Beyond first impressions: a new oxytrichid genus (Alveolata, Ciliophora, Hypotrichia) with stylonychine morphology and non-stylonychine ontogeny reveals a complex evolutionary identity

Abstract

Within the family Oxytrichidae, two major lineages are usually distinguished: the rigid-bodied Stylonychinae and the flexible, non-stylonychine oxytrichids. Members of each lineage typically exhibit constant, homologous morphological and ontogenetic traits. During a survey of freshwater, terrestrial, and semiterrestrial habitats in South Korea, we discovered a new oxytrichid species displaying a unique combination of features characteristic of both lineages. Morphologically, the species possesses a semi-rigid body and transverse cirri arranged in two distinct groups, a combination of traits restricted to members of Stylonychinae. Ontogenetically, however, it exhibits a defining feature of non-stylonychine Oxytrichidae: participation of the postoral ventral cirrus V/3 in primordia formation. Phylogenetic analyses based on single-gene and multi-gene datasets revealed an inconsistent placement of the new species. It clustered within the non-stylonychine Oxytrichidae in the 18S rRNA gene trees, within Stylonychinae in the 28S rRNA gene trees, and at the base of the Stylonychinae clade in the trees inferred from the ITS1–5.8S rRNA–ITS2 and concatenated three-gene datasets. Topology testing based on the 18S rRNA gene did not reject its inclusion in Stylonychinae. The combination of stylonychine-like morphology, non-stylonychine oxytrichid ontogeny, and an ambiguous but informative phylogenetic signal supports the establishment of a new genus, Rigidooxytricha gen. nov., which likely represents an intermediate evolutionary lineage bridging the stylonychine and non-stylonychine oxytrichid clades. This finding offers valuable insights into character evolution and phylogenetic relationships within the family Oxytrichidae.

Keywords

Character evolution / Integrative taxonomy / ITS2 secondary structure / Multi-gene phylogeny / Ontogenesis / Rigidooxytricha gen. nov.

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