Haplotype variability and identification of new functional alleles at the Rdg2a leaf stripe resistance gene locus

The barley Rdg2a locus confers resistance to the leaf stripe pathogen Pyrenophora graminea and, in the barley genotype Thibaut, it is composed of a gene family with three highly similar paralogs. Only one member of the gene family (called as Rdg2a) encoding for a CC-NB-LRR protein is able to confer resistance to the leaf stripe isolate Dg2. To study the genome evolution and diversity at the Rdg2a locus, sequences spanning the Rdg2a gene were compared in two barley cultivars, Thibaut and Morex, respectively, resistant and susceptible to leaf stripe. An overall high level of sequence conservation interrupted by several rearrangements that included three main deletions was observed in the Morex contig. The main deletion of 13,692 bp was most likely derived from unequal crossing over between Rdg2a paralogs leading to the generation of a chimeric Morex rdg2a gene which was not associated to detectable level of resistance toward leaf stripe. PCR-based analyses of genic and intergenic regions at the Rdg2a locus in 29 H. vulgare lines and one H. vulgare ssp. spontaneum accession indicated large haplotype variability in the cultivated barley gene pool suggesting rapid and recent divergence at this locus. Barley genotypes showing the same haplotype as Thibaut at the Rdg2a locus were selected for a Rdg2a allele mining through allele re-sequencing and two lines with polymorphic nucleotides leading to amino acid changes in the CC-NB and LRR encoding domains, respectively, were identified. Analysis of nucleotide diversity of the Rdg2a alleles revealed that the polymorphic sites were subjected to positive selection. Moreover, strong positively selected sites were located in the LRR encoding domain suggesting that both positive selection and divergence at homologous loci are possibly representing the molecular mechanism for the generation of high diversity at the Rdg2a locus in the barley gene pool.