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ates are united by the position of a well-defined oral 
cavity with a paroral and three oral polykinetids , 
called membranelles , although some included taxa 
are astomatous (Kozloff, 1954). The ophryoglenids 
share the organelle of Lieberkühn as the synapo-
morphy for the group (Canella & Rocchi-Canella, 
1964, 1976; Corliss; Lynn, Fromback, Ewing, 
& Kocan, 1991b). Ichthyophthirioides may have 
secondarily lost this organelle. We include the 
families Ichthyophthiriidae and Ophryoglenidae in 
this order. The tetrahymenids lack the organelle of 
Lieberkühn and do not demonstrate any synapo-
morphies at the morphological level. However, 
sequences of the SSUrRNA gene strongly separate 
the two orders and confirm their monophyly (Lynn 
& Strüder-Kypke, 2005; Miao et al., 2004b; Wright 
& Lynn, 1995). We confirm the placement of the 
Family Turaniellidae in the Order Tetrahymenida 
(Corliss, 1979) based on the similarities of its 
oral ultrastructure with that of the tetrahymenids 
(Lynn & Didier, 1978). The family now includes 
Colpidium because of similarities in division 
morphogenesis and ultrastructure (Iftode, Fryd-
Versavel, & Lynn, 1984). In addition to this family, 
the Order Tetrahymenida includes the following five 
families: the Curimostomatidae , the Glaucomidae , 
the Spirozonidae , the Tetrahymenidae , and the 
 Trichospiridae . 
 The tetrahymenids , and particularly the genus 
Tetrahymena , have been the focus of much sys-
tematic research. Corliss and Daggett (1983) 
reviewed the taxonomic and nomenclatural 
aspects of research on the “Tetrahymena pyri-
formis ” complex , noting that there is still a spe-
cies Tetrahymena pyriformis . Nanney and McCoy 
(1976) restricted this name to an amicronucleate 
form on the basis of isoenzyme features, and 
characterized 13 other species in the pyriformis
 complex , three of them amicronucleates and ten 
of them bona fide biological species, formerly 
called syngens . They relied heavily on isoenzyme 
variation among these taxa previously demon-
strated as distinct by Allen and Weremiuk (1971), 
Borden, Whitt, and Nanney (1973a, 1973b), and 
Borden, Miller, Whitt, and Nanney (1977). This 
biochemical characterization of species had been 
necessary because, as with the “Paramecium 
aurelia” complex , there is strong conservation 
of morphological form among Tetrahymena spe-
cies (Gates & Berger, 1976a; Nanney, Chen, 
& Meyer, 1978; Nanney, Cooper, Simon, & 
Whitt, 1980a). Multivariate techniques , however, 
have been successful at discriminating some 
strains (Gates & Berger, 1974). New species 
continue to be described based on combina-
tions of mating-type reactivity and isoenzyme 
patterns (Nanney et al., 1980a; Nyberg, 1981a;
Simon, Meyer, & Preparata, 1985). In addition 
to enzyme proteins, Tetrahymena species have 
been shown to vary in both ribosomal protein 
patterns (Cuny, Milet, & Hayes, 1979), surface 
proteins (Williams, Van Bell, & Newlon, 1980), 
and cytoskeletal proteins (Vaudaux, Williams, 
Frankel, & Vaudaux, 1977; Williams, Buhse, & 
Smith, 1984; Williams, Honts, & Dress, 1992). 
Williams et al. (1984) described a new species, 
Tetrahymena leucophrys in part based on the 
 cytoskeletal protein pattern. Williams (1984) drew 
attention to the conspicuous disjunction between 
morphological and molecular variation among 
“Tetrahymena pyriformis ” species: species are 
typically impossible to distinguish morphologi-
cally but demonstrate vast differences in cytoskel-
etal protein patterns. Meyer and Nanney (1987) 
concluded in their review of the isozyme approach 
to Tetrahymena that these molecules may be most 
useful in the future to analyze evolutionary proc-
esses, while systematic approaches will rely more 
heavily on nucleic acid sequences. 
 Allen and Li (1974) began the DNA approach to 
Tetrahymena taxonomy using DNA-DNA hybridi-
zations , and demonstrated deep divergences among 
species. Van Bell (1985), using sequences of 5 S 
and 5.8 S rRNA genes, showed that the latter gene 
had one nucleotide change between two species. 
Sogin, Ingold, Karlok, Nielsen, & Engberg (1986a) 
used complete sequences of the SSUrRNA gene to 
demonstrate that most Tetrahymena species could 
be distinguished from each other while Morin 
and Cech (1988) demonstrated that mitochondrial 
 large subunit (LSU) rRNA genes revealed similar 
phylogenetic relationships. Nanney, Meyer, Simon, 
and Preparata (1989) and Preparata et al. (1989) 
demonstrated congruence in the topologies of 
phylogenetic trees for evolution of Tetrahymena
species derived from nuclear 5 S, 5.8 S, SSU-, and 
LSUrRNA genes, which also broadly confirmed 
the major clusters based on isozyme variation. 
These major clusters – the so-called australis and 
borealis clades – were also confirmed by sequences 
of the amino-terminal portion of the histone H4 
gene (Sadler & Brunk, 1992) and telomerase RNA 
(Ye & Romero, 2002). What these phylogenetic 
trees thus clearly refuted was the assignment 
of the Tetrahymena species into three classical 
complexes, the “pyriformis” complex , “patula”
 complex of microstome-macrostome forms, and 
“rostrata ” complex of histophages (Corliss, 1970, 
1972c). Rather, it now appears that these three 
complexes represent similar life history strategies 
that have evolved by convergence (Nanney, Park, 
Preparata, & Simon, 1998; Strüder-Kypke, Wright, 
Jerome, & Lynn, 2001). Since true biological spe-
cies of Tetrahymena are known to have identical 
SSUrRNA genes, taxonomists have used sequence 
differences to identify new species within the genus 
(Jerome, Simon, & Lynn, 1996; Lynn, Gransden, 
Wright, & Josephson, 2000). Jerome and Lynn 
(1996) provided a riboprinting strategy to identify 
those species whose sequences were not identical, 
but this leaves us unable to differentiate several 
species. Brunk, Lee, Tran, and Li (2003) have now 
embarked on a program to completely sequence 
the mitochondrial genomes of tetrahymenines , and 
have demonstrated homology of the entire organel-
lar genomes of T. thermophila and T. pyriformis . 
Comparison of some mitochondrial genes sug-
gested greater divergences among species than was 
found with nuclear genes. Barcode sequencing of 
the cytochrome c oxidase subunit 1 (cox-1) gene 
has not only differentiated those species identical 
15.1 Taxonomic Structure 285
based on nuclear rRNA genes (Lynn & Strüder-
Kypke, 2006), but was shown to be a powerful 
identification tool (Chantangsi et al., 2007). 
 McCoy (1974b) has used the isoenzyme approach 
to examine “species” of Colpidium , and con-
firmed broadly earlier conclusions of morpholo-
gists (Jankowski, 1967b). Foissner and Schiffmann 
(1978) provided a slightly different vision of evolu-
tion within this genus, which remains to be tested 
by a molecular approach. Variation among strains 
of Ichthyophthirius multifiliis has also been dem-
onstrated in the surface antigens (Dickerson, Clark, 
& Leff, 1993) and the amino-terminal third of 
 histone H3 and H4 genes (Van Den Bussche, 
Hoofer, Drew, & Ewing, 2000). Whether this indi-
cates that this fish parasite is also a species com-
plex remains to be determined by future research. 
 The apostomes were placed by Corliss (1979) as 
an order in the Class KINETOFRAGMINOPHORA . 
However, Small and Lynn (1981, 1985) considered 
the similarities in the somatic kinetid of apos-
tomes and the general features of their life cycle 
to demonstrate oligohymenophorean affinities, and 
so established them as the Subclass Apostomatia 
in the Class OLIGOHYMENOPHOREA . This 
subclass can be characterized by several synapo-
morphies, including the presence of a rosette open-
ing and a polymorphic life cycle , often including 
 palintomy within cysts. If the rosette is absent, we 
assume that it has been secondarily lost. Jankowski

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