基于18S rDNA遗传距离与GC含量对游走类纤毛虫的分子系统学研究

为了揭示游走类纤毛虫的系统发生,对寄生于淡水鱼类的车轮虫科中的6种车轮虫进行了18S rDNA的测序并获得了9个序列。采用了最大似然法(ML)与贝叶斯法(BI)对GenBank中所有游走类纤毛虫的18S rDNA序列进行了系统树的构建,并首次将SPSS与18S rDNA遗传距离结合分析了游走类纤毛虫的系统发生。研究结果进一步证实了车轮虫属(Trichodina)的非单系发生与小车轮虫属 (Trichodinella) 的有效性。此外,研究结合18S rDNA 的GC含量与遗传距离分析提出了游走类纤毛虫科属及种间新的鉴定依据: 18S rDNA 的GC含量可用于游走类纤毛虫的科属区分,且与游走类纤毛虫的分化密切相关; 18S rDNA的遗传距离在游走类纤毛虫的不同阶元中具有一定的阈值范围,即通常种内遗传距离阈值范围为0.000-0.005,属种间阈值范围为0.005-0.150,当遗传距离大于0.150时,则达到了科间水平。

MOLECULAR PHYLOGENETIC EVIDENCES ON MOBILIDA BASED ON GENETIC DISTANCE AND GC CONTENT OF 18S rDNA USING BROAD TAXON SAMPLING

In order to uncover the real phylogeny of Mobilida ciliates, we sequenced six Trichodinidae species from freshwater fish hosts and acquired nine 18S rDNA sequences. A phylogeny of all Mobilida species with known 18S rDNA sequences was estimated by the maximum likelihood and Bayesian analyses. To our best knowledge, their genetic distance inferred from 18S rDNA was analyzed statistically by using SPSS for the first time. The results further confirmed the non-monophyly of the genus Trichodina and validity of the genus Trichodinella. Additionally, the present study provided 1) new diagnostic evidences combining GC content and genetic distance inferred from 18S rDNA data for family-genus or intraspecific identification of mobilid ciliates and 2) suggested that GC content of 18S rDNA could help to distinguish taxa within Mobi-lida at family or genus level and was closely related to the divergence of mobilid ciliates. The thresholds of genetic distances inferred from 18S rDNA data for different taxa were proposed to be 0.000-0.005 for intraspecific level, 0.005-0.15 for genus-species levels and higher than 0.15 for family level.