Psammomitra, AS THE RANK OF

Yi, Zhenzhen, Song, Weibo, Stoeck, Thorsten, Al-Rasheid, Khaled A. S., Al-Khedhairy, Abdulaziz A., Gong, Jun, Ma, Hongwei & Chen, Zigui, 2009, Phylogenetic analyses suggest that Psammomitra (Ciliophora, Urostylida) should represent an urostylid family, based on small subunit rRNA and alpha-tubulin gene sequence information, Zoological Journal of the Linnean Society 157 (2), pp. 227-236 : 233-234

SUPPORTING PSAMMOMITRA AS THE RANK OF FAMILY

The conclusion that Psammomitra should be assigned to Urostylida challenged the arrangement of Psammomitra into either Trachelostylidae ( Small & Lynn, 1985; Dini et al., 1995), Amphisiellidae ( Tuffrau & Fleury, 1994; Lynn & Small, 2002), or Oxytrichidae ( Kahl, 1932, 1935; Borror, 1972; Jankowski, 1979) by many investigators. Our SSrRNA gene sequence data ( Figs 2 , 3 ) clearly also rejected the proposal of Psammomitra being a uroleptid species as suggested by Song & Warren (1996) ( Figs 2 , 3 ), as the latter branched as a sister clade to some stichotrichids and sporadotrichids (e.g. Foissner et al., 2004; Schmidt et al., 2007), whereas the clade Holosticha – Psammomitra branched first from the urostylids at a very deep level. In addition, P. retractilis lacks caudal cirri and dorsomarginal kineties, which both are conspicuous in most uroleptid species ( Berger, 2006).

In our SSrRNA gene analyses ( Figs 2 , 3 ), an unexpected finding was that four genera in Holostichidae (sensu Berger, 2006) did not cluster together. Berger (2006) also considered that the diagnosis of Holostichidae (sensu Berger, 2006), because of the lack of apomorphies, only a combination of the most important plesiomorphies, indicated that the group is nonmonophyletic. Amongst these four genera, Pseudoamphisiella had a closer relationship with Prodiscocephalus , a species of Discocephalina, rather than with urostylids ( Figs 2 , 3 ), which is consistent with morphogenetic data ( Song, Warren & Hu, 1997) and a previous molecular phylogenetic study ( Yi et al., 2008). Figures 2 and 3 also showed that Anteholosticha grouped with other urostylids instead of the clade of Holosticha – Psammomitra , which was discrepant with the morphological information that Holosticha is more closely related to Anteholosticha than to Psammomitra . Morphologically, Psammomitra differs from the traditional ‘ Holosticha ’ (s. l.) in the following aspects ( Song & Warren, 1996): (1) the former is conspicuously contractile, tripartite in head, trunk with a long tail relative to the body shape (vs. almost noncontractile, generally nonpartite in Holosticha and Anteholosticha ); (2) midventral rows strongly shortened, i.e. extending only about half of the trunk length (vs. basically extend nearly the whole length of the body in Holosticha and Anteholosticha ); (3) the anterior membranelles are highly differentiated, i.e. forming a corona-shaped structure with several long, rigid, spine-like membranelles (vs. nonspecialized in the latter two genera), and (4) thigmotactic in behaviour (vs. never thigmotactic in the latter two genera).

In conclusion, considering the information revealed by morphological and SSrRNA gene data ( Fig. 2 ) that Psammomitra branched with Holosticha at a rather deep level, we believe that Psammomitra should be arranged in an isolated position near the Holostichidae but represent a taxon within the order Urostylida (sensu Berger, 2006) at the rank of family, i.e. Psammomitridae Jankowski, 1979 stat. nov.

SUBCLASS STICHOTRICHIA

ORDER UROSTYLIDA

Figures

Figure 2. Phylogenetic tree based on small subunit rRNA sequences showing the position of Psammomitra retractilis, by Bayesian inferences applying the GTR + G + I model. ‘-’ reflects disagreement between a method and the reference Bayesian tree at a given node. The fully supported (1.00/100%/100%) branches are marked with solid circles. Psammomitra is shaded black, and holostichids are enclosed in rectangles. Thick branches and arrows denote position of investigated species. The scale bar corresponds to five substitutions per 100 nucleotide positions. Infraciliature of Oxytricha and Uroleptus (from Foissner et al., 2004), Amphisiella (from Li et al., 2007), Trachelostyla (from Gong et al., 2006), and Holosticha (from Hu & Song, 2001) are also shown.

Figure 3. Maximum parsimony phylogeny of small subunit rRNA genes. Psammomitra is highlighted in black, and holostichids are enclosed in rectangles. Thick branches and arrows denote position of investigated species. Numbers on branches are values generated from 1000 bootstrap replicates.