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Genome Dynamics and Evolution of the Mla (Powdery Mildew) Resistance Locus in Barley

^a Interdepartmental Genetics Program and Department of Plant Pathology, Iowa State University, Ames, Iowa 50011-1020
^b Clemson University Genomics Institute, Clemson, South Carolina 29634
^c Corn Insects and Crop Genetics Research, United States Department of Agriculture, Agricultural Research Service, Iowa State University, Ames, Iowa 50011-1020

³ To whom correspondence should be addressed. E-mail rpwise{at}iastate.edu; fax 515-294-9420

Genes that confer defense against pathogens often are clustered in the genome and evolve via diverse mechanisms. To evaluate the organization and content of a major defense gene complex in cereals, we determined the complete sequence of a 261-kb BAC contig from barley cv Morex that spans the Mla (powdery mildew) resistance locus. Among the 32 predicted genes on this contig, 15 are associated with plant defense responses; 6 of these are associated with defense responses to powdery mildew disease but function in different signaling pathways. The Mla region is organized as three gene-rich islands separated by two nested complexes of transposable elements and a 45-kb gene-poor region. A heterochromatic-like region is positioned directly proximal to Mla and is composed of a gene-poor core with 17 families of diverse tandem repeats that overlap a hypermethylated, but transcriptionally active, gene-dense island. Paleontology analysis of long terminal repeat retrotransposons indicates that the present Mla region evolved over a period of >7 million years through a variety of duplication, inversion, and transposon-insertion events. Sequence-based recombination estimates indicate that R genes positioned adjacent to nested long terminal repeat retrotransposons, such as Mla, do not favor recombination as a means of diversification. We present a model for the evolution of the Mla region that encompasses several emerging features of large cereal genomes.

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