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means the macronuclear anlagen developing from these division products of the synkaryon can range from one up to 15. This is complicated by the fact that variable numbers of these division products may degenerate without development (Raikov, 1972). There is really insufficient breadth of analysis to draw any firm conclusions of patterns, if indeed there are any, in relation to the subclasses of oligo- hymenophoreans . Autogamy results from the fusion in one cell of the haploid meiotic products of the maturation divison of the micronucleus (Corliss, 1952; Diller, 1936). The progeny are thus homozygous, and this has been advantageous for the genetic explo- ration of Paramecium in that mutations can be brought to full expression by inducing autogamy . Tetrahymena thermophila cannot be induced to autogamy . However, geneticists can now achieve homozygosity in this species by matings with so- called star strains , for example, strain A * . During this process, called genomic exclusion , the star strain loses its micronucleus during meiosis . A migratory gametic nucleus is transferred to this star strain partner, after which both partners, now isogenic, become diploid by endoreduplication (Allen, 1967; Bruns, 1986). Conjugation and/or autogamy are now consid- ered crucial to the continued existence of strains of ciliates. Sonneborn (1954) originally showed their importance, demonstrating that periodic bouts of autogamy in a so-called \u201cMethuselah\u201d strain of Paramecium biaurelia extended its clonal life. Later the same phenomenon was demonstrated for Tetrahymena species (Corliss, 1965). Without these sexual processes, senescence sets in at from 200\u2013 350 cell divisions in members of the Paramecium \u201caurelia\u201d complex and up to 1,500 cell divisions in Tetrahymena (Takagi, 1988, 1999). A single known exception is the amicronucleate Tetrahymena pyri- formis , which has remained in culture for over 60 years: while it is \u201cgenetically dead\u201d, it is so-far physiologically immortal (Nanney, 1974). A variety of features indicates cells have entered senescence , among others: unequal distribution of macronu- clear DNA at cytokinesis , a decreased viability of progeny after conjugation, a decreased ability to form food vacuoles, and a decreased fission rate (Smith-Sonneborn, 1981; Takagi, 1988). Clonal life span is undoubtedly under genetic control as mutants with variations in the clonal life cycle have been discovered (Komori, Sato, Harumoto, & Takagi, 2005; Takagi, Suzuki, & Shimada, 1987). Environmental factors can influence longevity , including UV and other forms of ionizing radiation (Smith-Sonneborn, 1981). Conjugation is rarely observed in natural pop- ulations of oligohymenophoreans (Lucchesi & Santangelo, 2004). However, populations of Paramecium and Tetrahymena can be dominated by immature individuals, suggesting that sex may be quite frequent in nature (Doerder, Gates, Eberhardt, & Arslanyolu, 1995; Kosaka, 1991b), although a population dominated by senile individuals has also been discovered (Kosaka, 1994). Sonneborn (1957) also related breeding systems of Paramecium to characteristics of the life history of the species. He proposed an inbreeding- outbreeding continuum : extreme inbreeders would have two mating types, a short period of imma- turity, high fission rates, and local distributions, while extreme outbreeders would have the oppo- site set of characters (reviewed by Landis, 1986; Nyberg, 1988). At that time, some Paramecium \u201caurelia\u201d species represented the extreme inbreed- ers while Paramecium \u201cbursaria\u201d species repre- sented extreme outbreeders . Tetrahymena species with their multiple mating types would be con- sidered relative outbreeders . While this has been an attractive thesis, Nyberg (1988) concluded that there is contradictory data to refute it. Paramecium \u201cbursaria\u201d species, supposed extreme outbreed- ers , appear to be restricted in their geographic distributions while some Paramecium \u201caurelia\u201d species, typical inbreeders , are globally distrib- uted. Furthermore, Nyberg (1981b) demonstrated that continental geographic distances did not reduce the fertility of several Tetrahymena species while Przybos (1995) has demonstrated that North American and European isolates of \u201cinbreeding\u201d Paramecium triaurelia are not genetically isolated. Nevertheless, our ideas may be refined in the future as more molecular data accumulate. Stoeck, Przybos, and Schmidt (1998) have shown, using RAPD fingerprinting, that European populations of Paramecium sexaurelia , an extreme inbreeder , are more genetically isolated than populations of Paramecium triaurelia , a moderate inbreeder , consistent with Sonneborn\u2019s predictions. Stoeck et al. (2000a) have also used this approach to char- acterize P. novaurelia as a moderate inbreeder and P. pentaurelia as a weak inbreeder . Sonneborn (1957) provided evidence that the \u201cgenetic species\u201d of the Paramecium \u201caurelia\u201d complex were identical to the sibling species of the fruit fly Drosophila . Nevertheless, because of the relatively onerous task of operationally identifying a species of Paramecium \u201caurelia\u201d , Sonneborn (1957) was reluctant to name them as taxonomic species and instead chose to place them in syn- gens ( syn , Gr = same, gens , Gr = kind). A similar situation was soon discovered for the Tetrahymena \u201cpyriformis\u201d species complex (Elliott, 1973b; Gruchy, 1955; Nanney, 1980). While analyses of cortical patterns suggested that some species of tetrahymenine hymenostomes might be sepa- rated morphologically (Cho, 1971; Nanney, 1966, 1968), multivariate morphometric analyses finally demonstrated that four species of the P. \u201caurelia\u201d complex could be separated but others could not (Gates & Berger, 1976b; Powelson et al., 1975). The discoveries of isozyme variation among spe- cies of Paramecium by Tait (1970) and Allen, Byrne, and Cronkite (1971) and Tetrahymena (Allen & Weremiuk, 1971; Borden et al., 1973a, 1973b) were to provide an easy operational method to distinguish \u201cgenetic species\u201d. These results lead Sonneborn (1975) and Nanney and McCoy (1976) to establish nominate species for the syngens of Paramecium \u201caurelia\u201d and Tetrahymena \u201cpyri- formis\u201d . DNA fingerprinting is now being used to distinguish species of Paramecium (Skotarczak et al., 2004; Stoeck et al., 1998), and to demonstrate that other morphological species of Paramecium , such as Paramecium duboscqui , are probably also species complexes (Fokin et al., 1999). While it had been difficult to morphologi- cally resolve free-living species of Paramecium and Tetrahymena , morphological variability even among populations of symbiotic species has been well established. For example, mobiline peritrichs on breeding carp showed statistically significant seasonable variability (Kazubski & Migala, 1968); scuticociliate endosymbionts of sea urchins (Lynn & Berger, 1972, 1973) and bivalves (Berger & Hatzidimitriou, 1978) showed statistically signifi- cant variation on a number of traits between host populations; and apostome symbionts showed sig- nificant variation among host crustaceans (Landers, Zimlich, & Coate, 1999). These variations are likely due to a combination of factors, including invasion of the host by one to a few founders and adaptive responses to differing host environments (Berger & Hatzidimitriou, 1978). This dramatic morphological variation is contrasted with genetic uniformity in some symbionts from around the world: isolates of Orchitophrya stellarum have apparently identical nuclear genotypes in different starfish hosts from around the world (Goggin & Murphy, 2000). Whether this holds for cytoplasmic genes, such as those from mitochondria, awaits future research. 15.7 Other Features Oligohymenophoreans