NBS-containing genes frequently confer disease resistance to angiosperm species. Due to their vital impact on crop production and unraveling the pathogen-host biology, extensive studies have been carried out to elucidate their regulatory mechanism and evolutionary history. While features such as high evolutionary rate and diversified selection pressure on distinct functional domains have been verified by phylogenetic studies based on gene sequence, the evolutionary mechanism for gene copy number variation across species (ranging from ~50 genes to ~600 genes) is still blurred. We identified ~350 and ~190 NBS-containing genes in two closely related monocots: Sorghum bicolor and Zea mays, respectively, each of which has its NBS genes grouped into sub-families regarding the domain components. We employ well-designed graphs which integrate evolutionary information including the phylogenetic tree, genomic location, synteny blocks and gene domain structure as well as comparative genomics methods to present the genomic-level gene-copy-number dynamics of this gene family in the two species. Finally, we discuss several evolutionary processes which may provide explanations for the dynamics of this gene family.
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