Spirostomum teres, Claparede & Lachmann, 1858

Chi, Yong, Duan, Lili, Luo, Xiaotian, Cheng, Ting, Warren, Alan, Huang, Jie & Chen, Xiangrui, 2020, A new contribution to the taxonomy and molecular phylogeny of three, well-known freshwater species of the ciliate genus Spirostomum (Protozoa: Ciliophora: Heterotrichea), Zoological Journal of the Linnean Society 189 (1), pp. 158-177 : 173-175

COMMENTS ON SPIROSTOMUM TERES

As a model organism in ecological studies, this well-known species has been reported on numerous occasions ( Hobbs et al., 1983; Twagilimana et al., 1998; Madoni, 2000). The Qingdao population of S. teres closely resembles other populations with respect to its body size and shape, the number of somatic kineties, macronucleus shape and the arrangement of cortical granules ( Table 3). Furthermore, it clusters with other populations of S. teres in the SSU rDNA tree ( Fig. 8 ). Its identity is, therefore, not in doubt. Only one other species of Spirostomum , namely S. caudatum , has an

Abbreviations: BL, body length; BW, body width; CG, cortical granules; CV, contractile vacuole; NA, not available; OL, oral length; SK, somatic kineties. References: [1] Boscaro et al. (2014); [2] Fernandes et al. (2016) (identified following in vivo observation and protargol staining); [3] Schmidt et al. (2007) (clonal cultures, identificied following in vivo observation and histological staining methods); [4] Shazib et al. (2014) (clonal cultures, identified following in vivo observation and protargol staining); [5] present work; [6] ( Shazib et al., 2016) (clonal cultures, identified following in vivo observation and protargol staining).

* data from staining cells; ** the column unit is %.

Abbreviations: BL, body length; BW, body width; CG, cortical granules; CV, contractile vacuole; NA, not available; OL, oral length; SK, somatic kineties. References: [1] Boscaro et al. (2014); [2] Schmidt et al. (2007) (clonal cultures, identified following in vivo observation and histological staining methods); [3] Shazib et al. (2016) (clonal cultures, identified following live observation and protargol staining); [4] Shazib et al. (2014) (clonal cultures, identified following in vivo observation and protargol staining); [5] Fernandes & da Silva Neto (2013); [6] Fernandes et al. (2016) (identified following in vivo observation and protargol staining); [7] present work.

* data from living cells (μ m); ** the column unit is %.

ovoidal macronucleus. However, S. teres can be clearly distinguished from S. caudatum by the shape of the posterior end of the body (bluntly rounded vs. tail-like) ( Jang et al., 2012).

There are several independent branches of S. teres in the SSU rDNA tree ( Fig. 8 ), which is consistent with previous studies ( Fernandes & da Silva Neto, 2013; Boscaro et al., 2014; Chen et al., 2017). Also, there are up to 37 unmatched nucleotides among the SSU rDNA sequences of different populations ( Fig. 9 ). Morphologically, S. teres can be easily distinguished from its congeners by having the ovoidal macronucleus and regular cylindrical body shape. Some sequences used in the phylogenetic tree unfortunately lack morphological information ( Fig. 9 ; Table 5), so it is, therefore, unclear if S. teres is a species complex.

PHYLOGENETIC ANALYSES

In the SSU rDNA tree, Spirostomum is most closely related to Anigsteinia , which is consistent with previous studies and supports the morphological classification that places these genera in the same family ( Chen et al., 2018; Lynn, 2008; Shazib et al., 2014; Yan et al., 2015, 2016). Spirostomum species are divided into two clades. One clade contains S. yagiui , S. dharwarensis , S. teres and S. caudatum , all of which have a single macronucleus, either ellipsoidal or vermiform in shape. The other clade consists of S. minus , S. semivirescens , S. ambiguum and S. subtilis , all of which have a moniliform macronucleus. This grouping pattern supports the assertion that macronuclear shape is a phylogenetically informative character for determining evolutionary relationships in the genus Spirostomum ( Boscaro et al., 2014) . Otherwise, evolutionary relationships among species of Spirostomum remain largely unresolved, because molecular data does not correspond well with the morphological information. For example, the 15 populations of S. minus are divided into two completely separate clusters ( Fig. 8 ), but we find almost no morphological differences among these populations ( Table 4). Populations of S. teres are likewise split into several different subclades. These findings are consistent with those of previous studies suggesting the existence of cryptic species within S. minus and S. teres ( Fernandes & da Silva Neto, 2013; Boscaro et al., 2014).

In order to gain a better understanding of phylogenetic relationships within the genus Spirostomum , detailed morphological information is required for all species, in particular the number of adoral membranelles and the structure of the paroral membrane. In addition, the majority of deeper nodes in the present SSU rDNA tree are not resolved, thus analyses based on multiple gene markers will be needed to address this.

Figures

Figure 8. Maximum likelihood (ML) tree based on SSU rDNA sequences of heterotrichous ciliates. Members of the class Karyorelictea are the outgroup taxa. Numbers near branches show the bootstrap values from ML and posterior probabilities of BI. Disagreements between ML and BI are shown by asterisks. The species newly sequenced in this study are shown in red bold font. The scale bar corresponds to 2 substitutions per 100 nucleotide positions.

Figure 9. Nucleotide differences among populations of Spirostomum minus, S. subtilis, S. teres and unidentified Spirostomum species based on SSU rDNA sequences. The numbers in the header indicate the unmatched site positions. The species newly sequenced in this study are shown in red. The red and blue boxes indicate clades I and II in Figure 8, respectively. Sites not available are shown by asterisks.