Living morphology and molecular phylogeny of oligohymenophorean ciliates associated with freshwater turbellarians

Three freshwater turbellarian species (Dugesia gonocephala, Girardia tigrina, and Polycelis felina), belonging to the order Tricladida, were examined for the presence of ciliates. Living morphology and phylogenetic position of the isolated ciliates were studied using light microscopy and molecular phylogenetic methods. Three ciliate species, all from the highly diverse class Oligohymenophorea, were detected: Haptophrya planariarum from the subclass Astomatia, Urceolaria mitra from the subclass Peritrichia, and Tetrahymena sp. from the subclass Hymenostomatia. Each of these ciliates is specialized for different parts of the turbellarian bodies: H. planariarum lives in the pharynx and rami of the intestine, U. mitra colonizes the body surface, and Tetrahymena sp. attacks open wounds and feeds on the mesenchyme. Astomes and peritrichs isolated from turbellarians are placed deeper in 18S rRNA gene phylogenies than their relatives isolated from annelids and mollusks. On the other hand, Tetrahymena sp. isolated from turbellarians is classified comparatively deeply within the family Tetrahymenidae, suggesting that the phylogeny of tetrahymenids does not correlate with that of their obligate/facultative host groups. Nevertheless, the reconstruction of ancestral traits corroborated the hypothesis that histophagy was already a life history trait of the progenitor of the subclass Hymenostomatia to which Tetrahymena belongs.