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a typical conjugation , although modi- fied by a bizarre mutual exchange of portions of macronuclei. Other apostomes engage in what has been called syndesmogamy or zygopalintomy : two trophonts encyst together, undergo synchronized palintomic divisions, and ultimately conjugate as tomites (Chatton & Lwoff, 1935a; Minkiewicz, 1912; Raikov, 1972). Conjugating endosymbionts may often be smaller than the average trophont, suggesting that they might undergo preconjugation divisions (Kazubski, 1963; Raikov, 1972). Paramecium and Tetrahymena served as the early model organisms for exploration of the genetics of the ciliates, and these efforts have been complemented and amplified by research on spirotrich genetics ( see Chapter 7 ). There is a very large literature on these two genera, representing the Subclasses Peniculia and Hymenostomatia , which we have used very selectively to touch on the characteristics of these sexual processes for the Class OLIGOHYMENOPHOREA . Late in the 19th century, Maupas (1889) discovered the clonal life cycle of ciliates in his investigations of nuclear phenomena in Paramecium . During this clonal life cycle , cells of a clone pass through stages characterized as immature , adolescent , mature , and senescent , dependent upon the number of fissions since the last conjugation and upon their ability to engage in conjugation at the moment (Hiwatashi, 1981; Sonneborn, 1957; Takagi, 1999). As with other ciliates, oligohymenophoreans can be induced to conjugate after moderate starva- tion and if they are sexually mature (Bruns, 1986; Fujishima, 1988). Relatively little research, other than that of a descriptive nature, has been focused on members of the other oligohymenophorean subclasses. Finley (1936) was able to predictably induce conjugation in Vorticella species, but only after they had encysted and excysted . Conjugation can also be induced in Paramecium by chemical means (Cronkite, 1974) and by mixing a single mating type with detached cilia from mature cells of the complementary mating type (Miyake, 1964). In Paramecium , the natural mating-type substances aiding agglutination are very likely proteins, and in complementary mating types they may even have a precursor-derivative relationship (Xu et al., 2000). Sonneborn (1937) discovered complementary mating types of Paramecium \u201caurelia\u201d species while Elliott and Nanney (1952) reported a similar phenomenon in Tetrahymena . These discoveries opened the way to exploration of the genetics of ciliates (Nanney, 1980; Sonneborn, 1947). The binary mating-type system in Paramecium with its two complementary mating types is probably the ancestral state in the oligohymenophoreans , and from it the multiple mating-type systems described for Paramecium bursaria and Tetrahymena thermophila have likely evolved (Bleyman, 1996; Miyake, 1996). Exhaustive sampling of species in these two genera has not expanded the number of mating types beyond a maximum of 8 and 7 respectively, in dramatic contrast to the poten- tially hundreds of mating types recorded for some stichotrichs (Doerder et al., 1996; see Chapter 7 ). Jankowski (1972b) has demonstrated that Paramecium putrinum also has a multiple mat- ing-type system . Mating type in binary systems has been further classified into three categories. In the A or caryonidal mating-type system , each developing macronucleus following conjugation is independently determined to express either mating type. In the B or clonal mating-type system , the parental macronucleus epigenetically determines the expression of the mating type by the new macronucleus. In the C or genotypic mating-type system , the mating type expressed is under genotypic control (Bleyman, 1996; Simon & Orias, 1987; Sonneborn, 1977). A variety of environmental factors can influence the expression of the ultimate mating type in Tetrahymena with temperature and nutrition being important vari- ables (Arslanyolu & Doerder, 2000; Doerder et al., 1996). One of the most interesting environmental influences is the circadian rhythm of mating-type expression in Paramecium multimicronucleatum (Barnett, 1966). While the complete analyses of the genomes of Paramecium and Tetrahymena may eventually resolve the matter, the mating-type alleles of Tetrahymena exhibit serial dominance at a single locus, explained by a model for somatic DNA rearrangements during macronuclear devel- opment (Orias, 1963, 1981). Those of Paramecium bursaria are explained by combinations of alleles at two or three different loci (Siegel, 1963). In the oligohymenophoreans , the molecules signalling the readiness for mating are firmly bound to the cell surface of the ciliate, either on the ciliary membranes or the plasma membrane; Miyake (1996) calls this type, gamone-carrying . Mature Tetrahymena prepare for cell fusion in two stages, called initiation (Bruns & Brussard, 1974) and costimulation (Finley & Bruns, 1979). In contrast, mature Paramecium agglutinate upon first contact after which the cells prepare for con- jugation (Fujishima, 1988; Nanney, 1980). Other morphological differentiations occur in cells prior to fusion. In Tetrahymena , a region of the cor- tex anterior to the oral region becomes smooth and flattened. This tip transformation prepares the cells for fusion (Wolfe & Grimes, 1979). In Paramecium , a broad region of cortex anterior and posterior to the oral region becomes decili- ated prior to cell fusion (Watanabe, 1978). Cell membranes ultimately fuse to enable transfer of the pronuclei, at the anterior end in Tetrahymena and at the paroral cone region in Paramecium (Fujishima; Inaba, Imamoto, & Suganuma, 1966; Wolfe, 1985). In Tetrahymena , microtubules have been implicated in the movements of micronuclei prior to formation of the gametic nuclei (Nakajima, Ishida, & Mikami, 2002), and in the formation of the complex, microtubular-microfilamentous trans- fer basket that envelopes the migratory pronucleus in each partner, enabling transfer (Orias, Hamilton, & Orias, 1983). As with other ciliates, the micronuclei of oligohymenophoreans undergo typically three maturation divisions with the exception that some peritrich micronuclei may only undergo two matu- ration divisions (Raikov, 1972). These patterns are confirmed in more recent reports for scuticocili- ates (Coppellotti, 1990), hymenostomes (Martín- Gonzalez, Serrano, & Fernández-Galiano, 1984), and peritrichs (Sola, Guinea, & Fernández-Galiano, 1989a). Raikov (1972) noted that the \u201ccrescent stage\u201d is probably typical of micronuclear meio- sis in peniculines , scuticociliates , hymenostomes , and some peritrichs , but a \u201cparachute stage\u201d , found in nassophoreans , has been observed as examples, in one astome , one apostome , and in Paramecium putrinum (Jankowski, 1972b, Raikov, 1972). The \u201ccrescent stage\u201d micronucleus is prob- ably elongated by microtubular growth (Suganuma & Yamamoto, 1992). The chromosomes appear to be arranged in parallel with their telomeres aggre- gated near one end of the developing \u201ccrescent\u201d, 15.6 Nuclei, Sexuality and Life Cycle 323 324 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA which may facilitate the pairing of homologues (Loidl & Scherthan, 2004). There is either mutual exchange of gametic nuclei, or in the case of total conjugation in peritrichs , the fusion of the single \u201cmigratory\u201d gametic nucleus of the microconju- gant with the single \u201cstationary\u201d gametic nucleus of the macroconjugant . This makes the exconju- gants isogenic. The zygotic nucleus or synkaryon then undergoes typically two or three divisions, although in some Frontonia species it may divide only once while in some Paramecium species it may divide four times (Raikov, 1972). This