transcription suggests that siskins have many highly expressed MHC-I genes

Passerine birds belong to the most species rich bird order and are found in a wide range of habitats. The extremely polymorphic adaptive immune system of passerines, identified through their major histocompatibility complex class I genes (MHC-I), may explain some of this extreme radiation. Recent work has shown that passerines have higher numbers of MHC-I gene copies than other birds, but little is currently known about expression and function of these gene copies. Non-passerine birds have a single highly expressed MHC-I gene copy, a pattern that seems unlikely in passerines. We used high-throughput sequencing to study MHC-I alleles in siskins (Spinus spinus) and determined gene expression, phylogenetic relationships and sequence divergence. We verified between six and 16 MHC-I alleles per individual and 97% of these were expressed. Strikingly, up to five alleles per individual had high expression. Out of 88 alleles 18 were putatively non-classical with low sequence divergence and expression, and found in a single phylogenetic cluster. The remaining 70 alleles were classical, with high sequence divergence and variable degrees of expression. Our results contradict the suggestion that birds only have a single dominantly expressed MHC-I gene by demonstrating several highly expressed MHC-I gene copies in a passerine.

Major histocompatibility complex class II DAB alleles associated with intestinal parasite load in the vulnerable Chinese egret (Egretta eulophotes)

The maintenance of major histocompatibility complex (MHC) polymorphism has been hypothesized to result from many mechanisms such as rare-allele advantage, heterozygote advantage, and allele counting. In the study reported herein, 224 vulnerable Chinese egrets (Egretta eulophotes) were used to examine these hypotheses as empirical results derived from bird studies are rare. Parasite survey showed that 147 (65.63%) individuals were infected with 1-3 helminths, and 82.31% of these infected individuals carried Ascaridia sp. Using asymmetric polymerase chain reaction technique, 10 DAB1, twelve DAB2, and three DAB3 exon 2 alleles were identified at each single locus. A significant association of the rare allele Egeu-DAB2*05 (allele frequency: 0.022) with helminth resistance was found for all helminths, as well as for the most abundant morphotype Ascaridia sp. in the separate analyses. Egeu-DAB2*05 occurred frequently in uninfected individuals, and individuals carrying Egeu-DAB2*05 had significantly lower helminth morphotypes per individual (HMI) (the number of HMI) and the fecal egg count values. Further, the parasite infection measurements were consistently lower in individuals with an intermediate number of different alleles in the duplicated DAB loci. Significantly, heterozygosity within each DAB locus was not correlated with any parasite infection measurements. These results indicate that the diversity in MHC Egeu-DAB gene is associated with intestinal parasite load and maintained by pathogen-driven selection that probably operate through both the rare-allele advantage and the allele counting strategy, and suggest that Egeu-DAB2*05 might be a valuable indicator of better resistance to helminth diseases in the vulnerable Chinese egret.