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Long duplication of 18S ribosomal DNA in Cynoglossus lineolatus (Pleuronectiformes: Cynoglossidae): novel molecular evidence for unequal crossing over model
GONG Li, SHI Wei, YANG Min, SI Lizhen, KONG Xiaoyu
2016, 35(12): 38-50. doi: 10.1007/s13131-016-0957-z
Keywords: nrDNA, Cynoglossus lineolatus, tandem repeat, pseudogene, polymorphism, non-concerted evolution
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.