Protofontonia, Wang & Lu & Sun & Li & Zhao & Ni & Fan, 2025

DISCUSSION

The new lineage in the order Peniculida

In previous studies, all the peniculid sequences were divided into seven major lineages in phylogeny analyses, corresponding to the four monophyletic families: Lembadionidae , Paranassulidae , Stokesiidae , and Parameciidae , and the three distantly related Frontoniidae clades (Clade I, II, and III in the present study) ( Zhao et al. 2016, Xu et al. 2018, Fokin 2019, Serra et al. 2021, Sun et al. 2021). The present study increases the species’ sampling in phylogenetic analyses and confirms all the previous reported lineages, However, a new clade ( Frontoniidae Clade IV), comprising the soil species F. depressa , is discovered in the phylogenetic analyses using three nuclear sequences ( SSU rDNA, LSU rDNA, and ITS1- 5.8S rDNA-ITS2) separately and the concatenated sequences as well as using the COI gene. Although the new clade branches early among all the Frontonia - like peniculids, its phylogenetic position is not fully resolved. The AU-test rejects the hypotheses of its close relationship with Frontoniidae clades I and III and four clades clustering together using SSU rDNA and concatenated sequences datasets; while the hypotheses that it clusters with Frontoniidae clade II, it clusters with Paranassulidae , and it clusters as an inner or outer taxon of Paranassulidae are not rejected (Supporting Information, Table S 1; Fig. S 1). Meanwhile, the sole species of the new clade, F. depressa , has been revealed to have a frontal kinety that is formed by the interruption of the rightmost ophryokinety. This feature makes it different from all the other Frontonia species ( Foissner 1987, Stout 1956; Supporting Information, Fig. S6 View Figure 6 ). Considering that Frontonia is a genus in urgent need of revision ( Serra et al. 2021), the genus Protofrontonia nov. gen. and P. depressa nov. comb. are proposed to split F. depressa from the genus Frontonia . Considering the uncertain position of Protofrontonia and its Frontonia -like morphology in general, it is placed in the family Frontoniidae as an incertae sedis for the time being. The taxonomic summary for the new genus is given as follows.

Diagnosis: Small peniculid with a Frontonia -like ciliary pattern, relatively few somatic kineties and nematodesmata associated somatic kinetids, and an incomplete ophryokinety. Single contractile vacuole. Cyst wall smooth without a papula and emergence pore. Terrestrial habitat.

Etymology: A composite of the Greek prefix ‘ proto ’ (primitive) and the generic name ‘ Frontonia ’.

Type species: Protofrontonia depressa ( Kahl 1931) nov. comb..

ZooBank registration of Protofrontonia : urn:lsid:zoobank.org:act: E1 AB4395-DA3D-405F-8AC0-5CF62F5BA273.

ZooBank registration of Protofrontonia depressa : urn:lsid: zoobank.org:act:42C83E00-84D6-44D3-AF70-E1C1 422AA6FE.