Contemporary evolution of the viral-sensing TLR3 gene in an isolated
vertebrate population
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 the TLR3C allele in the original
population, and that similar declines in the TLR3C allele frequency
occurred in all the derived populations. Individuals (of both sexes)
with the TLR3CC genotype had lower survival, and males - but not females
- that carry the TLR3C allele had significantly lower lifetime
reproductive success than those with only the TLR3A allele. These
results indicate that positive selection, caused by an as yet unknown
agent, is driving TLR3 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 TLR3C allele in the populations
over an extended period of time.