Purifying selection in the toll-like receptors of song sparrows Melospiza melodia

Variation in immune gene sequences is known to influence resistance to infectious diseases and parasites, and hence survival and mate choice, across animal taxa. Toll-like receptors (TLRs) comprise one essential gene family in the vertebrate innate immune system, and recognize evolutionarily conserved structures from all major microorganism classes. However, the causes and consequences of TLR variation in passerine birds remain largely unexplored. We examined seven TLR genes in song sparrows (Melospiza melodia), a species that is studied across North America. We then examined sequences from four unduplicated TLRs (TLR1LB, TLR3, TLR4 and TLR15) from birds in two parts of the species’ range (N = 27, N = 6), tested for evidence of selection, and conducted pilot analyses of the role of TLR heterozygosity in survival. We identified 45 SNPs: 19 caused changes in amino acid sequences, and two of these were likely deleterious. We found no evidence of codon-level episodic positive selection, but...