Tetrahymena australis (Protozoa, Ciliophora): A Well‐Known But “Non‐Existing” Taxon – Consideration of Its Identification, Definition and Systematic Position

Abstract

A cryptic species of the Tetrahymena pyriformis complex, Tetrahymena australis, has been known for a long time but never properly diagnosed based on taxonomic methods. The species name is thus invalid according to the International Code of Zoological Nomenclature. Recently, a population isolated from a freshwater lake in Wuhan, China was investigated using live observations, silver staining methods and gene sequence data. This organism can be separated from other described species of the T. pyriformis complex by its relatively small body size, the number of somatic kineties and differences in sequences of two genes, namely the small subunit ribosomal RNA (SSU rRNA) and the mitochondrial cytochrome c oxidase subunit I (cox1). We compared the SSU rRNA gene sequences of all available Tetrahymena species to reveal the nucleotide differences within this genus. The sequence of the Wuhan population is identical to two sequences of a previously isolated strain of T. australis (ATCC #30831). Phylogenetic analyses indicate that these three sequences (X56167, M98015, KT334373) cluster with Tetrahymena shanghaiensis (EF070256) in a polytomy. However, sequence divergence of the cox1 gene between the Wuhan population and another strain of T. australis (ATCC #30271) is 1.4%, suggesting that these may represent different subspecies.A wide variety of protists, including hymenostome ciliates, have relatively simple body structures with few morphological characters for species circumscription, which makes them difficult to separate from each other (Borden et al. 1977;Simon et al. 2008). Early taxonomists identified and classified such organisms based only on morphological and cell cycle characters. As a consequence, independent species were sometimes misidentified as morphologically similar species within a morphospecies complex (for instance, Paramecium aurelia). The development of modern methods, especially gene sequencing and phylogenetic analyses, has improved this situation and the taxonomic positions of several such species have been clarified in recent years (Quintela-Alonso et al. 2013;Zhao et al. 2016).Cryptic species of ciliates were first discovered by Sonneborn (1939) for the model organism P. aurelia Ehrenberg, 1838. Cryptic species have since been found in a variety of ciliate genera including Aspidisca, Colpidium, Euplotes, Glaucoma, Oxytricha, Paramecium, Pseudourostyla, Stylonychia, Tetrahymena, Tokophrya, and Uronychia (Dini and Nyberg 1993;Przybos 1986;Simon et al. 2008). The probable wide occurrence of cryptic speciation has been cited as evidence for the underestimation of ciliate species diversity (Foissner et al. 2007).Tetrahymena pyriformis (Ehrenberg, 1830) Lwoff, 1947 is another example now known to be a species complex (Nanney and McCoy 1976). Species of Tetrahymena were previously assigned to the genera Leucophyrs or Glaucoma until the establishment of the genus Tetrahymena by Furgason (1940). Using mating type tests, Gruchy (1955) first discovered the heterogeneity of T. pyriformis by describin...

Redescription of a Hymenostome Ciliate, Tetrahymena setosa (Protozoa, Ciliophora) Notes on its Molecular Phylogeny

“…; Liu et al. ), it can be easily distinguished by the presence of a caudal cilium (vs. absent in T. australis , T . shanghaiensis and T .…”

“…; Liu et al. ; Nanney and McCoy ; Strüder‐Kypke et al. ); (2) concerning the upper “ pyriformis ‐ setosa ‐clustering” group in the red frame, T. setosa (AF364041) is probably in fact T. pyriformis , since Strüder‐Kypke et al.…”

Barcodes Reveal 48 New Species ofTetrahymena,Dexiostoma, andGlaucoma: Phylogeny, Ecology, and Biogeography of New and Established Species

“…The barcodes also were used to identify wild isolates as T. australis in a re‐description of this species (Liu et al. ). The few exceptions to barcoding success appear to be due to mislabeling or misidentification of certain stock strains, a situation partially remedied here and discussed in more detail below.…”

Molecular Phylogeny and Taxonomy of a New Freshwater Hymenostomatid from Northeastern China, with the Establishment of a New Genus Anteglaucoma gen. n. (Protista, Ciliophora, Oligohymenophorea)

“…( P – R ) Tetrahymena australis (from Liu et al. ,b). M1–3, membranelles 1, 2 and 3; PK , postoral kineties; PM , paroral membrane; x, X‐body.…”

“…; Corliss , ; Frankel ; Liu et al. ; McCoy ). Despite these problems, research on hymenostomatids is comparatively advanced with more than half of the genera recognized by Jankowski () and Lynn () investigated or reinvestigated with modern methods (Chen et al.…”