Genomic organization, rapid evolution and meiotic instability of NBS-encoding genes in a new fruit crop "chestnut rose"

From chestnut rose, a promising fruit crop of Rosa species, powdery mildew disease resistant and susceptible genotypes and their F1 progenies were used to isolate NBS-encoding genes using 19 degenerate primer pairs and an additional cloning method called overlapping extension amplification. A total of 126 genes were harvested; of these 38 were from resistant parent, 37 from susceptible, and 51 from F1 progenies. Phylogenetic tree was constructed and revealed that NBS sequences from parents and F1 progenies tend to form mixture and are well distributed among the branches of the tree. Mapping of these NBS genes suggested their organization in the genome as "tandem duplicated cluster" and to a less extent "heterogeneous cluster". Intra-specific polymorphisms and inter-specific divergence were detected by Southern blotting with NBS-encoding genes as probes. Sequencing on nucleotide level revealed even more intra-specific variation: for R4 gene 9.81% of the nucleotides are polymorphic. Amino acid sites under positive selection were detected in the NBS region. Some NBS-encoding genes were meiotically unstable, which may due to recombination and deletion events. Moreover, a transponson-like element was isolated in the flanking region of NBS gene, implying a possible role for transponson in the evolution history of resistance genes.

Key Words: chestnut rose, genomic organization, meiotic instability, rapid evolution, resistance gene

Online ISS 1091-6490
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