title: The Evolution of Innate Immune Genes: Purifying and Balancing Selection on β-Defensins in Waterfowl.
abstract: In disease dynamics, high immune gene diversity can confer a selective advantage to hosts in the face of a rapidly evolving and diverse pathogen fauna. This is supported empirically for genes involved in pathogen recognition and signalling. In contrast, effector genes involved in pathogen clearance may be more constrained. β-Defensins are innate immune effector genes; their main mode of action is via disruption of microbial membranes. Here, five β-defensin genes were characterized in mallards (Anas platyrhynchos) and other waterfowl; key reservoir species for many zoonotic diseases. All five genes showed remarkably low diversity at the individual-, population-, and species-level. Furthermore, there was widespread sharing of identical alleles across species divides. Thus, specific β-defensin alleles were maintained not only spatially but also over long temporal scales, with many amino acid residues being fixed across all species investigated. Purifying selection to maintain individual, highly efficacious alleles was the primary evolutionary driver of these genes in waterfowl. However, we also found evidence for balancing selection acting on the most recently duplicated β-defensin gene (AvBD3b). For this gene, we found that amino acid replacements were more likely to be radical changes, suggesting that duplication of β-defensin genes allows exploration of wider functional space. Structural conservation to maintain function appears to be crucial for avian β-defensin effector molecules, resulting in low tolerance for new allelic variants. This contrasts with other types of innate immune genes, such as receptor and signalling molecules, where balancing selection to maintain allelic diversity has been shown to be a strong evolutionary force.
authors.affiliation: Centre for Ecology and Evolution in Microbial Model Systems, Linnaeus University, Kalmar, Sweden joanne.chapman@lnu.se jonas.waldenstrom@lnu.se.
authors.forename: Joanne R
authors.initials: JR
authors.lastname: Chapman
authors.affiliation: Molecular Ecology & Evolution Lab (MEMEG), Department of Biology, Lund University, Lund, Sweden.
authors.forename: Olof
authors.initials: O
authors.lastname: Hellgren
authors.affiliation: Centre for Ecology and Evolution in Microbial Model Systems, Linnaeus University, Kalmar, Sweden.
authors.forename: Anu S
authors.initials: AS
authors.lastname: Helin
authors.affiliation: Department of Biology, University of Konstanz, Konstanz, Germany.|Department of Migration and Immuno-Ecology, Max Planck Institute for Ornithology, Radolfzell, Germany.
authors.forename: Robert H S
authors.initials: RH
authors.lastname: Kraus
authors.affiliation: Wildfowl & Wetlands Trust, Slimbridge, Gloucestershire, United Kingdom.
authors.forename: Ruth L
authors.initials: RL
authors.lastname: Cromie
authors.affiliation: Centre for Ecology and Evolution in Microbial Model Systems, Linnaeus University, Kalmar, Sweden joanne.chapman@lnu.se jonas.waldenstrom@lnu.se.
authors.forename: Jonas
authors.initials: J
authors.lastname: Waldenström
mesh_terms: D000483:Alleles; D000595:Amino Acid Sequence; D000818:Animals; D046090:Anseriformes / Q000235:genetics* / Q000276:immunology*; D023181:Antimicrobial Cationic Peptides / Q000235:genetics; D019143:Evolution, Molecular; D020440:Gene Duplication; D014644:Genetic Variation; D007113:Immunity, Innate / Q000235:genetics; D005810:Multigene Family / Q000235:genetics; D010802:Phylogeny; D012641:Selection, Genetic; D023083:beta-Defensins / Q000235:genetics* / Q000276:immunology
chemical_list: D023181:Antimicrobial Cationic Peptides; D023083:beta-Defensins
keywords: Antimicrobial peptides; avian immune system; ecoimmunology; host defense peptides; host-pathogen dynamics.