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.