Tetrahymena

• Taxonomic-nomenclatural consequences: Again, as in the case of the aurelia complex, the steps taken by Nanney and co-workers were laudable. The taxonomic situation here, however was not as directly solvable as with Paramecium. As a result, many biologists, even numerous ("non-genetical") tetrahymenologists, whose interests in these small hymenostome ciliates represent a diversity of fields, are apparently (still) confused. It was not possible to name all involved species after numbered sygens in the Tetrahymena pyriformis case. There was either inadequate information to assign a species designation to each syngen or, as in the case of syngen 6 and 8, it was dicovered that the members of separate syngenic groups could interbreed with production of viable clones of progeny. (This would cause gaps in the latinized numbering system). Slefing strains have caused additional confusion. Furthermore the long-known strains of amicronucleate tetrahymenas (often with mixed-up strain designations) have added a confounding parameter that did not exist in the case of the old "Paramecium aurelia". Finally, in the present case, the name Tetrahymena pyriformis itself has been preserved, appropriately assigned to one of the classical asexual strains or phenosets. (ref. ID; 7704)
  • Names of the species involved. Currently, there are 17 species comprising the entire Tetrahymena pyriformis complex (shortenable to the "pyriformis complex" when it clear that these Tetrahymena are the topic). Unlike the situation for the P. aurelia complex, the recognition of possibly many more may be predicted. Of the 17 described to date, four embrace solely asexual (amicronucleate) strains: 10 are the sexual "sibling" species recognized by Nanney and McCoy; and three are additional "genetic" species recently named by Nyberg (1981). (ref. ID; 7704)
    The amicronucleate problem. For many years, all members of "Tetrahymena pyriformis" (often popularly but incorrectly known as "Glaucoma pyriformis" or "Tetrahymena geleii") established in laboratory culture turned out to be strains without a macronucleus. Because of the ease with which these ciliates were culturable axenically, they early became ideal in physiological and biochemical researches of a great variety of kinds, thus rapidly surprassing Paramecium and all others as the most popular protozoa in such work. Strain designations had often been assigned to various populations isolated in one laboratory or another; it thus came as a shock to learn, in recent years, that many "pure" lines had/have become mixed contaminated, or mislabeled with free exchange and transfer between workers over thousands of tetrahymena generations. This was verified when comparison by electrophoretic mobilities, or use of other biochemical properties, was made using subcultures of cultures obtained from different laboratories, cultures that bore the same designations but turned out to be quite different? On occasion, worse still, cultures that carried entirely different labels were shown to be the same strain. Such corrective investigations have resulted in recognition of five phenotypic sets (phenosets), replacing the now unreliable clusters of old strain designations, of "amic" tetrahymenas (Bordern et al. 1973; McCoy 1975; Nielsen and Andronis 1975). Four of these "amic" phenosets, viz., A, B, C, and E, were recognized as separate species by Nanney and McCoy (1976). The name Tetrahymena pyriformis (Ehrenberg, 1830) Lwoff, 1947 -see discussion of its complicated nomenclatural history in Corliss (1953) and Corliss and Dougherty (1967)- was arbitrarily retained to include especially the celebrated "GL" strains of Andre Lwoff, isolated and established in axenic culture in 1922 and maintained in various laboratories ever since; it is a member of the phenoset called "A" by Borden et al. (1973). The strain "E" used by Corliss (1971) in his deposition in the U.S. National Museum of neotype material for T. pyriformis is also in phenoset A, so no provision of the International Code of Zoological Nomenclature has been broken. T. pyriformis remains the type-species of the genus Tetrahymena Furgason, 1940. But notice that, since 1976, this species can no longer be identified simply by possession of the morphological characteristics that, in the main, are also found in many other members of the entire pyriformis complex: it must also show the properties that separate phenoset A from four other phenosets and from some 15 micronucleated syngens. The three other species of "amics", the ones named as new by Nanney and McCoy (1976), are : T. elliotti (=phenoset B); T. furgasoni (=phenoset C); and T. lwoffi (=phenoset E). Since nothing can be known about restricted or unrestricted gene flow in populations that cannot breed, Nanney (in Nanney and McCoy 1976) has defined an asexual species as "a population with approximately the same amount of molecular (genetic) heterogeneity as a sexual species" and one that is "discontinuous in that heterogeneity from other sexual and asexual species". (ref. ID; 7704)
  • The "genetic" species. The 10 sibling species recognized and formally named by Nanney and McCoy (1976), from among the dozen syngens known at that time, are as follows: T. americanis, T. australis, T. borealis, T. canadensis, T. capricornis, T. cosmopolitanis, T. hyperangularis, T. pigmentosa, T. thermophila, T. tropicalis. The most extensively studied species in genetic research to date has been T. thermophila, under its former designation of variety or syngen 1 of "T. pyriformis" sensu lato (Sonneborn 1974). Its newer name is based on its tolerance for high temperatures (e.g., up to 41 degrees C). The specific names of the other species reflect either some unusual character or the geographical area in which the organisms have been found. Tetrahymena americanis is the most commonly collected species in North America, and to date it has never been found elsewhere in the world. Recently, Nyberg (1981) has described what he considers to be three additonal"biological" species assignable to the pyriformis complex. Two were new mating groups collected from the wild that would have been syngens 13 and 14 if that had been the only system in vogue in 1981. The other was old syngen 5, found by Nyberg in natural populations, which Nanney and McCoy (1976) had cautiously declined to name as new. These three most recent pyriformis complex species, formally named by Nyberg in his 1981 paper, are: T. hegewischi, T. nipissingi, and T. sonneborni. (ref. ID; 7704)
    [Characteristics used to separate species]: The widespread existence of amicronucleate ("genetically dead") strains, not to mention selfers, has confounded the problem. Enzyme analysis has become a powerful tool; but, for the sexual forms, breeding tests are still held to be of great importance too (e.g., see Nyberg 1981). Cryopreserved strains are, again, available (see below).
  • Isozyme patterns. Once again, the electrophoretic mobilities of the isozymes represent an excellent and convenient method for discrimination of individual species among the 17 Tetrahymena species currently comparising the pyriformis complex. The early work (Allen and Weremiuk 1971; Borden et al. 1973), on both asexual and sexual strains, set the stage for more recent studies (e.g. Borden et al. 1977; Nanney, Cooper et al. 1980; Nyberg 1981) that, in the main, have confirmed the data ammassed and the conclusions drawn by such pioneers as Bordern. Also SDS polyacrylamide gel electrophoresis of cytoskeletal proteins has supported some preceived relationships based on isoenzyme analysis and suggested that closer examination of other relationships may result in some taxonomic changes (Vaudaux et al. 1977; Williams 1983; Williams and Buhse 1983). Nanney, Cooper et al. (1980) raise some important points with respect to possible problems associated with "perfect" reproducibility of isozyme analysis results: choice, and source, of enzyme systems: factors of enzyme polymorphism; preference of isozyme used as the standard mobility reference; wide molecular divergence of the strains under comparative study; number of enzymes surveyed; and even the technical and procedural methodologies employed by different laboratories and/or different personnel. Details of procedures can be neither given nor discussed here. Techniques, as well as the result and data obtained, must be studied firsthand in the papers cited above by any new worker wishing to compare his or her fresh results, on either known or unknown strains, with findings in the earlier literature. Having authenticated reference strains now available on request (e.g., from ATCC) is of inestimable value to such new investigators: they should take full advantage of it. (ref. ID; 7704)
  • Morphological features. In general, morphological features of the various members of the pyriformis complex are too similar, or too variable both within and among species, to serve reliably in diagnosis of a given species or of a population of unknown tetrahymenas collected from the wild. One of the most interesting corticotypic characters in Tetrahymena, however, but one requiring good silver impregnation of multiple specimens, is its set of contractile vacuole pores, and their position may come to be quite useful in species determination (McCoy 1975; Nanney 1967, 1971; Nanney, Nyberg et al. 1980). The CVP positions are given as percentages of the distance around the organism from the first kinety (ciliary meridian) to the mean location of the pores themselves. The figures on such measurements are remarkably constant in some populations (e.g., see Frankel 1972). But, to date, they have been considered risky to use for species separation, especially for amicronucleate members of the complex (Nanney and McCoy 1976). Differences in body size have seemed valuable in the past in separation of some of the classical "amic" strains from others (Corliss 1953; Loefer 1952); but such a feature is not recommended for species discrimination among the current membership of the whole pyriformis complex. Use of combinations or of a "constellation" of morphological characters has also been suggested by the workers just cited (Corliss 1973; Loefer 1967), but such a recommendation was/is insufficient for application to the present overall problem. The total number of kinetosomes are tetrahymenid ciliate at a particular time in the cell cycle, under controlled laboratory growth conditions, may represent another feature of comparative value in species identification. However, the ranges in numbers overlap for certain (former) syngens of the pyriformis complex (Nanney et al. 1978; Nanney and Chow 1974), making this character one of questionable reliability. As in the case of Paramecium, and for ciliates in general, morphogenetic and ultrastructural features are of limited taxonomic value at the lower levels of classification (Corliss 1979, 1980), particularly at the sibling-species level. However, there is a vast and continually growing literature on Tetrahymena in areas of morphologenesis and ultrastructure, with many papers emphasizing both "form" and "function". Conceivably, some day some data on cortical patterns (as in the case of the contractile vacuole pores) and on the fine structure of the kinetid may prove to be worthwhile in identification of cryptic species. (ref. ID; 7704)
  • Ecological and biogeographical characteristics. While multiple strains of certain species within the pyriformis complex have been found within relatively restricted areas of the world (Elliott 1970, 1973; McCoy 1975; Nyberg 1981), deliberate attempts to make an exhaustive search of any region by systematic collecting have not been carried out. Concerning the kinds of fresh-water niches preferred, our information to date is also fragmentary. In short, our knowledge of the natural distribution of species of the T. pyriformis complex is even worse than it is for members of the P. aurelia complex. It is doubtful if collection site can ever be of broad value in taxonomically differentiating such cryptic ciliate species, but the carrying out of extensive collections would still be worthwhile for biogeographical reasons. (ref. ID; 7704)
  • Other features of possible use. Certain molecular approaches to taxonomy are more feasible for species of Tetrahymena than for Paramecium: for example, determination of DNA base ratios and nucleic acid hydridization experimentation. Comparative studies on the G+C content of species of the pyriformis complex have indeed been carried out (see Allen and Gibson 1973); and Nanney and McCoy (1976) included such data in their "chief characteristics" chart. Allen and Li (1974) have done some nucleotide sequencing work. Their results are important in revealing the large evolutionary discontinuities among species of the complex (Nanney 1983), but the approach would be neither so convenient nor feasible to apply in routine taxonomic separation of groups at the species level. Immunological procedures, important for other reasons in their own right (in ciliate genetics, etc.), also appear not to be suitable for taxonomic use in the case of species belonging to the pyriformis complex (Loefer and Scherbaum 1963). Immobilization antigens in Tetrahymena show an unsuitably large degree of genetic polymorphism (Nanney and McCoy 1976). Although all "sexual" species of the complex appear (or are assumed) to have a haploid number of five micronuclear chromosomes, idiogramming them at anaphase (Tiedtke 1982) might reveal interesting differences of value in the taxonomy of these tetrahymenas. Various physiological and biochemical traits (generally studied for non-taxonomic reasons) have been found to differ among certain pyriformis strains (Hill 1972; Holz et al. 1959; Levy 1973; Loefer 1967; McCashland and Johnson 1957; Nyberg 1974). But the data from such investigations, to date, are not generally applicable to the problem of recognizing the singular uniqueness of each species in the complex. In the recent work by Nyberg (1981), however, a much large variance in temperature tolerance was found among these species than in any of the wild stocks within a species: this represented a "physiological distinctness" of considerable potential taxonomic value at the species-level. Yet there was overlap in possible pairwise comparisons among the species, as Nyberg admits; and he states, further, his belief that "knowing the temperature tolerance of stock is not sufficient information for (its) unambiguous identification". Life cycle characteristics (in the genetic sense) are of possible diagnostic utility, but most of them need further study and extension to include the more "neglected" species before their potential value can be realized. Of course, as in the case of the P. aurelia species, "fruitful mating" remains an ideal - albeit sometimes difficult phenomenon to bring about, occasionally unreliable, and, of course, impossible with limited material - criterion for species identication within the sexual or syngenic moiety of the T. pyriformis complex. Mating type determination data (Sonneborn 1974) are useful but incomplete. Such additional attributes as the length of period of sexual immaturity, behavior of heterozygotes during vegetative growth, and behavior of the macronucleus during conjugation are still too little known to be of predictable taxonomic value, as Nanney and McCoy (1976) acknowledge. (ref. ID; 7704)
• Availability of Strains: For authenticated material, as in the case of paramecium stocks, the two repositories most trustworthy are the American Type Culture Collection in Maryland, and the Culture Centre of Algae and Protozoa in Cambridge, England. A complete set of the members of the pyriformis complex is available only from the ATCC; in fact, the cryogenically preserved material there includes not only the 17 named tetrahymenas of this complex but also strains of phenoset D and of syngens 6 and 8 (the last two at present combined under the name of the much studied species, T. pigmentosa). Still additional groups and strains of the pyriformis complex, ones not yet formally described as separate species, are included among ATCC's holdings. A warning to researchers not to obtain "pedigreed" strains from sources outside the two collections cited is made in earnest, if one wishes to be absolutely certain of having a reliable reference strain for some precise investigation. Over the years, most unfortunately, inadvertent "mix-ups" have occurred in handling cultures, particularly of the early, now classical, amicronucleate strains: cultures have become contaminated (e.g., with a faster growing strain) and/or strain labels have been lost or interchanged, as already mentioned briefly above. To add to the confusion, cultures, bacterized or axenic, have been generously exchanged back and forth between laboratories, with no way of knowing where, by whom, or when mix-up may have occurred, so that tracing the "true" history of a given strain has now generally become completely impossible. To add to the dilemmna, in a few cases cultures have been known to have been replaced with fresh collections from the wild that were then assumed by the collector to be identical with the "old" strain!. (ref. ID; 7704)
• Proposed practical solutions: As suggested in various of the preceding paragraphs, the problem of proper identification of species assignable to the (still growing) T. pyriformis complex is greater than it is in the case of the P. aurelia ciliates for at least these reasons: the pyriformis organisms are more heterogenous genetically, their breeding data are not always as reliable, some strains are amicronucleate, some are persistent selfers, and finally their very diversity and more widespread distribution requires more extensive study to insure accuracy in differentiating species within the overall group (Nanney 1983). Yet, since tetrahymenas are used in considerably greater number of kinds of biological studies than paramecia, they more often need -and on behalf of a greater variety of workers- to be correctly recongized taxonomically. (ref. ID; 7704)
  • For field biologists. Hydrobiogists or survey-ecologists of fresh-water habitats are bound to encounter tetrahymenas, small (one-third length of a paramecium and thus of much less volume) and relatively undistinguished though these ciliates are. They may be either amicronucleate or (potentially) sexual strains: this ought to be determined first, if other general characteristics have already suggested that the organism belongs to the pyriformis complex. Either phase microscopy should be employed on living material or, preferably, specimens should be brought to a laboratory, cultured on a simple proteose-peptone medium (e.g., 1% or 2% with the antibiotic streptomycin, 500 ug/liter, added to keep down bacterial growth), and fixed and impregnated with silver by one of several possible techniques (see references in Corliss 1973, 1979). Unless an electrophoretic study can be carried out on the organism to prove the fact, it should not be identified as "Tetrahymena pyriformis (Ehrenberg, 1830) Lwoff, 1947, or named by any one of the earlier junior synonyms, such as "T. geleii", "Glaucoma pyriformis", or "G. pyriformis". If the ciliate is amicronucleate, perhaps the best that can be done today (unless electrophoretic data are obtained) is to call it "Tetrahymena sp.", with a footnote that it appears to be an amicronucleate member of the T. pyriformis complex. However, see our Addendum, in which the attractive concept of a "supraspecies" (proposed by Genermont and Lamote 1980) is discussed: using that alternative solution, one could label his or her material as 'Tetrahymena suprasp. pyriformis". If electrophoresis is used, then assignment to the proper phenoset (Nanney and McCoy 1976) can be made, unless the organism represents an entirely new form. If the population is micronucleated, then, with or without evidence of conjugation in the collected material, it would be best to call it, again, "Tetrahymena sp.", with the added note that presumably it is either a member of one of the already described "mating" species of the pyriformis complex or a new sibling species (but also see our Addendum, below, as mentioned in the preceding paragraph). The proper full specific name should be used, of course, if breeding analysis and/or electrophoretic evidence support(s) the choice of one of the known cryptic micronucleated species over any of the others. If the organism has been found in the hemocoel of some freshwater insect larva (sometimes adult, too), or in some other host, the problem is compounded. Amicronucleate members of the pyriformis complex adapt well to many such "parasitic" habitats; but a different already known species of Tetrahymena outside the pyriformis group may well be involved or even an altogether new species (e.g., see Batson 1983; Corliss 1960, 1961, 1972, 1973; Corliss et al. 1979; Corliss and Coats 1976; Golini and Corliss 1981; Lynn et al. 1981). Once again, as for Paramecium, standard protozoological field guides are of no aid in distinguishing the (now) real T. pyriformis from its many sibling species, micro- and amicronucleate forms alike. In fact, the protozoan ecologists' indispensable "bible", Kahl (1931), is particularly misleading since species of "Tetrahymena", a name not yet invented until 1940, are mixed with other groups, notably Glaucoma. (ref. ID; 7704)
  • For laboratory experimentalists. Exactly as advised in the case of the P. aurelia complex, workers who plan to carry out sophisticated experiments with results to be meaningful in comparison with those of other studies must use only properly identified material. If available old strains are suspect in any way or for any reason, then fresh cultures of authenticated material should be obtained from such reliable collections as the ATCC or the CCAP. If other, or newly isolated, strains are employed, they should be identified by enzyme analyses and given the appropriate designation or scientific name. If the organism does not "fit" into a known group, it might be best not to use it; but do not discard it! A culture should be sent to a recognized collection or to an experienced tetrahymenologist so that a comparative study can possibly be made to determine its probable relationship to other species in the complex. It should be noted here that electrophoretic studies have not yet been carried out on the dozen or so other micronucleated species of the whole genus Tetrahymena that are not members of the pyriformis complex (Corliss 1973, 1979). The results of such investigations, once available, may prove to be very interesting from comparative taxonomic and evolutionary points of view. (ref. ID; 7704)
  • For writeres, teachers, and editors. It is a disservice to science to knowingly perpetuate (as well as to perpetrate it in the first place) and erroneous taxonomic name in association with an organism, whether one does it in a published paper or, as a teacher, in a classroom or laboratory full of unsuspecting students. Such false biological information makes impossible the comparison of results with those of other investigators who have accurately identified their material. In the Materials and Methods sections of a paper, the exact name of the organism (including any appropriate strain or phenoset designation) should appear, an it source as well. Editors should be willing to help authors who are unsure of the exact information required. (ref. ID; 7704)
• Concluding remarks on the T. pyriformis problem: There is (still) a species of the genus Tetrahymena named T. pyriformis. But it is now (since 1976) restricted to an amicronucleate ciliate sharing enzyme electrophoretic characteristic of only phenoset A of the entire pyriformis complex. Other "amics" are similarly assignable to other phenosets (B-E), three which (B, C, E) now bear separate taxonomic names of their own. Older strain designations for long-standing cultures of amicronucleate tetrahymenas are suspect and are thus no longer reliable: do not use them. The micronucleated (sexual, syngenic) former members of Tetrahymena pyriformis sensu lato now have their own separate taxonomic names, 10 established by Nanney and McCoy (1976) and three, more recently, by Nyberg (1981). It is highly likely that still more such species await naming, following discovery and proper characterization (by breeding test and/or appropriate enzyme analysis). It is advisable, as in the case of forms beloning to the Paramecium aurelia complex, that only authenticated strains (both "amic" and sexual ones are now available in the cryopreserved state) be used in comparative research work of a sophisticated genetic or biochemical nature. The "pyriformis-like" populations of tetrahymenas found "in the wild" in field work (e.g., during ecological or hydrobiological studies) are probably best labeled only as "Tetrahymena sp." with mention of their likely association with the pyriformis complex, unless/until precise laboratory identification tests can be carried out on them. As an alternative, a "supraspecific" name could be used, as disccused in our Addendum, below. (ref. ID; 7704)
• Addendum: We are greatful to the Redacteur for permitting us to append a short section here concerning an excellent point brought to our attention by one of the reviewers of our manuscript, discussion of which however, would be difficult to incorporate directly to the pages of any preceeding sections. In effect, an alernative practical solution has been suggested for the problem of what name to use in identification of some sibling (jumelle) or syngenic species when sophisticated breeding or molecular techniques are not avaiable to the investgator (e.g., a field ecologist). The very recently proposed taxonomic-nomenclatural category of "supraspecies" -well described by its originators Genermont and Lamotte (1980), who acknowledge their indebtedness to nomenclaturist G. Bernardi- nicely links together taxonomically any group of species difficult to separate, one from the other, morphologically or ecologically. The term is not to be confused with the restrictive, little used, quite different category of "superspecies" (see Mayr 1969). Thus, instead of listing a population of organisms as "Paramecium sp." or "Tetrahymena sp.", indicating total ignorance of the precise (sibling) specific name to use and seemingly leaving only the genus as the first level of their identification, one could write: "Paramecium suprasp. aurelia" or "Tetrahymena suprasp. pyriformis". Usage of such terminology would clearly provide the reader with a "morphological type", as it were, but at an infrageneric (even infrasubgeneric) level. When/if the specific name is known, the new category could still be employed: for example, a biochemist might be working with Tetrahymena (suprasp. pyriformis) thermophila. In a way, the supraspecific category would replace the longer expressions "pyriformis complex" or "member of the pyriformis complex", the non-taxonomic notations recommended in a preceding section of this paper to field biologists, to be used as footnotes (in lists, tables, etc.) to the name Tetrahymena sp. One of the advantages of the idea is that it releases the term "complex" for informal usage at a still lower group level. Recall that Williams and Buhse (1983) have suggested that among the micronucleated members of the "T. pyriformis complex" one can recognize three well defined groups: the T. americanis complex, the T. borealis complex, and the T. thermophila complex. There appear to be very few disadvantages to this proposed practical solution, that is, using the supraspecies concept described above. Yet one, naturally, is that its very newness will, at first, make it unfamiliar and therefore possibly a little confusing to many readers. Also, the closeness of the spelling of the two words "supraspecies" and "superspecies" is unfortunate. From a taxonomic-nomenclatural point of view, however, we endorse with enthusiasm its future consideration by protologists. With respect to its usage with the two groups of ciliate sibling species under discussion in the present paper, only new serious problem arises. I is caused by the necessity (see Genermont and Lamotte 1980) of having a "nominate" species within each supraspecies; or, to put it another way, the Latin name of the supraspecies must be identical to the name of one (presumably the oldest) species included in the group. This causes no trouble in the case of Tetrahymena. T. (suprasp. pyriformis) pyriformis works very well. But for Paramecium, as discussed on earlier pages, there is no longer any taxonomic species P. aurelia and thus, legalistically, there can, at the moment, be no P. (suprasp. aurelia) aurelia! In their paper, our good French colleagues suggest rejection of Sonneborn's first binomen, P. primaurelia, replacing it with "P. aurelia", which would then allow a taxonomic-nomenclatural treatment parallel to that given for Tetrahymena above. But the matter cannot be handled so simply. Further discussion of this particular knotty and complex problem, however, is beyond the scope of the present paper. Its final proper resolution would involve, at the least, a carefully prepared lengthly pertition to the Intenational Commission on Zoological Nomenclature for future formal consideration by the commissioners. Unfortunately, at the present time, this restrict our giving complete support to the otherwsie attactive "alternative practical solution" discribed in this Addendum. (ref. ID; 7704)

1. Tetrahymena americanis (ref. ID; 65, 3882, 4005, 4035, 4147, 7115, 7390)
   Syn; Tetrahymena pyriformis syngen 2
2. Tetrahymena asiatica Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper) reported author and year? (ref. ID; 65, 7115, 7390)
   Syn; Tetrahymena pyriformis-complex
3. Tetrahymena australis (ref. ID; 65, 3882, 4005, 4035, 4147, 7115, 7390)
   Syn; Tetrahymena pyriformis syngen 11
4. Tetrahymena bergeri Roque, de Puytorac & Savoie (ref. ID; 3789) reported author and year? (ref. ID; 191, 7115)
5. Tetrahymena borealis (ref. ID; 65, 190, 3882, 4005, 4035, 7115, 7390)
   Syn; Tetrahymena pyriformis syngen 3
6. Tetrahymena canadensis (ref. ID; 65, 190, 3882, 4005, 4035, 4147, 7115, 7390)
   Syn; Tetrahymena pyriformis syngen 7
7. Tetrahymena capricornis (ref. ID; 65, 3882, 4005, 4035, 7115, 7390)
   Syn; Tetrahymena pyriformis syngen 12
8. Tetrahymena caudata Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper) reported author and year? (ref. ID; 7115)
   See; Tetrahymena patula-complex
9. Tetrahyemna chironomi (ref. ID; 7267)
10. Tetrahymena corlissi Thompson, 1955 (ref. ID; 3789) reported year? (ref. ID; 3698) reported author and year? (ref. ID; 65, 191, 7115)
11. Tetrahymena cosmopolitanis (ref. ID; 3882, 7115, 7390)
    Syn; Tetrahymena pyriformis syngen 4
12. Tetrahymena dimorpha (ref. ID; 191)
13. Tetrahymena edaphoni Foissner (ref. ID; 4842)
14. Tetrahymena elliotti Nanney & McCoy, 1976 (ref. ID; 6884) reported author and year? (ref. ID; 65, 3882, 4035), 7390
    See; Tetrahymena pyriformis-complex
15. Tetrahymena empidokyrea Jerome, Simon & Lynn, 1996 (ref. ID; 7267) reported author and year? (ref. ID; 7115)
16. Tetrahymena farleyi (ref. ID; 7115)
17. Tetrahymena furgasoni Nanney & McCoy, 1976 (ref. ID; 3983) reported author and year? (ref. ID; 65, 3882, 4035, 4075, 7390)
    Syn; Tetrahymena pyriformis strain W (ref. ID; 3977, 3983)
18. Tetrahymena hegewischi (ref. ID; 65, 154, 4005, 7115, 7390)
    Syn; Tetrahymena pyriformis syngen 5
19. Tetrahymena hyperangularis (ref. ID; 65, 190, 3882, 4005, 4035, 7115, 7390)
    Syn; Tetrahymena pyriformis syngen 10
20. Tetrahymena leucophrys Williams, Buhse Jr. & Smith, 1984 (ref. ID; 4119 original paper, 4147) reported author and year? (ref. ID; 65, 7115)
21. Tetrahymena limacis (Warren) (ref. ID; 1618, 3789) reported author and year? (ref. ID; 65, 7115)
22. Tetrahymena lwoffi (ref. ID; 3882)
23. Tetrahymena malaccensis Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper) reported author and year? (ref. ID; 65, 190, 7115, 7390)
    See; Tetrahymena pyriformis-complex
24. Tetrahymena mimbres (ref. ID; 65, 4395, 7115, 7390)
25. Tetrahymena mobilis (ref. ID; 7115)
26. Tetrahymena nanneyi Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper) reported author and year? (ref. ID; 65, 190, 4611, 7115, 7390)
    See; Tetrahymena pyriformis-complex
27. Tetrahymena nipissingi (ref. ID; 65, 154, 4005, 7115, 7390)
    Syn; Tetrahymena pyriformis syngen 14
28. Tetrahymena paravorax Corliss, 1957 (ref. ID; 7539, 7602) reported year? (ref. ID; 3828) reported author and year? (ref. ID; 65, 191, 3893, 7115)

    ---

    Tetrahymena patula-complex

    Tetrahymena caudata Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper)
29. Tetrahymena patula (ref. ID; 65, 190, 191, 7115)
30. Tetrahymena patula (Ehrenberg, 1830) Corliss, 1951 (ref. ID; 3959) reported year? (ref. ID; 1335, 1618, 2100)
    Syn; Leucophrys patula Ehrenberg (ref. ID; 1618)
31. Tetrahymena patula (Kidder) (ref. ID; 1308)
    Tetrahymena silvana Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper)

    ---

32. Tetrahymena pigmentosa (ref. ID; 65, 190, 3882, 4005, 4035, 7115, 7390)
    Syn; Tetrahymena pyriformis syngen 6, 8

    ---

    Tetrahymena pyriformis-complex

    • Tetrahymena pyriformis syngen 1=Tetrahymena thermophila (ref. ID; 3873, 3882, 3905, 4035)
    • Tetrahymena pyriformis syngen 2=Tetrahymena americanis (ref. ID; 3882, 4005, 4035, 4147)
    • Tetrahymena pyriformis syngen 3=Tetrahymena borealis (ref. ID; 3882, 4005, 4035)
    • Tetrahymena pyriformis syngen 4=Tetrahymena cosmopolitanis (ref. ID; 3882)
    • Tetrahymena pyriformis syngen 5=Tetrahymena hegewischi (ref. ID; 4005)
    • Tetrahymena pyriformis syngen 6=Tetrahymena pigmentosa (ref. ID; 3882, 4005, 4035)
    • Tetrahymena pyriformis syngen 7=Tetrahymena canadensis (ref. ID; 3882, 4005, 4035, 4147)
    • Tetrahymena pyriformis syngen 8=Tetrahymena pigmentosa (ref. ID; 3882, 4005, 4035)
    • Tetrahymena pyriformis syngen 9=Tetrahymena tropicalis (ref. ID; 3882, 4005, 4035)
    • Tetrahymena pyriformis syngen 10=Tetrahymena hyperangularis (ref. ID; 3882, 4005, 4035, 4147)
    • Tetrahymena pyriformis syngen 11=Tetrahymena australis (ref. ID; 3882, 4005, 4035, 4147)
    • Tetrahymena pyriformis syngen 12=Tetrahymena capricornis (ref. ID; 3882, 4005, 4035)
    • Tetrahymena pyriformis syngen 13=Tetrahymena sonneborni (ref. ID; 4005)
    • Tetrahymena pyriformis syngen 14=Tetrahymena nipissingi (ref. ID; 4005)
    • Tetrahymena pyriformis strain W=Tetrahymena furgasoni (ref. ID; 3977, 3983)
    • Tetrahymena asiatica Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper)
    • Tetrahymena elliotti (ref. ID; 4611)
    • Tetrahymena lwoffi (ref. ID; 4611)
    • Tetrahymena malaccensis Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper)
    • Tetrahymena nanneyi Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper)
      Syn; Leucophrys pyriformis Ehrenberg, 1830 (ref. ID; 4611); Saprophilus oviformis Kahl, 1926 (ref. ID; 4611); Tetrahymena geleii Furgason, 1940 (ref. ID; 4611)
    [ref. ID; 3882]
    The ciliated protozoa contain clusters of sibling species that are difficult to identify morphologically but that can be readily differentiated at the molecular level. Their fundamental structural similarity indicates a common origin, but the molecular diversity within some of the species clusters, and particularly within the Tetrahymena pyriformis complex, is so great as to suggest only a remote common ancestor. The unexpected contrast between the results of molecular and morphological analyses of this ciliate complex raises significant evolutionary and taxonomic questions. (ref. ID; 3882)
    [ref. ID; 4147]
    Elliott and coworkers collected strains of the Tetrahymena pyriformis complex from diverse geographic areas (Elliott 1973) and demonstrated that 12 "biological species" were included. When the study of isozymes (Borden, Miller, Whitt & Nanney 1977) permitted differentiation of these groups without resorting to tester strains, they were accorded specific binominal designations (Nanney & McCoy 1976). (ref. ID; 4147)
    [ref. ID; 5923]
    Morphological changes during the growth cycle of axenic and monoxenic. (ref. ID; 5923)
    [ref. ID; 7390]
    It is difficult to distinguish species in the Tetrahymena pyriformis complex even when multivariate analyses are applied (Gates and Berger 1974, 1976) because the species are morphologically similar. In fact, the ciliate literature is plagued by misidentification of ciliates from the genus Tetrahymena (for reviews see (Corliss and Coats 1976; Meyer and Nanney 1987). Species in the T. pyriformis complex are known to exhibit differences in temperature tolerance (reviewed in Nyberg 1981), rRNA arrangement (Sogin et al. 1986), types of mating inheritance, and other life cycle characteristics (Meyer and Nanney 1987). The riboprinting strategy (PCR/RFLP) provides an alternative means to mating experiments and isoenzyme analysis for identifying and distinguishing species in the T. pyriformis complex. (ref. ID; 7390)

33. Tetrahymena pyriformis (Ehrenberg) (ref. ID; 3698, 5462), (Ehrenberg) Lwoff (ref. ID; 1219, 1335, 1618, 1629, 2100, 2245) or (Ehrenberg) Lwoff, 1947 (ref. ID; 3560) reported author and year? (ref. ID; 65, 190, 191, 5923, 7115, 7390, 7699)
    Syn; Glaucoma pyriformis (Ehrenberg) Schewiakoff (ref. ID; 1219); Tetrahymena geleii Furgason (ref. ID; 1618)

    ---

    Tetrahymena rostrata-complex

    [ref. ID; 3789]
    As a member of the rostrata complex, T. corlissi is most closely associated with T. rostrata and T. bergeri. The 3 organisms share with the other members of the rostrata complex as ovoid micronucleus and a histophagous or parasitic habit. Moreover, by their possession of a caudal cilium and a basically similar life cycle, these 3 are distinguished from T. limacis and T. stegomyiae. This life cycle essentially is summarized as follows: (a) histophagous and/or parasitic trophont; (b) dividing, free-swimming or encysted (i.e. tomont) trophont; (c) active theront; (d) if food is present, back to trophont; (e) if food is absent, a resting stage; (f) from resting stage and in the presence of food, back to active theront and then to trophont. The tomont is not an obligate stage in any of these species. Tetrahymena bergeri is not known to produce a resting cyst and possibly lacks a resting stage. The resting cysts of T. corlissi and T. rostrata have different shapes. The sturdy resting cyst of the latter is apparently formed more readily and by a greater proportion of the population. (ref. ID; 3789)

34. Tetrahymena rostrata (Kahl) (ref. ID; 1618, 3789) or (Kahl, 1926) Corliss, 1952 (ref. ID; 4861) reported author and year? (ref. ID; 65, 7115)

    ---

35. Tetrahyemna rotunda (ref. ID; 7267)
36. Tetrahymena setifera (ref. ID; 191, 646, 3638, 4108)
    See; Tetrahymena setosa (ref. ID; 3638, 4108)
37. Tetrahymena setosa (ref. ID; 65, 3638, 4108, 7115)
    Syn; Tetrahymena setifera (ref. ID; 3638, 4108)
38. Tetrahymena shanghaiensis (ref. ID; 7115)
39. Tetrahymena sialidos (ref. ID; 7267)
40. Tetrahymena silvana Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper) reported author and year? (ref. ID; 65, 7115)
    See; Tetrahymena patula-complex
41. Tetrahymena sonneborni (ref. ID; 65, 154, 4005, 7115, 7390)
    Syn; Tetrahymena pyriformis syngen 13
42. Tetrahymena stegomyiae (Keilin) (ref. ID; 3789)
43. Tetrahymena thermophila (ref. ID; 65, 190, 191, 3648, 3894, 3980, 3981, 7115, 7390, 7667)
    Syn; Tetrahymena pyriformis syngen 1 (ref. ID; 3873, 3882, 3894, 3905, 3924)
44. Tetrahymena tropicalis (ref. ID; 65, 190, 3882, 4005, 4035, 7115, 7390)
    Syn; Tetrahymena pyriformis syngen 9
45. Tetrahymena vorax Kidder, 1941 (ref. ID; 4123, 4210, 4217), (Kidder et al., 1940) Kidder, 1941 (ref. ID; 388) or (Kidder, Lilly & Claff) (ref. ID; 1618) reported author and year? (ref. ID; 65, 191, 7115, 7612)
    Syn; Glaucoma vorax Kidder, Lilly & Claff (ref. ID; 1618)

Tetrahymena americanis (ref. ID; 65, 3882, 4005, 4035, 4147, 7115, 7390)

Synonym

Descriptions

Tetrahymena asiatica Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper) reported author and year? (ref. ID; 65, 7115, 7390)

Synonym

Descriptions

Tetrahymena australis (ref. ID; 65, 3882, 4005, 4035, 4147, 7115, 7390)

Synonym

Descriptions

Tetrahymena bergeri Roque, de Puytorac & Savoie (ref. ID; 3789) reported author and year? (ref. ID; 191, 7115)

Descriptions

Tetrahymena borealis (ref. ID; 65, 190, 3882, 4005, 4035, 7115, 7390)

Synonym

Descriptions

Tetrahymena canadensis (ref. ID; 65, 190, 3882, 4005, 4035, 4147, 7115, 7390)

Synonym

Description

Tetrahymena capricornis (ref. ID; 65, 3882, 4005, 4035, 7115, 7390)

Synonym

Descriptions

Tetrahymena caudata Simon, Meyer & Preparata, 1985 (ref. ID; 4147 original paper) reported author and year? (ref. ID; 7115)

See

Descriptions

Tetrahymena corlissi Thompson, 1955 (ref. ID; 3789) reported year? (ref. ID; 3698) reported author and year? (ref. ID; 65, 191, 7115)

Descriptions

Tetrahymena cosmopolitanis (ref. ID; 3882, 7115, 7390)

Synonym

Descriptions

Tetrahymena elliotti Nanney & McCoy, 1976 (ref. ID; 6884) reported author and year? (ref. ID; 65, 3882, 4035, 7390)

Remarks

Tetrahymena empidokyrea Jerome, Simon & Lynn, 1996 (ref. ID; 7267) reported author and year? (ref. ID; 7115)

Descriptions