线纹舌鳎(Cynoglossus lineolatus)18S rRNA基因长片段复制:不等交换模型(unequal crossing over model)罕见的分子证据

尽管18S rRNA基因序列极其保守，但是在一些种类中仍然发现其存在序列多态性。本研究在线纹舌鳎18S rRNA基因中发现了3种差异显著的序列类型（Type A，B和C），说明其为非协同进化，而不是严格遵循协同进化方式。基于以上三种类型序列GC含量、二级结构、最小自由能等差异，我们推测Type A和B可能是假基因类型。此外，在Type A类型中还发现了一段长达189 bp的重复序列。据我们所知，如此长的重复序列，这在硬骨鱼类核糖体基因中还是首次报道。通过比较分析产生重复序列常见的模型，我们认为不等交换模型最有可能解释该重复序列的形成。本研究结果不仅为不等交换模型提供了罕见的分子证据，而且为鱼类18S rRNA基因研究奠定了很好的基础。

Long duplication of 18S ribosomal DNA in Cynoglossus lineolatus (Pleuronectiformes: Cynoglossidae): novel molecular evidence for unequal crossing over model

Although 18S rDNA sequence is extremely conservative, the polymorphism still has been found in few species. In the present study, three types (Type A, B and C) of 18S rDNA sequence coexisted in Cynoglossus lineolatus genome, suggesting a non-concerted evolution process, rather than a strictly concerted evolution fashion. Based on the differences of sequence variation, GC content, secondary structure and minimum free energy, Types A and B were speculated as the potential pseudogenes. Additionally, a fascinating finding was a 189-bp duplication of 18S rDNA in Type A sequence. To our knowledge, this is the first report on such a long duplication in teleostean ribosomal DNA. Compared with several theories accounting for the formation of tandem repeats, the unequal crossing over model was thought to be the most likely mechanism to generate the 189-bp duplication of 18S rDNA. These results not only provide a novel molecular evidence for the unequal crossing over model, but also benefit for the further study on 18S rDNA in fishes.