Drift, not selection, shapes toll-like receptor variation among oceanic island populations.
Understanding the relative role of different evolutionary forces in shaping the level and distribution of functional genetic diversity among natural populations is a key issue in evolutionary and conservation biology. To do so accurately genetic data must be analysed in conjunction with an unambiguous understanding of the historical processes that have acted upon the populations. Here, we focused on diversity at toll-like receptor (TLR) loci, which play a key role in the vertebrate innate immune system and, therefore, are expected to be under pathogen-mediated selection. We assessed TLR variation within and among 13 island populations (grouped into three archipelagos) of Berthelot's pipit, Anthus berthelotii, for which detailed population history has previously been ascertained. We also compared the variation observed with that found in its widespread sister species, the tawny pipit, Anthus campestris. We found strong evidence for positive selection at specific codons in TLR1LA, TLR3 and TLR4. Despite this, we found that at the allele frequency level, demographic history has played the major role in shaping patterns of TLR variation in Berthelot's pipit. Levels of diversity and differentiation within and across archipelagos at all TLR loci corresponded very closely with neutral microsatellite variation and with the severity of the bottlenecks that occurred during colonization. Our study shows that despite the importance of TLRs in combating pathogens, demography can be the main driver of immune gene variation within and across populations, resulting in patterns of functional variation that can persist over evolutionary timescales.
School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK.|Grupo Ecología y Evolución de Vertebrados, Instituto de Biología, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia, Calle 70 No. 52-21, Medellin, Colombia.
Catalina
C
Gonzalez-Quevedo
School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK.
Lewis G
LG
Spurgin
Research Unit of Biodiversity (UO-CSIC-PA), Oviedo University, Campus of Mieres, Research Building, 5th Floor. C/Gonzalo Gutiérrez Quirós, s/n, 33600 Mieres, Asturias, Spain.
Juan Carlos
JC
Illera
School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK.
David S
DS
Richardson
D000818:Animals; D019143:Evolution, Molecular; D005787:Gene Frequency; D040961:Genetic Drift; D005828:Genetics, Population; D062312:Islands; D018895:Microsatellite Repeats; D008969:Molecular Sequence Data; D046109:Passeriformes / Q000235:genetics*; D010802:Phylogeny; D012641:Selection, Genetic; D051193:Toll-Like Receptors / Q000235:genetics*
D051193:Toll-Like Receptors
Anthus berthelotii; Berthelot's pipit; bottleneck; founder effects; genetic drift; genetic variation; selection; toll-like receptors