Abstract
Understanding where and how genetic variation is maintained within populations is important from an evolutionary and conservation perspective. Signatures of past selection suggest that pathogen-mediated balancing selection is a key driver of immunogenetic variation, but studies tracking contemporary evolution are needed to help resolve the evolutionary forces and mechanism at play. Previous work in a bottlenecked population of Seychelles warblers (Acrocephalus sechellensis ) show that functional variation has been maintained at the viral-sensing Toll-like receptor 3 (TLR3) gene. Here, we characterise evolution at this TLR3 locus over a 25-year period within the original remnant population of the Seychelles warbler, and in four other derived, contained populations. Results show a significant and consistent temporal decline in the frequency of theTLR3 C allele in the original population, and that similar declines in the TLR3 C allele frequency occurred in all the derived populations. Individuals (of both sexes) with the TLR3 CC genotype had lower survival, and males - but not females - that carry theTLR3 C allele had significantly lower lifetime reproductive success than those with only theTLR3 A allele. These results indicate that positive selection, caused by an as yet unknown agent, is drivingTLR3 evolution in the Seychelles warblers. No evidence of heterozygote advantage was detected. However, whether the positive selection observed is part of a longer-term pattern of balancing selection (through fluctuating selection or rare-allele advantage) cannot be resolved without tracking the TLR3 Callele in the populations over an extended period of time.