| Tetrahymena in the Real Word |
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Tetrahymena Biogeography
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| Table of Contents A. The Nature of Ciliate Species B. The Evolutionary Dispersion of Tetrahymena Species C. The Collections of Tetrahymena Strains D. The World Wide Distribution of Species - Table 1 - Global Sites E. The North American Distribution of Common Species - Table 2 - North American Collecting Sites |
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A. The Nature of Ciliate Species Bland Finlay and colleagues (Finlay et al., 1996) have contended for some time, and reiterated more recently (Finlay, 2002), that “There are strong indications that protozoa and other microbial eukaryotes in general do not have biogeographies ...”. The implication of the assertion is not simply that the geographical distributions of ciliate species have not been ascertained, but that, in fact, no meaningful biogeography exists: ciliate species are cosmopolitan. In whatever part of the world a ciliate naturalist goes, the same general habitats yield the same morphologically indistinguishable species. While the cosmopolitan distribution of the major morphological types of ciliates is incontestable, the conclusion is faulted on two different bases. Wilhelm Foissner (1999), for example, claims that careful examination of cortical features in ciliates provides far more significant distinctions than are revealed in routine cytological analysis. He suspects that the slightly different morphotypes are associated with slightly different habitats. Of equal significance is the fact that the “lumping” of similar morphotypes denies the significance of the cryptic species long known in ciliates. Lumping flies in the face of fragmentary but hardly negligible evidence for genetic differences among phenotypically similar protists in different habitats and in different regions of the world. This issue may be of considerable importance in our understanding of global biodiversity, since the significance of protist biodiversity is not restricted to the naming of the ciliated protozoa, but also concerns their roles in the natural economy. Moreover, the issue, as Finlay points out, is not restricted to ciliated protozoa but comprehends the entire kingdom of eukaryotic protists. Over the last half century considerable effort has been focused on the genetic systems of the tetrahymenid ciliates, beginning with the discovery of mating types in a tetrahymena by Elliott and Hayes (1953), and the demonstration by Gruchy (1955) of multiple isolated mating systems (genetic species) among morphologically similar tetrahymena strains. As time went on, more and more genetic species were described (Elliott, 1973; Nanney and McCoy, 1976; Simon, in preparation). Controversy over how to name cryptic protist species began early, even before mating types were discovered in tetrahymena. It has continued to develop, as biologists try to understand the ecological and evolutionary significance of microbial sibling species. A first advocate of the reality and the first interpreter of the significance of cryptic ciliate species was Tracy Sonneborn, who had earlier discovered such species in another ciliate - Paramecium aurelia (Sonneborn, 1939). Though Sonneborn steadfastly contended that the genetically isolated mating systems were distinctive genetic systems, playing different roles in natural settings, he was sympathetic with the problems they raise for systematists. For this reason he proposed not to designate the siblings by Latin binomials, but to keep such formal designations for the morphospecies. He developed the neologism “syngen” to refer to a genetically isolated population under the morphospecies umbrella (Sonneborn, 1957). His argument for rejecting technical species names was that the loss of living reference strains could result in the loss of investigators’ ability to identify the species of newly acquired strains. He promised, however, to supply Latin names as soon as molecular techniques permitted a strain to be identified without the use of living standards. Eventually the sibling species of the P. aurelia complex could be distinguished by molecular techniques and Sonneborn (1975) dutifully assigned Latin species names to 14 syngens originally defined solely by mating tests. Ernst Mayr (1957), as the publicist, historian and defender of the “Modern Evolutionary Synthesis” (Provine, 1986), was incensed at Sonneborn’s refusal to apply formal taxonomic names to the paramecium species when he (Mayr) thought the appropriate time. He contended that Sonneborn had no understanding of evolution (See Schloegel, 1999). Mayr’s failure to permit microbial systems to come into the “Modern Synthesis” on their own terms, probably petrified that synthesis in the intellectual sediments of the 1940s, and prevented the merging of genetic and evolutionary theory to include the microbial mating systems that were just beginning to be probed in the 1930s and 40s. That narrow anthropic perspective not only blighted population genetics in ciliates and other eukaryotic protists, but it eventually made the “modern” synthesis obsolete, not directly applicable to the history of life before the advent of the eukaryotes (see Part III: Protists and the Origin of Species) or to the evolutionary economie still manifested in the prokaryotic world in modern times. The continuing resistance of many evolutionists to microbial and molecular contributions is exemplified in the exchange between Ernst Mayr (1998) and Carl Woese (1998). Now that global climate changes are recognized as hazardous to the health of the entire biosphere, more serious attention is being directed to a correct inventory of the species on earth (May, 1988), and to a better understanding of their ecological roles. Molecular techniques reveal much more about the denizens of the dirt and of the deeps (Pace, 1992), and demonstrate whole clades of extant protists that are yet to be seen in the laboratory (Lopez-Garcia et al. 2001; Swung Yeo Moon et al. 2001). Molecular techniques provide the basis for assessing more reliably the relationships, the molecular diversity of ciliate species and, in particular, the biogeography of the tetrahymenids. |
| B. The Evolutionary Dispersion of Tetrahymena Species The evolutionary distances among the tetrahymena species and the distances to other ciliates and other protists (Brunk et al. 1990; Preparata et al. 1989; Sogin et al. 1987; Liepe et al., 1994) have been ascertained by examining conservative nucleic sequences. The largest number of tetrahymena strains have been assessed using a single short sequence of the D2 region of the large rRNA molecule (Nanney et al., 1998). Though this shorter region was chosen for economy of effort, the results are generally comparable to those obtained with smaller numbers of species but longer and more conserved sequences. The phylogenetic tree (Fig. 1) provides the basis for assembling the species into five major groups. The T. thermophila Group consists of four species separated by a considerable distance from all the other species. Three of the species have been crossed; one is asexual and defined solely on the basis of significant differences in conserved molecules. The T. pyriformis Group consists of seven species, none of which has been successfully bred in the laboratory. Both their placement in the same group and their designation as distinct species are based on the analysis of highly conserved molecules. The T. tropicalis Group contains five molecularly distinct species for which we have little breeding information. Elliott and Hayes (1955) reported 5 mating types karyonidally inherited in T. tropicalis, and this is confirmed by Simon (in press). The T. borealis Group is an evolutionary cluster with six species, two of which have been crossed in the laboratory and manifest karyonidal inheritance. When mating tests are not available, species status is awarded on the basis of differences in rRNA sequences. Strains of the americanis Group usually retain their micronuclei under laboratory conditions and, hence, are more readily crossed in the laboratory than those in some other groups. Breeding studies reveal a large group of genetically isolated species with minimal evolutionary distances. Indeed, ten species in the americanis group show no differences in their D2 regions, even though breeding tests clearly establish their genetic separation. By analogy, strains assigned to the same species in other Groups, on the basis of molecular similarities, may need to be separated if mating tests become available. Mating type determination in the americanis species appears to be directly genetic, with a series of multiple alleles showing peck-order dominance. Figure 1. An evolutionary tree for tetrahymena species based on nucleic sequence of the D2 region of the large ribosomal RNA molecule (Nanney et al., 1998). |
| C. The Collections of Tetrahymena Strains Our information concerning the biogeography of strains of tetrahymena, and their genetic and evolutionary relationships, is based primarily on fresh water collections made by Elliott’s laboratory in Central and South America, Australia and the Pacific Islands, and to Europe (Elliott, 1973), and by the Nanney laboratory (Nyberg, 1981; Simon et al., 1985 and unpublished). They are supplemented by information from the Doerder lab (1995,1996), though somewhat different collecting methods were employed. Samples of significant strains have been deposited in the American Type Culture Collection (The Simon-Nanney Tetrahymena Collection) where they may be obtained. The Illinois laboratory is no longer able to supply strains. Despite the fact that the original Woods Hole strains of T. thermophila (the species then called “T. pyriformis”} came from a briney pond in Massachusetts (Elliott and Hayes, 1953), the tetrahymenas we are primarily concerned with here are fresh-water protists; most of the collecting sites have been lakes, streams and roadside ditches. The methods of sampling varied. Sometimes water samples were inoculated into proteose-peptone tubes and incubated with antibiotic before being examined. Sometimes samples traveled a considerable distance before they reached a laboratory. Once the samples reached the lab, single cells were isolated directly from a water sample into antibiotic/peptone or into bacterized Cerophyl (rye grass infusion). Many cells that appear to be tetrahymenas under a dissecting scope fail to yield cultures when isolated into the media provided. The fact that many of the cultures that do thrive in the laboratory have been subjected to antibiotic treatment before examination limits their use in assessing natural symbionts that they might have harbored. The fact that many laboratory cultures are found to be amicronucleate does not necessarily indicate that they lacked micronuclei in their native habitats. In the early years of collecting, when the search was primarily for breeding stocks, many strains were discarded quickly if preliminary cytological examination showed no micronuclei. Some strains remained in the lab years before being subjected to critical analysis. Some collecting sites yielded many viable isolates. Usually strains isolated from a single sample (2-5 oz.) belonged to the same species, but they often have different genetic traits (See Doerder et al. 1995,1996). On the other hand, strains of two or more different species were sometimes isolated from a single sample. As many as 4-5 species were rarely recovered from a single sample e.g. from a sandy shore of Lake Michigan or from the banks of a Malaysian river. A single site may thus occasionally appear in the records more than once. This observation alone indicates that tetrahymena cryptic species, at least in their dispersive phase, may be sympatric. The evolutionary analysis includes some strains collected in very different circumstances and these are excluded from the geographical analysis. The special habitats in which they were found gave rise to the expectations that the organisms are evolutionarily distant and deserving of generic or at least specific distinction. Ichthyoptheris multifiliis (ick), for example, is a well-known tetrahymena parasitic on fish in aquaria. Lambornella clarkii (Edgeter et al. 1986) is found parasitizing anopholine mosquitoes in tree stumps in California. Batson (1983) describes remarkable cytological behavior in a species he called T. dimorpha, collected in simulid larvae. T. chironomus was collected originally from chironomid fly larvae. Reports have been received of T. rostrata in snails in Poland. Strains from these special habitats are sometimes available in laboratory cultures and have been sequenced, but their geographic origins are of uncertain significance. Even after being established in a laboratory strain, origins and histories may become blurred. Many strains have clearly been mislabeled and their ultimate sources mistaken (Borden, Whitt and Nanney, 1973). Some doubt still exists, for example, about which of the modern lab strains labeled GL are actually derived from Andre Lwoff’s original 1923 isolation. Despite uncertainty about where some of these strains originated, and about how representative they are of natural populations, these laboratory strains, some 600 in number, provide the only data base available. The information on collected strains was often “published” in abstracts of meetings, grant applications and other out-of-the-way places. Some of it lies buried in student reports and lab notebooks. Ellen Simon has undertaken the onerous task of assembling the information from the published literature and from the Nanney laboratory records (uiuc.edu, 1959-2001) and is preparing the compiled data for a comprehensive publication. She permits us to summarize some of the major observations. |
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D. The World Wide Distribution of Species. The total number of collection sites yielding viable laboratory cultures in our analysis comes to 617 (100% See Table 1). Of these, 522 (84.6%) were in North America, including Canada, USA, Mexico , Central America and the Caribbean. Some parts of North America were much more intensely sampled than others, e.g. the Northeast as compared to the Southwest. Only 95 sites (15.4%) represent the rest of the world. Though some samples came from South America, Europe, China, Australia, Africa and the Pacific Islands, no claim can be made that these samples adequately represent the global distribution of tetrahymenids. Two species of the T. thermophila Group are common in North America. T. thermophila comes from 54 sites (including the Pennsylvania ponds studied by Doerder), as does T. elliotti. T. elliotti, however, has been recovered from 8 widely scattered sites around the world, while T. thermophila has been found only in the northeastern and northcentral parts of North America. T. malaccensis was isolated by Simon only from two sites in Malaysia. Six of the 7 species of the T. pyriformis Group have been isolated in North America, from a total of 40 sites, and three were collected from 8 sites elsewhere, in Europe and in Malaysia. Sixty North American sites, mainly in the south, yielded species of the T. tropicalis Group. The six global sites of this species included Hawaii and Thailand. Eighty North American collections yielded species of the T. borealis Group. The largest number (54) were of T. borealis, and T. canadensis was next (18). Outside North America, T. borealis was found once in Germany, and T. canadensis was found in Malaysia. The most frequently encountered group in our sample is the T. americanis Group, which contains 50% of all the characterized strains and about 7/8 of the strains collected in North America (including Central America and the Caribbean). This group contains 17 species, making it the most speciose group and generally the most genetically compact. The most common species is T. americanis americanis, recovered 107 times in North America, but only 4 times elsewhere in the world (China). The next most frequent species in this group is T. australis, collected first by Elliott (1973) in Australia. It has been collected only 47 times total, but it is the most frequently collected species (18 times) outside North America. It has been found in North America 29 times. The americanis group includes the only known case of partial mating compatability between tetrahymena species. In some combinations of mating types, conjugation has been observed between T. pigmentosa pigmentosa and T. pigmentosa europigmentosa (orginally syngen 6 and syngen 8), though viable progeny have not been recovered. The european subspecies has only been reported once in North America, while the american subspecies is primarily restricted to North America. The global distribution of the major evolutionary groups of tetrahymena shows no dramatic evidence for geographic bias. (Table 1) The geographic ratio of North American/total global collection sites) is 84.6%. The ratio for the borealis cluster is slightly higher (92%), while the ratio for the americanis cluster is a little lower (78%). The ratios for the thermophila and pyriformis clusters (89% and 83%) are near the global mean . The distribution of the common species in North America, however, shows that particular species have distinctive ranges. (Table 2). |
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