Integrative Analyses Reveal Sigmohymena gen. nov. and the Ninth Pattern of Undulating Membranes in Hypotrichia (Protista, Ciliophora): Insights From Polygenic Analysis on Undulating Membranes Evolution

Hypotrichs, a highly specialised group of ciliates, display extraordinary morphological complexity. A novel hypotrichous ciliate, Sigmohymena sinica gen. nov., spec. nov., collected from China, was investigated based on morphology, morphogenesis, and molecular phylogeny. Sigmohymena gen. nov. is characterised by the following features: undulating membranes in the Sigmohymena pattern; the presence of a short amphisiellid median cirral row, buccal cirrus, cirrus III/2, and transverse cirri; the absence of postperistomial cirri, caudal cirri, and cirri to the left of the anterior portion of the amphisiellid median cirral row (originating from anlage IV); one right and one left marginal row; three dorsal kineties. We provide new sequences of Sigmohymena sinica gen. nov., spec. nov. and three other species to conduct phylogenetic analyses using both single‐gene and multi‐gene datasets, aiming to clarify evolutionary relationships among related species. The results support the assignment of Sigmohymena gen. nov. within the family Amphisiellidae. The phylogenetic topology is basically supported by the features of undulating membranes. Additionally, due to the unique characteristic of the undulating membranes of the species, the Sigmohymena pattern is revealed and defined as follows: the paroral curves rightwards and then upwards distally, while the endoral is relatively short and positioned at the level of the posterior portion of the paroral.