What Is Present at Common Bean Subtelomeres? Large Resistance Gene Clusters, Knobs and Khipu Satellite DNA

Abstract

In plants, the largest class of resistance (R) gene encodes nucleotide-binding leucine-rich repeat (NB-LRR) proteins. This multigene family is often organized in clusters of tightly linked genes. In the common bean (Phaseolus vulgaris L.) genome, most of the well-characterized large R gene clusters are not randomly distributed since they are often located at the ends of the linkage groups (LG), suggesting that this location is favorable for R gene proliferation. In addition, terminal knobs (heterochromatic blocks) are present at most chromosome (Chr) ends of P. vulgaris, and we have identified a satellite DNA referred to as khipu that is a component of most of them. Plasticity of subtelomeres has been described in various organisms such as yeast and human but is not well documented in plants. In common bean, the B4 cluster of R gene was shown to derive from the Co-2 R gene cluster through an ectopic recombination between non-homologous chromosomes in subtelomeric regions. These unusual features of common bean genome (subtelomeric localization of large NB-LRR sequences, the presence of terminal knobs, and plasticity of subtelomeres) have been investigated with the availability of the complete common bean genome sequence.