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

Abstract

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.