Multi-gene-based investigation on the molecular phylogeny of the hypotrichous family Strongylidiidae (Protista, Ciliophora), with notes on the ontogeny of a new genus and new species

Ciliates in the subclass Hypotrichia have long been difficult to classify as they are one of the most polymorphic and highly differentiated groups, leading to their systematics remaining unresolved. Phylogenetic relationships within the hypotrich family Strongylidiidae have been ambiguous due to discordance between the morphological and genetic data. In this study, a new strongylidiid genus Heterouroleptus is established, mainly based on the novel mode of origin of the ventral cirral rows: left ventral cirral row (LVR) originates from frontal–ventral–transverse cirral anlagen (FVTA) Ⅲ (anterior portion), Ⅳ (middle portion), and Ⅴ (rear portion); right ventral cirral row comes from the entire FVTA Ⅵ. A new species, Heterouroleptus weishanensis gen. nov., sp. nov., is investigated along with the morphometric and molecular data from a population of Strongylidium wuhanense. Eight new sequences and nuclear gene markers (single-gene and multi-gene) are provided to analyze the phylogenetic relationships of strongylidiids, with the COI gene utilized to uncover further genetic information at species level and below. The results reveal that: (1) Strongylidiidae is monophyletic and has a close relationship with Dorsomarginalia; (2) Heterouroleptus gen. nov. forms a clade that is sister to all the other strongylidiids; (3) Hemiamphisiella Foissner, 1988 and Pseudouroleptus Hemberger, 1985 should not be synonyms, and both genera should be subdivided due to their variable morphological characteristics; (4) LVR originating from three anlagen is a plesiomorphy of Strongylidiidae. The discovery of the origin of the LVR not only contributes to the establishment of the genus Heterouroleptus, but also helps to improve the diagnosis of the family Strongylidiidae.