Migratory birds have a lifestyle that creates exposure to a wide range of pathogens, making them ideal model systems to study the evolution of immune genes, including toll-like receptors (TLRs). TLRs are part of the innate immune system and recognize conserved patterns of pathogens, including viruses. We investigated the evolution of the ectodomain (ECD) of two TLRs (TLR3 and TLR7) involved in virus recognition in three migratory wader species (C. alba, C. ruficollis, A.interpres), as well as across other avian groups. Our results revealed that the inferred relationships among avian TLR3 and TLR7 ECDs do not match the phylogeny of birds. Furthermore, we showed that although both loci are mostly under purifying selection, the evolution of avian TLR3 ECD is also driven by episodic diversifying selection.
We discovered that TLR7 is duplicated in all three wader species, as well as in other Charadriiformes, Cuculiformes and Passerines. The duplication appears to be ancestral for each order, and the duplicated copies appear to be undergoing concerted evolution.
Significantly higher proportions of non-synonymous mutations were detected in TLR7 than in TLR3 across the three wader species. In addition, while the phylogenetic relationships of wader TLR7 matched those of the three species, initial analysis of TLR3 showed potential associations with the species’ ecology, such as exposure to pathogens like avian influenza virus.