Avian β-defensin variation in bottlenecked populations: the Seychelles warbler and other congeners

Abstract

b-defensins are important components of the vertebrate innate immune system responsible for encoding a variety of anti-microbial peptides. Pathogen-mediated selection is thought to act on immune genes and potentially maintain allelic variation in the face of genetic drift. The Seychelles warbler, Acrocephalus sechellensis, is an endemic passerine that underwent a recent bottleneck in its last remaining population, resulting in a considerable reduction in genome-wide variation. We genotyped avian b-defensin (AvBD) genes in contemporary (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) and museum samples of the Seychelles warbler to investigate whether immunogenetic variation was lost through this bottleneck, and examined AvBD variation across four other Acrocephalus species with varying demographic histories. No variation was detected at four of the six AvBD loci screened in the post-bottleneck population of Seychelles warbler, but two silent nucleotide polymorphisms were identified at AvBD8 and one potentially functional amino-acid variation was observed at AvBD11. Variation in the Seychelles warbler was significantly lower than in the mainland migratory congeneric species investigated, but it similar to that found in other bottlenecked species. In addition, screening AvBD7 in 15 museum specimens of Seychelles warblers sampled prior to the bottleneck revealed that this locus possessed two alleles previously, compared to the single allele in the contemporary population. Overall, the results show that little AvBD variation remains in the Seychelles warbler, probably as a result of having low AvBD diversity historically rather than the loss of variation due to drift associated with past demographic history. Given the limited pathogen fauna, this lack of variation at the AvBD loci may currently not pose a problem for this isolate population of Seychelles warblers, but it may be detrimental to the species' long-term survival if new pathogens reach the population in the future.

Toll‐like receptor variation in the bottlenecked population of the Seychelles warbler: computer simulations see the ‘ghost of selection past’ and quantify the ‘drift debt’

“…A previous study showed that five of seven TLR loci remained polymorphic in the Seychelles warbler population (Gilroy et al ., ,b). According to our simulations, we can reject neutral evolution for the three most polymorphic loci ( TLR1LB, TLR3 and TLR15; P < 0.001 for all), but not for the two less diverse loci ( TLR1LA and TLR5; P = 0.991, 0.964, respectively; Tables and ).…”

“…The following TLR loci were amplified: TLR1LA, TLR1LB, TLR3, TLR4, TLR5, TLR15 and TLR21, in 22‐30 individuals, as detailed in Gilroy et al . (,b).…”

“…We assume there is no recombination and that variation is solely introduced by mutation. This was based on previous tests using genetic algorithm recombination detection (GARD) analyses (Pond et al ., ) on sequences for all TLR loci in the Seychelles warbler, which failed to detect any evidence of recombination using site‐by‐site analysis (Gilroy et al ., ,b).…”

“…Individuals were diploid, and the fitness ( w ) of an individual with a homozygous locus was given by w = 1 − S , with S being the selection coefficient. Based on preliminary findings (Gilroy et al ., ,b), we explored the parameter space across selection coefficients 0.0 ≤ S ≤ 0.1. The fitness of heterozygote individuals was equal to unity.…”

“…The Seychelles warbler, Acrocephalus sechellensis , is an island endemic passerine species that went through a population bottleneck of less than 30 individuals on a single island during the last century (Collar & Stuart, ). In a previous study, we characterized variation at TLR genes in this population (Gilroy et al ., ,b). We found that despite the considerable losses of genomewide variation due to the bottleneck (Spurgin et al ., ), considerable polymorphism remained at five different TLR loci ( TLR1LA, TLR1LB, TLR3, TLR5 and TLR15 ), while two loci were monomorphic ( TLR4 and TLR21 ).…”