The correspondence between genotype at bacterial and viral-facing Toll-like receptors and preen oil chemical composition in song sparrows

Mate choice can optimize immune genotypes of offspring and thus reinforce parasite-mediated selection on immune genes. Information about genotype at class II major histocompatibility complex (MHC) is conveyed through body odour, possibly mediated by symbiotic odour-producing bacteria. This can influence mate choice: many jawed vertebrates prefer mates dissimilar to themselves (‘compatible’) at MHC. However, whether odour conveys information about genotype at other immune loci remains unknown. I assessed whether preen oil chemical composition (a proxy for body odour in birds) varies with genotype at viral-facing (TLR3) and bacterial-facing (TLR4) Toll-like receptors in song sparrows (Melospiza melodia). Pairwise distance of preen oil chemical composition did not covary in most cases with pairwise genetic distance at either TLR3 or TLR4, nor did heterozygosity at either locus predict preen oil chemical diversity. Despite evidence of nonrandom mating at TLR3, the mechanism by which song sparrows assess genetic similarity at TLR loci remains elusive.