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IN BRIEF

Complexities of R Gene Evolution in Arabidopsis

neckardt{at}aspb.org

Many plant disease resistance (R) genes belong to the large class of nucleotide binding site leucine-rich repeat (NBS-LRR) genes. Bakker et al. (pages 1803–1818) resequenced the LRR region of 27 R genes in 96 Arabidopsis accessions and compared single nucleotide polymorphisms (SNPs) in this region to a genome-wide distribution of SNPs based on randomly distributed fragments. The LRR regions were found to be highly polymorphic for protein variants but not necessarily for synonymous changes, suggesting that in these instances, alleles are not maintained as polymorphisms for long time periods. Polymorphisms present in seven R genes showed weak to moderate indications of balancing selection, whereas little evidence could be found for directional selection or selective sweeps. The data suggest a model of transient or frequency-dependent selection maintaining protein variants at a locus for variable time periods.

This study provides a broad overview of R gene evolution that represents an important step toward a more complete understanding of this important process in higher plant populations. Historically, there has been a widespread notion of plants and pathogens as engaging in a coevolutionary arms race. The work of Bakker et al. is consistent with our current state of knowledge that R gene evolution follows more complex pathways, and different R genes evolve in different ways depending on the intricacies of their specific selective advantages.

View larger version (114K): [in this window] [in a new window]   Arabidopsis plants growing in the field are infected by a wide variety of bacterial and fungal pathogens. The presence of particular R gene alleles confers resistance to certain pathogens in Arabidopsis and other plants.

Related articles in Plant Cell:

A Genome-Wide Survey of R Gene Polymorphisms in Arabidopsis

Erica G. Bakker, Christopher Toomajian, Martin Kreitman, and Joy Bergelson
Plant Cell 2006 18: 1803-1818. [Abstract] [Full Text]  

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