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stichotrichs (e.g., Urostyla , Pseudourostyla , Heckmann, 1965; Raikov, 1972, Takahashi, 1974). Cells are typically of similar size although cells of different size have been shown to mate preferentially under some conditions (Gold & Pollingher, 1971). Laval-Peuto (1983) observed cultured forms of Favella to mate even when the loricae of cells were of different types. Fusion of cells typically occurs near the oral regions often accompanied by disassembly of cortical alveoli and some oral structures and assembly of microtu- bules and microfilaments (Dallai & Luporini, 1989; Geyer & Kloetzel, 1987a, 1987b; Laval-Peuto, 1983; Rosati, Verni, & Dini, 1998). As in the Class Oligohymenophorea (see Chapter 15 ), microtu- bules and microfilaments are presumably involved in the positioning and movement of the gametic nuclei . Completion of macronuclear maturation and post-conjugation morphogenesis (see below) in hypotrichs and stichotrichs may take several days during which feeding does not occur. During this time, energy may be supplied by programmed autophagocytosis of cell constituents (Sapra & Kloetzel, 1975). As in other ciliates, spirotrichs are induced to conjugate by a variety of factors, with reduction in food resources being a dominant one (e.g., Adl & Berger, 2000; Gates & Ramphal, 1991). Their sexual life cycle is characterized by a period of immaturity in which conjugation cannot effectively occur, a period of maturity or conjugation compe- tence , and a period of senescence (Miyake, 1996; Smith-Sonneborn, 1981). The development to matu- rity is not an instantaneous switch in all cells (Dini & Nyberg, 1994), while the length of the immaturity period can be determined by cytoplasmic factors (Dini, Bleyman, & Giubbilin, 1990). The intensity of mating reactivity may show daily rhythms in some Euplotes species (Gates & Ramphal, 1991; Miyake & Nobili, 1974). The breeding systems of spirotrichs are gener- ally multipolar , that is with many more than two mating types: many more than 100 mating types have been recorded in the stichotrich Stylonychia (Ammermann, 1982; Ammermann & Schlegel, 1983); and up to 38 have been recorded in Euplotes species (Dini & Luporini, 1979, 1985; Heckmann, 1964; Kimball, 1942; Nobili, 1966). On the other hand, bipolar breeding systems have been observed in other Euplotes species (Katashima, 1959) and its hypotrich relative Aspidisca (Dini et al., 1987). Strains isolated from geographically distant localities may show varying intensities of mating reactivity , but this has not resulted in genetic par- titioning as there is still clear evidence of gene flow (Ammermann et al., 1989; Kusch, Welter, Stremmel, & Schmidt, 2000; Mollenbeck, 1999; Valbonesi, Ortenzi, & Luporini, 1992). Preparation for conjugation in stichotrichs and hypotrichs may be mediated by chemical signals, called pheromones or gamones , which diffuse through the medium and induce cells of com- plementary mating type to prepare for conjuga- tion (Esposito, Ricci, & Nobili, 1976; Heckmann & Kuhlmann, 1986; Luporini & Miceli, 1986; Luporini, Vallesi, Miceli, & Bradshaw, 1995). Secretion of multiple pheromones appears to occur sequentially as the cells mature sexually (Kuhlmann & Heckmann, 1989). The pheromones are proteins whose genes have been sequenced (Miceli, La Terza, Bradshaw, & Luporini, 1992; Raffioni, Luporini, & Bradshaw, 1989). Several models now exist to explain how pheromones from different mating types may interact with each other to induce mating reactivity in cells of complemen- tary mating types (Luporini & Miceli; Miyake, 1996; Ortenzi et al., 2000). Pheromones may also function in an autocrine fashion, stimulating mito- sis in some Euplotes species (Luporini, Alimenti, Ortenzi, & Vallesi, 2005). There is suggestive evidence that pheromones may also attract cells of the complementary mating type (Kosaka, 1991a) and influence the locomotory behavior of cells 7.6 Nuclei, Sexuality and Life Cycle 171 172 7. Subphylum 2. INTRAMACRONUCLEATA: Class 1. SPIROTRICHEA by eliminating rhythmic, spontaneous membrane depolarizations (Stock, Kruppel, Key, & Lueken, 1999). While diffusible pheromones appear to be the rule among hypotrichs and stichotrichs , we do not know how widespread they are within the Class SPIROTRICHEA . Indeed, there are some species of Euplotes in which the signal substances appear to be firmly bound to the cell surface (Heckmann & Siegel, 1964). There are typically three prezygotic divisions of the micronucleus of spirotrichs , but as few as two and as many as four have been observed (Raikov, 1972). In some Euplotes species, the gametic nuclei may not be sister nuclei. Thus, the two exconju- gants will not be isogenic (Baird & Klobutcher, 1988; Katashima, 1960; Kuhlmann & Heckmann, 1991) nor will autogamous forms be homozygous (Dini, Raikov, & Bracchi, 1999). Once fertilization has occurred, cells separate when the fusion zone is resorbed and/or contrac- tile processes operate (Geyer & Kloetzel, 1987a). During conjugation and the long postconjugation period, hypotrichs typically undergo two rounds of cortical reorganization while stichotrichs undergo three rounds of cortical reorganization (Ng, 1990; Tuffrau, Fryd-Versavel, & Tuffrau, 1981; Tuffrau, Tuffrau, & Genermont, 1976). The first reorganization is correlated with separation of the conjugants . Since the cytostome is non-functional, this neces- sitates additional rounds of reorganization. Ng (1990) has analyzed these processes in terms of developmental heterochrony in which he believes the sexual cycle overlaps the preceding asexual cycle . The first reorganization is similar to asexual development in that it bears similarities to asexual cortical development and the micronucleus is dis- pensable to this process (Ng, 1990; Zou & Ng, 1991). In the hypotrich Euplotes , the second corti- cal reorganization requires the presence of mater- nal macronuclear fragments (Fidler, Jayaraman, & Kloetzel, 1985), while in stichotrichs a macro- nuclear anlage and micronuclei are necessary to proceed through both the second and third cortical reorganizations (Lu, Shi, & Ng, 1991; Ng, 1990). In Paraurostyla weissei , which undergoes total conjugation , an exconjugant zygocyst is formed as the entire infraciliatures of both donor and recipient cells are resorbed. Nevertheless, there are still three rounds of cortical reorganization (Frontczak-Baniewicz & Jerka-Dziadosz, 1992). Retention of the interphase or vegetative pattern of superficial cortical microtubules indicates that the \u201ccell pattern\u201d is retained throughout this complex conjugation process (Fleury & Laurent, 1994). Whether these differences in numbers of cortical reorganizations during and following conjugation are phylogenetically significant is an open ques- tion. Ng (1990) noted that the stichotrich Kahliella apparently undergoes only two cortical reorganiza- tions (Fleury & Fryd-Versavel, 1982). Could this reflect it\u2019s presumed basal position in the adaptive radiation of this subclass? And we do not yet know how many cortical reorganizations oligotrichs and choreotrichs might undergo. Senescence follows the maturity period of the life cycle, and is characterized by reduced growth rate and increased mortality (Smith-Sonneborn, 1981). Clones can be rejuvenated by undergoing conjugation . Hypotrichs and stichotrichs typically have long immaturity and maturity periods , which means that they are typically outbreeding organ- isms: the probability of encountering cells of like type being decreased the longer the cell has a chance to disperse (Nanney, 1980; Sonneborn, 1957). In circumstances where complementary mating types are not present, some hypotrichs may undergo intraclonal