title: Polymorphism and evolution of ribosomal DNA in tea (Camellia sinensis, Theaceae).
abstract: Ribosomal DNA (rDNA) repeats often exhibit a high level of within-species homogeneity, but intra-individual polymorphism of rDNA has been found in Camellia species. In order to reveal the evolutionary pattern of rDNA repeats in tea [Camellia sinensis (L.) O. Kuntze], we identified the 45S rDNA loci, estimated their copy number, and cloned partial regions of them from different PCR products and from digested genomic DNA. The results show that there are 3 loci (6 sites) containing 45S rDNA in the tea genome; they are located at the ends of the short arms of 6 chromosomes and consist of a maximum of ∼6500 repeat units. On one hand, 164 sequences of the 26S rDNA cloned from PCR products contained 147 haplotypes, consisting of 62% pseudogenes, 24% putative functional genes, and 14% PCR-mediated recombinants, suggesting that the 45S rDNA of tea maintains an extremely high level of polymorphism and divergence; on the other hand, rDNA fragments cloned directly from genomic DNA exhibited a very high level of homogeneity: only one of 33 rDNA fragments was from a pseudogene. These results demonstrate that although over 60% of the 26S rDNA sequences identified in PCR products belong to pseudogenes, most 45S rDNA repeats are functional genes and have undergone concerted evolution. In this study, strong PCR bias and PCR-mediated recombination greatly increased the apparent proportion of pseudogenes in PCR products. Phylogenetic analysis and genetic divergence values for 26S rDNA sequences obtained in this study show that many pseudogenes have originated independently from functional genes at different times, and despite thus escaping from concerted evolution, they have failed to be eliminated from the tea genome over a long period, some of them having even produced addition copies by rapid expansion. Importantly, our study suggests that in order to determine the true pattern of evolution of rDNA it is necessary to combine data from more than one method rather than relying only on sequences from PCR products.
authors: Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, School of Life Sciences, Fudan University, Shanghai 200433, China; Shanghai Information Center for Life Sciences, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China.
authors: Jing
authors: J
authors: Xu
authors: Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, School of Life Sciences, Fudan University, Shanghai 200433, China.
authors: Ying
authors: Y
authors: Xu
authors: Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, School of Life Sciences, Fudan University, Shanghai 200433, China.
authors: Takahiro
authors: T
authors: Yonezawa
authors: School of Life Sciences, Wuhan University, Wuhan 430072, China.
authors: Lijia
authors: L
authors: Li
authors: Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, School of Life Sciences, Fudan University, Shanghai 200433, China.
authors: Masami
authors: M
authors: Hasegawa
authors: Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, School of Life Sciences, Fudan University, Shanghai 200433, China.
authors: Fan
authors: F
authors: Lu
authors: Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, School of Life Sciences, Fudan University, Shanghai 200433, China.
authors: Jiakuan
authors: J
authors: Chen
authors: Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, School of Life Sciences, Fudan University, Shanghai 200433, China. Electronic address: wjzhang@fudan.edu.cn.
authors: Wenju
authors: W
authors: Zhang
mesh_terms: D028241:Camellia sinensis / Q000235:genetics*; D056905:Chromosome Breakpoints; D004275:DNA, Ribosomal / Q000235:genetics*; D019143:Evolution, Molecular; D018745:Genome, Plant / Q000235:genetics; D006239:Haplotypes / Q000235:genetics; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D011110:Polymorphism, Genetic / Q000235:genetics*; D011544:Pseudogenes / Q000235:genetics; D011995:Recombination, Genetic / Q000235:genetics; D013662:Tea / Q000235:genetics
chemical_list: D004275:DNA, Ribosomal; D013662:Tea
keywords: Camellia sinensis; Concerted evolution; PCR bias; Polymorphism; Pseudogenes; rDNA