Rubrioxytricha, Berger, 1999

Kouser, Farzana, Liao, Lijian & Hu, Xiaozhong, 2024, Morphology and molecular phylogeny of a Chinese population of Rubrioxytricha guamensis Kumar et al., 2018 (Ciliophora: Hypotrichia), Journal of Natural History 58 (21 - 24), pp. 688-701 : 694

Phylogenetic position of genus Rubrioxytricha

With the newly sequenced Rubrioxytricha guamensis included in the phylogenetic analysis, the monophyly of the genus Rubrioxytricha is corroborated, which is consistent with previous studies ( Chen et al. 2015, 2017; Kumar et al. 2018a). However, Luo et al. (2021) argued for the paraphyly of the genus because of the nesting of Pseudogastrostyla flava and Polystichotrix monilata , although it was not statistically supported by the ML (59) analysis. In the current study, the sister relationship between these two species and the genus Rubrioxytricha is fully supported (ML/BI, 100%/1.00). It can be explained by the high level of morphological similarity among them: they all have a flexible body, one marginal row on each side of the body, and brightly coloured cortical granules. However, Rubrioxytricha differs significantly from Polystichothrix and Pseudogastrostyla by having a typical oxytrichid pattern of 18 FVT cirri (vs more than 18 FVT cirri in short lines) ( Fan et al. 2015; Luo et al. 2017, 2021). Furthermore, in terms of dorsal ciliature, the Rubrioxytricha –Polystichothrix– Pseudogastrostyla clade demonstrates great variation: (1) Pseudogastrostyla shares four DK with R. guamensis , R. haematoplasma and R. guangzhouensis ; (2) Polystichothrix shares five DK with R. ferruginea and R. indica ; (3) six DK are present in R. tsinlingensis ; (4) Pseudogastrostyla shares one caudal cirrus with R. guamensis , R. haematoplasma , R. indica and R. guangzhouensis ; (5) one or two caudal cirri occur in R. ferruginea and three caudal cirri are present in R. tsinlingensis ; (6) Polystichothrix lacks caudal cirri ( Berger 1999; Naqvi et al. 2006; Chen et al. 2015; Fan et al. 2015; Luo et al. 2017; Kumar et al. 2018a). The branch consisting of species within the genus Rubrioxytricha and Polystichothrix– Pseudogastrostyla is closely related to Ponturostyla enigmatica and Pseudocyrtohymena koreana as shown in the previous reports ( Chen et al. 2017; Kumar et al. 2018a; Luo et al. 2021). Morphologically, Ponturostyla differs significantly from the genus Rubrioxytricha in: (1) having multiple fragmentations (vs absence) of DK 3; (2) more than one (vs one) left and right marginal rows; and (3) absence (vs presence) of caudal cirri ( Song 2001). Pseudocyrtohymena differs from the genus Rubrioxytricha in: (1) the arrangement of the undulating membranes (in Cyrtohymena vs Australocirrus pattern); and (2) the slightly higher number of adoral membranelles (38–53 vs 21–48) ( Jung et al. 2015). Thus, the close grouping in phylogenetic analyses might be attributed to the lack of molecular sequences of related species. A detailed investigation of the morphology and the gene sequences of related and yet-tobe-discovered species will clarify further the phylogenetic positions of this group of ciliates.