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MBE Advance Access published online on September 17, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn208
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© 2008 The Authors
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Research Article

Balancing selection at a frog antimicrobial peptide locus: fluctuating immune effector alleles?

Jacob A. Tennessen and Michael S. Blouin

Department of Zoology, Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331

Please address correspondence to: Jacob A. Tennessen, Phone: (541)737-4360, Fax: (541)737-0501, E-mail: tennessj{at}science.oregonstate.edu

Received for publication June 5, 2008. Revision received August 11, 2008. Accepted for publication September 11, 2008.

Balancing selection is common on many defense genes, but it has rarely been reported for immune effector proteins such as antimicrobial peptides. We describe genetic diversity at a brevinin-1 antimicrobial peptide 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, non-neutral 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. Antimicrobial peptide loci could be important sites of adaptive genetic diversity, with consequences for host-pathogen coevolution and the ability of species to resist disease epidemics.

Key Words: Rana pipiens • Rana blairi • Rana palustris • antimicrobial peptide • balancing selection • fluctuating selection


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