Originally published as Genetics Published Articles Ahead of Print on February 3, 2008.

Genetics, Vol. 178, 2081-2091, April 2008, Copyright © 2008
doi:10.1534/genetics.107.086421

Genomic Organization, Rapid Evolution and Meiotic Instability of Nucleotide-Binding-Site-Encoding Genes in a New Fruit Crop, "Chestnut Rose"

Qiang Xu*, Xiaopeng Wen{dagger} and Xiuxin Deng*,1

* National Key Laboratory of Crop Genetic Improvement, Huazhong Agricultural University, Wuhan 430070, Hubei, People's Republic of China and {dagger} Guizhou Key Laboratory of Agricultural Bioengineering, Guizhou University, Guiyang 550025, Guizhou, People's Republic of China

1 Corresponding author: National Key Laboratory of Crop Genetic Improvement, Huazhong Agricultural University, Lion St. no.1, Wuhan 430070, People's Republic of China.
E-mail: xxdeng{at}mail.hzau.edu.cn

From chestnut rose, a promising fruit crop of the Rosa genus, powdery mildew disease-resistant and susceptible genotypes and their F1 progeny were used to isolate nucleotide-binding-site (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 a resistant parent, 37 from a susceptible parent, and 51 from F1 progeny. A phylogenetic tree was constructed, which revealed that NBS sequences from parents and F1 progeny tend to form a mixture and are well distributed among the branches of the tree. Mapping of these NBS genes suggested that their organization in the genome is a "tandem duplicated cluster" and, to a lesser extent, a "heterogeneous cluster." Intraspecific polymorphisms and interspecific divergence were detected by Southern blotting with NBS-encoding genes as probes. Sequencing on the nucleotide level revealed even more intraspecific variation: for the 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 transposon-like element was isolated in the flanking region of NBS genes, implying a possible role for transposon in the evolutionary history of resistance genes.