The neotypification of Frontonia vernalis (Ehrenberg, 1833) Ehrenberg, 1838 and the description of Frontonia paravernalis sp. nov. trigger a critical revision of frontoniid systematics

Abstract

Background Among Oligohymenophorea (Ciliophora, Alveolata) the subclass Peniculia stands as one of the most well-known groups. Frontonia is the largest genus of Peniculia, and its representatives are spread in any type of water bodies as well as in soil. At a first glance, Frontonia species exhibit an overall similar morphology, and form a well-recognizable taxon of ciliates. Despite the general morphological homogeneity, the phylogenetic analysis based on the 18S rDNA sequencing showed that Frontonia is a non-monophyletic group. The systematics of this genus should be deeply reviewed, although additional issues complicate the task solving. First, type species of the genus is not yet clearly established, and no type material is available. In this context, the situation of F. vernalis, one of the first Frontonia ever described, is somehow puzzled: the description of this ciliate made by Ehrenberg (in 1833 and 1838) contains several inaccuracies and subsequent misidentifications by other authors occurred. Moreover, the 18S rDNA sequence of a putative F. vernalis is available on GenBank, but no morphological description of the correspondent specimens is provided; thus, in our opinion, it should be only prudently associated with F. vernalis or at least indicated as “F. vernalis”. Results In the present work, we provide the neotypification of F. vernalis newly found in Italy, presenting its multidisciplinary description and its neotype material. Similarly, we describe a novel species bearing Chlorella-like endosymbionts, Frontonia paravernalis sp. nov., retrieved in two far distant locations (Italy, Russia). A critical discussion on the status of Frontonia taxonomy and phylogeny is also presented, based on the 18S rDNA sequencing of both these two newly collected species and other 14 frontoniids isolated in different parts of the world. Finally, in the present study F. leucas was neotypified and proposed as the type species of the genus. Conclusions Green frontoniids form a monophyletic clade of freshwater organisms characterized by having a single contractile vacuole and bearing intracytoplasmatic Chlorella-like symbionts. With the neotypification of F. vernalis and F. leucas a fundamental step in Frontonia systematics was taken, and the bases for further taxonomic studies were laid.

Taxonomy and molecular phylogeny of a new freshwater ciliate Frontonia apoacuminata sp. nov. (Protista, Ciliophora, Oligohymenophorea) from Qingdao, PR China

“…The polyphyly of Frontonia was recognized in 2007 by Andreoli et al [55,56]. Since then, several studies on the molecular phylogeny of this genus have confirmed this finding [10,17,25,41,54,57]. Our phylogenetic trees also show the polyphyly of Frontonia, as members of Frontonia are separated into three phylogenetically distinct groups (Fig.…”

“…nov. MW243109 differs from its nine most closely related Frontonia sequences by 17-151 nucleotides, with the sequence identity ranging from 90.8% to 98.9 % (Table 2). The two most closely related sequences are F. atra MT040844 and F. minuta MT040846, both of which were submitted by Serra et al [17]. However, they differ from the F. apoacuminata sequence by 17 and 18 nucleotides, respectively, which corresponds to a sequence identity of 98.9 % (Fig.…”

“…Frontonia is one of the most diverse genera in the order Peniculida Fauré-Fremiet in Corliss, 1956, and morphological criteria for species identification include patterns of the oral and somatic ciliature, characteristics of the contractile vacuole, and features of the micronuclei [3,7,[9][10][11]17]. In addition, phylogenetic analyses using small subunit ribosomal RNA (SSU rRNA) gene sequences are widely used for clarifying the systematics of peniculids [18][19][20].…”

“…The continuous reports of new Frontonia species in the past two decades suggest that there is a significant undiscovered diversity necessitating further investigations of this genus, especially in rarely sampled habitats. More than 50 nominal species have been reported worldwide [6][7][8][9][10][11][12][13][14][15][16][17]. Many species have been briefly described with simple outline sketches and lack crucial diagnostic information, such as fine details of the ciliary pattern, that is now regarded as essential for ciliate species circumscription and identification.…”

Ciliated protozoa from industrial WWTP activated sludge: a biodiversity survey including trophic interactions and redescription ofBakuella subtropica(Spirotrichea, Hypotrichia) according to Next Generation Taxonomy

“…Moreover, an attempt was also made for understanding the influence of oxygen concentration level on ciliate community in the specific WWTP condition by separately collecting samples from the nitrification tank (which is regarded as an oxic environment) and the denitrification tank (which is considered to be an anoxic environment). Finally, among the various retrieved ciliate species, Bakuella subtropica was redescribed based on the recently proposed taxonomic standard approach, i.e., the next generation taxonomy (NGTax) (Serra et al, 2020), and a critical revision of the genus was conducted, in line with previous papers from our group (e.g., Serra et al, 2021a;Serra et al, 2021b),with a proposal of some taxa synonymization.…”