Evolution of innate and adaptive immune genes in a non-model waterbird, the common tern

Toll-like receptors (TLRs) and the Major Histocompatibility Complex (MHC) are the key pathogen-recognition genes of vertebrate immune system and they have a crucial role in the initiation of innate and adaptive immune response, respectively. Recent advancements in sequencing technology sparked research on highly duplicated MHC genes in non-model species, but TLR variation in natural vertebrate populations has remained little studied and comparisons of polymorphism across both TLRs and MHC are scarce. Here, we aimed to compare variation across innate (four TLR loci) and adaptive (MHC class I and class II) immune genes in a non-model avian species, the common tern Sterna hirundo. We detected relatively high allelic richness at TLR genes (9-48 alleles per locus), which was similar to or even higher than the estimated per locus allelic richness at the MHC (24-30 alleles at class I and 13-16 alleles at class II under uniform sample sizes). Despite this, the total number of MHC alleles across all duplicated loci (four class I and three class II) was much higher and MHC alleles showed greater sequence divergence than TLRs. Positive selection targeted relatively more sites at the MHC than TLRs, but the strength of selection (dN/dS ratios) at TLRs was higher when compared to MHC class I. There were also differences in the signature of positive selection and recombination (gene conversion) between MHC class I and II (stronger signature at class II), suggesting that mechanisms maintaining variation at the MHC may vary between both classes. Our study indicates that allelic richness of both innate and adaptive immune receptors may be maintained at relatively high levels in viable avian populations and we recommend a transition from the traditional gene-specific to multi-gene approach in studying molecular evolution of vertebrate immune system.