Phylogenetic study on three Metacineta species gives a new look at subclass Suctoria systematics

Suctoria is a subclass of ciliates (Phyllopharyngea, Suctoria) distinguished by the presence of a specialized feeding apparatus called tentacles and the absence of ciliature during most of their life cycle. This group of ciliates, particularly the order Exogenida, is poorly studied from a molecular phylogenetic perspective, with only a dozen small subunit (SSU) rRNA gene sequences available in public databases. In this study, we examined three species of suctorians from the genus Metacineta, formerly belonging to the order Exogenida. Using scanning electron microscopy and differential interference contrast microscopy, the examined species were redescribed and their SSU rRNA gene sequences were obtained. The morphological study refined the structural features of the calyx in representatives of the genus and a hypothetical scheme of calyx evolution was proposed based on morphological evidence and new molecular data. In addition, the application of an ultrastructural approach allowed clarification of the calyx structure in Metacineta alata, which possesses unique crests, instead of the typical clefts found in the majority of congeners. Our phylogenetic analysis showed that Metacineta is nested within the order Endogenida instead of clustering with the exogenid members. This new finding raises a question about the validity of the current classification system of the subclass Suctoria based on budding patterns and suggests necessity for a comprehensive reevaluation.