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MBE Advance Access originally published online on August 24, 2005
Molecular Biology and Evolution 2005 22(12):2480-2490; doi:10.1093/molbev/msi247
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© The Author 2005. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oupjournals.org

Research Article

Genetic Diversity and the Evolutionary History of Plant Immunity Genes in Two Species of Zea

David A. Moeller and Peter Tiffin

Department of Plant Biology, University of Minnesota, Saint Paul

E-mail: ptiffin{at}umn.edu.

Plant pathogenesis-related genes (PR genes) code for enzymes, enzyme inhibitors, and other peptides that confer resistance to pathogens and herbivores. Although several PR genes have been the subject of molecular population genetic analyses, a general understanding of their long-term evolutionary dynamics remains incomplete. Here we analyze sequence data from 17 PR genes from two closely related teosinte species of central Mexico. In addition to testing whether patterns of diversity at individual loci depart from expectations under a neutral model, we compared patterns of diversity at defense genes, as a class, to nondefense genes. In Zea diploperennis, the majority of defense genes have patterns of diversity consistent with neutral expectations while at least two genes showed evidence of recent positive selection consistent with arms-race models of antagonistic coevolution. In Zea mays ssp. parviglumis, by contrast, analyses of both defense and nondefense genes revealed strong and consistent departures from the neutral model, suggestive of nonequilibrium population dynamics or population structure. Nevertheless, we found a significant excess of replacement polymorphism in defense genes compared to nondefense genes. Although we cannot exclude relaxed selective constraint as an explanation, our results are consistent with temporally variable (transient or episodic) selection or geographically variable selection acting on parviglumis defense genes. The different patterns of diversity found in the two Zea species may be explained by parviglumis' greater distribution and population structure together with geographic variation in selection.

Key Words: coevolution • disease resistance • innate immunity • inducible defense • natural selection • population structure


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