Phylogeny of the Stichotrichia (Ciliophora; Spirotrichea) Reconstructed with Nuclear Small Subunit rRNA Gene Sequences: Discrepancies and Accordances with Morphological Data

Abstract

The Stichotrichia, known as an especially various and taxonomically difficult group, were intensely studied with morphological, morphogenetic, and molecular methods in the last years. Nevertheless, a consistent classification is lacking and several important questions about the phylogenetic relationships within this group remain unsolved. In order to gain deeper insights into these relationships, the nuclear small subunit rRNA genes of seven species of the Stichotrichia, representatives of the families Oxytrichidae, Amphisiellidae, and Pseudourostylidae, were phylogenetically analysed. Although our analyses resulted in a poor resolution of the phylogenetic relationships, some conclusions can be drawn. Firstly, following the current classification systems the Oxytrichidae as well as their subfamilies seem to be paraphyletic and the basic 18 FVT cirral pattern has been modified several times independently. Secondly, sequence analyses of several Oxytricha species resulted in a high molecular diversity, which does not support monophyly of this genus. Thirdly, several families of the order Urostylida (Urostylidae, Pseudokeronopsidae, and Pseudourostylidae) also do not form monophyletic groups.

Comparative phylogenetic study of Stichotrichia (Alveolata: Ciliophora: Spirotrichea) based on 18S-rDNA sequences

“…In our trees, both sequences of this species formed a monophylum, but with <50% bootstrap support, compatible with the study by Foissner and Stoeck (2006). The close relationship of Cyrtohymena, the Onychodromopsis flexilis population from Salzburg, Oxytricha ferruginea [Cyrtohymena ferruginea sensu Foissner (1989)] and Paraurostyla weissei was also similar to what was reported in a previous study (Schmidt et al, 2007).…”

“…Engelmanniella mobilis and Oxytricha lanceolata consistently formed a monophylum in our trees, agreeing with the Bayesian tree in Schmidt et al (2007), but with bootstrap values close to 50% and branching at highly discrepant points. In the MPC, they formed a clade with Halteria grandinella, which in both trees branched off from different, but nearby points in relation to the monophyletic group formed by the Antartic population of Onychodromopsis flexilis, Oxytricha granulifera 2 and Oxytricha longigranulosa (both equal after aligned and trimmed), and Rigidothrix goiseri.…”

“…This group is often recovered in molecular phylogenies (e.g., Bernhard et al, 2001;Hewitt et al, 2003;Foissner et al, 2004;Schmidt et al, 2007;Shao et al, 2007;Yi et al, 2008a,b) and so they were in both our trees, however, with discrepant placement of Steinia sphagnicola and Sterkiella histriomuscorum. Nevertheless, stylonychine external affinities did not follow the same pattern in both trees.…”

“…Strict consensus of both topologies (Figure 3) did not depict a monophyletic Stichotrichia due to the aforementioned position of Pseudoamphisiella spp and because the Holosticha spp + Oxytricha saltans group was included in the core Stichotrichia only in the MPC. The position of Holosticha outside a major lineage of stichotrichs was already described by Schmidt et al (2007); however, they did not include euplotid hypotrichs in their analyses, so the branching T.S. Paiva et al pattern was slightly different.…”

“…The most remarkable are: i) the placement of Halteria grandinella among the flexible-body oxytrichids (Bernhard et al, 2001;Strüder-Kypke and Lynn, 2003), referred to as the "Halteria paradox" (see Foissner et al, 2007); ii) the separation of Uroleptus from the urostylids (Croft et al, 2003;Hewitt et al, 2003;Foissner et al, 2004;Stoeck, 2006, 2008), which led to the "Convergent Evolution of Urostylids and Uroleptids" (CEUU) hypothesis, and the polyphyly of the non-uroleptid urostylids, suggested in subsequent studies (Schmidt et al, 2007;Yi et al, 2008a,b); iii) the polyphyly of Oxytricha (Schmidt et al, 2007), and iv) the splitting of the Stichotrichia, which were polyphyletic in Yi et al (2008b).…”