title: Balancing selection at a frog antimicrobial peptide locus: fluctuating immune effector alleles?
abstract: Balancing selection is common on many defense genes, but it has rarely been reported for immune effector proteins such as antimicrobial peptides (AMPs). We describe genetic diversity at a brevinin-1 AMP locus in three species of leopard frogs (Rana pipiens, Rana blairi, and Rana palustris). Several highly divergent allelic lineages are segregating at this locus. That this unusual pattern results from balancing selection is demonstrated by multiple lines of evidence, including a ratio of nonsynonymous/synonymous polymorphism significantly higher than 1, the ZnS test, incongruence between the number of segregating sites and haplotype diversity, and significant Tajima's D values. Our data are more consistent with a model of fluctuating selection in which alleles change frequencies over time than with a model of stable balancing selection such as overdominance. Evidence for fluctuating selection includes skewed allele frequencies, low levels of synonymous variation, nonneutral values of Tajima's D within allelic lineages, an inverse relationship between the frequency of an allelic lineage and its degree of polymorphism, and divergent allele frequencies among populations. AMP loci could be important sites of adaptive genetic diversity, with consequences for host-pathogen coevolution and the ability of species to resist disease epidemics.
authors.affiliation: Department of Zoology, Oregon State University, OR, USA. tennessj@science.oregonstate.edu
authors.forename: Jacob A
authors.initials: JA
authors.lastname: Tennessen
authors.forename: Michael S
authors.initials: MS
authors.lastname: Blouin
mesh_terms: D000595:Amino Acid Sequence; D029845:Amphibian Proteins / Q000737:chemistry / Q000235:genetics* / Q000276:immunology*; D000818:Animals; D023181:Antimicrobial Cationic Peptides / Q000737:chemistry / Q000235:genetics* / Q000276:immunology*; D005787:Gene Frequency; D008969:Molecular Sequence Data; D011898:Ranidae / Q000145:classification / Q000276:immunology*; D016415:Sequence Alignment
chemical_list: D029845:Amphibian Proteins; D023181:Antimicrobial Cationic Peptides; C078413:brevinin-1 protein, Rana
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