Phylogenetic analysis and taxonomic distinction of six genera of pathogenic scuticociliates (Protozoa, Ciliophora) inferred from small-subunit rRNA gene sequences.

Pathogenic scuticociliates, which are common in the haemolymph or tissues of maricultured animals and often cause serious diseases, are a species-rich assemblage with mostly unresolved systematic relationships, especially in some less-studied groups. In the present study, we sequenced the small-subunit rRNA gene of six species of scuticociliates, Uronemella parafilificum, Metanophrys sinensis, Parauronema longum, Cohnilembus verminus, Porpostoma notata and Ancistrum crassum, the last two of which have not been studied previously using molecular analyses. Phylogenetic trees were constructed using Bayesian inference, maximum-likelihood and maximum-parsimony methods to assess the inter- and intra-generic relationships of scuticociliates. Results revealed the following: 1) Porpostoma did not cluster with the Philasteridae, Cohnilembidae or any other family of the order Philasterida; 2) sequences of Uronemella parafilificum and Uronemella filificum showed a difference of 1.02% (15 nt sites), revealing a close relationship between them; 3) the approximately unbiased test rejected monophyly of both Metanophrys and Parauronema, indicating that the terminal position of the anterior end of the paroral membrane and the structure of membranelle 1 are unreliable characters for distinction of genera in this group of scuticociliates; 4) Ancistrum crassum grouped with Boveria subcylindrica, showing a close phylogenetic relationship between the orders Thigmotrichida and Pleuronematida; and 5) Parauronema longum, Cyclidium plouneouri and Cyclidium porcatum should be removed from their currently assigned genera.