Resistance genes in the Triticeae and the dynamics of divergence before duplication

Our understanding of inducible plant defense responses was greatly increased by the characterization of disease resistance genes from a variety of plant-pathogen interaction models. In the paper we aimed to characterize conserved motifs in the NBS domains, to determine their evolutionary fate. Using combined iterative data-mining approaches we effectively expanded the dataset, but the number of NBS-LRR sequences obtained was much lower than projected. We interpret this as evidence supporting low basal transcription levels as expected for R genes situated at the very start of signal amplification cascades. Motif analysis showed all key motifs of the NBS-LRR CNL domain present, with no evidence for TNL type sequences as previously observed for monocotyledonous taxa. Significant overlap between Triticeae CNL members and CNL R genes from other taxa was found. Three recently diverged clades of paralogous NBS-core sequences for barley was tested for gene-conversion events, but none was found, contradicting results from Arabidopsis where even ectopic gene-conversion events were detected. Non-synonymous to synonymous substitution rate ratio tests for comparing evolution of recent paralogous and homeologous duplications showed that the NBS-core domain of three barley paralogue clades were under strong purifying selection in contrast to the wheat go35 CNL gene, where four different non-synonymous substitutions were found. Only synonymous differences were seen between sequences obtained from two diploid wheat ancestors (A. tauschii and A. speltoides). We consider this as evidence supporting a divergence-before-duplication model of R gene evolution.