四照花亚属的nrDNA ITS致同进化不完全

Incomplete Concerted Evolution of nrDNA ITS in Cornus subg. Syncarpea (Cornaceae)

摘要:

四照花亚属(Cornus subg. Syncarpea)隶属于山茱萸科山茱萸属(Cornus),我国该亚属共有5种8亚种。为探讨四照花亚属nrDNA ITS序列的致同进化不完全现象及假基因产生的可能原因,分析了该亚属4种(每种1～2个居群)共21个个体的nrDNA ITS序列。结果表明,这些类群的nrDNA ITS存在多态性,通过分析这些nrDNA ITS克隆序列的G+C含量、5.8S保守基序和二级结构最小自由能,推测其可能存在假基因。系统发育研究结果显示所有nrDNA ITS序列分成5个分支,同一个体的不同拷贝被分别置于两个甚至多个分支中,且不同分支显示了不同种间关系。四照花亚属物种个体内部存在nrDNA ITS不完全致同进化,可能归咎于不完全的世系分选(incomplete lineage sorting)、种间杂交或多倍化等进化事件,从而导致基因组内nrITS区序列出现多态性,同时也导致难以通过外部形态来划分亚属内种间界限。

Abstract:

The subgenus Syncarpea, which belongs to genus of Cornus (Cornaceae), includes five species and eight subspecies in China. nrDNA ITS sequences of 21 individuals from 4 species, with 1 to 2 populations for each species, in the subgenus were amplified and used to test the existence of incomplete concerted evolution and pseudogenes. Intra-genomic polymorphism was detected in the internal transcribed spacer (including ITS1, 5.8S and ITS2) regions of these species. The analyses of G+C content, 5.8S conserved motifs and minimum free energy of 5.8S secondary structures suggested that there were putative pseudogenes of nrDNA in these species. Phylogenetic analyses of the sequences resulted in five clades with variants from the same accession commonly being placed in two or more clades and species were grouped differently among clades. These results demonstrated the existence of incomplete concerted evolution of ITS within individuals of Syncarpea, which could be explained by incomplete lineage sorting, possible hybridization or polyploidy and resulted in the polymorphism of ITS and the difficulty in morphology-based species delineation in the subgenus.