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Originally published as Genetics Published Articles Ahead of Print on December 18, 2006.
Genetics, Vol. 175, 1307-1319, March 2007, Copyright © 2007
doi:10.1534/genetics.106.063602
Natural Variation in the Pto Disease Resistance Gene Within Species of Wild Tomato (Lycopersicon). II. Population Genetics of Pto
Laura E. Rose*,1,
Richard W. Michelmore
and
Charles H. Langley
* Section of Evolutionary Biology, University of Munich (LMU), Martinsried 82152, Germany, The Genome Center and the Department of Plant Sciences and The Center for Population Biology, University of California, Davis, California 95616
1 Corresponding author: Section of Evolutionary Biology, University of Munich, Grosshadernerstrasse 2, Martinsried 82152, Germany.
E-mail: rose{at}zi.biologie.uni-muenchen.de
Disease resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) in the host species Lycopersicon esculentum, the cultivated tomato, and the closely related L. pimpinellifolium is triggered by the physical interaction between the protein products of the host resistance (R) gene Pto and the pathogen avirulence genes AvrPto and AvrPtoB. Sequence variation at the Pto locus was surveyed in natural populations of seven species of Lycopersicon to test hypotheses of host–parasite coevolution and functional adaptation of the Pto gene. Pto shows significantly higher nonsynonymous polymorphism than 14 other non-R-gene loci in the same samples of Lycopersicon species, while showing no difference in synonymous polymorphism, suggesting that the maintenance of amino acid polymorphism at this locus is mediated by pathogen selection. Also, a larger proportion of ancestral variation is maintained at Pto as compared to these non-R-gene loci. The frequency spectrum of amino acid polymorphisms known to negatively affect Pto function is skewed toward low frequency compared to amino acid polymorphisms that do not affect function or silent polymorphisms. Therefore, the evolution of Pto appears to be influenced by a mixture of both purifying and balancing selection.
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