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.