Physiological condition reflects polymorphism at the toll-like receptors in a colonial waterbird

Toll-like receptors (TLRs) are a crucial component of vertebrate innate immune response. Despite their importance, associations of TLR diversity with fitness-related traits have rarely been examined in wild animal populations. Here, we tested for associations of TLR polymorphism with physiological condition in a colonial waterbird, the Black-headed Gull (Chroicocephalus ridibundus). Physiological condition and polymorphism at 4 TLR loci were assessed in 60 gulls from a breeding colony in northern Poland. We found that blood hemoglobin and plasma albumin concentrations were positively associated with TLR diversity across all genotyped loci. Plasma concentrations of albumin and triglycerides were also associated with the presence of specific TLR variants and locus-specific diversity. All significant associations between physiological condition and TLRs were primarily apparent at the level of nucleotide, rather than amino acid allelic variants. Although the exact molecular mechanisms responsible for these associations require further investigation, our study provides strong correlational support for links between TLR diversity and physiological condition in a wild avian population, and it adds to the growing, but still modest, body of evidence for the fitness-related consequences of individual TLR repertoire in wild birds. LAY SUMMARY Toll-like receptors (TLRs) initiate innate immune response by recognition of conserved pathogen-associated molecular patterns. We tested for associations between TLR diversity and fitness-related traits (physiological condition) in the Black-headed Gull. Physiological condition was associated with both TLR diversity and presence of specific TLR variants, but these associations were mostly prevalent at the nucleotide, rather than amino acid, level. Our results raise the question on the adaptive significance of synonymous variation at TLR genes.