Does intergenic recombination play an important role in evolution of alleles at the RB late blight resistance locus?

Dimitre S. Mollov, Maria J. Sanchez, and James M. Bradeen

Department of Plant Pathology, University of Minnesota, 495 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, USA

The RB gene imparts resistance to all known races of the potato late blight pathogen Phytophthora infestans. Like many plant resistance (R) genes cloned to date, RB exists within a cluster of related paralogous loci.  Intergenic recombination between clustered paralogous R genes has been hypothesized as an important mechanism for the emergence of unique R alleles. The RB cluster is instable in BAC clones, evidence that intergenic recombination in this gene cluster can occur in vitro.  However, sequence analysis of the RB cluster predicts conservation of RB alleles with little intergenic recombination in planta. We have completed a survey of RB alleles present in a collection of S. bulbocastanum genotypes by amplifying RB orthologs using Long Range-PCR. Surprisingly, we identified two S. bulbocastanum genotypes, from the same population, yielding larger than expected RB fragments. Sequence analysis revealed structural rearrangement between the RB coding region and a paralogous locus.  Recovery of the recombinant RB allele from two genotypes from the same population suggests a common evolutionary origin and documents the occurrence of an in planta rearrangement of the RB gene cluster.  We continue to explore the functional and evolutionary significance of our findings.