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Stomatogenesis in Sorogena is of this type, and occurs by prolifera- tion within and between about six posterior, right ventral somatic kineties (Fig. 12.5) (Bardele et al., 1991). This is followed by a clockwise rotation of the field and differentiation of paroral dikinetids on the anterior and ultimately right side of the primor- Fig. 12.5. Division morphogenesis in the Class COLPODEA . A The merotelokinetal stomatogenesis of Colpoda occurs within a division cyst and begins with complete dedifferentiation of the oral structures ( a , b ). Kinetosomal proliferation then occurs at the anterior ends of several somatic kineties ( c ) and the right and left oral structures dif- ferentiate from different subsets of these somatic kineties ( d ) (from Foissner, 1993a). B The pleurotelokinetal pattern is more widespread within the class, exemplified here by Sorogena . Kinetosomal proliferation begins on several kineties in the posterior right region of the body ( a ). A paroral and oral polykinetids begin to differentiate along the anterior and posterior borders of the primordial field, respectively ( b ). As the field rotates clockwise ( c ) and migrates anteriorly ( d ), these become the right and left oral structures respectively. (from Bardele et al., 1991.) dial field and several oral polykinetids on the poste- rior and ultimately left side of the primoridial field (Fig. 12.5). This basic pattern has been observed in Platyophrya (Dragesco et al., 1977; Grolière, 1975b) and Woodruffia (= Woodruffides ) (de Puytorac et al., 1979b; Prelle, 1963). A similar pattern has also been reported for Bryometopus (Wirnsberger et al., 1985b) and Cyrtolophosis (Díaz, Martín- González, Borniquel, & Gutiérrez, 2000). There is as yet no study of bryophryid stoma- togenesis while stomatogenesis of bursariomorphids has only been studied once. Perez-Paniagua, de Puytorac, and Savoie (1980) demonstrated that Bursaria undergoes a pleurotelokinetal stomatogenesis by kinetosomal replication within and between a number of right posterior somatic kineties. Similar to colpodids , the left oral polykinetids are composed of dikinetids aligned to make two rows to which a third is added to complete stomatogenesis . However, these never fuse, as in colpodids , but remain as a series of separated adoral polykinetids . The right oral structures derive from a more disordered kineto- somal proliferation that eventually develops into the multiple and oblique but parallel files of dikinetids of the \u201cparoral\u201d of this ciliate (Foissner, 1996b). 12.6 Nuclei, Sexuality and Life Cycle Colpodeans typically have a single macronucleus . Depending upon cell size , larger colpodeans may have many micronuclei (e.g., Colpoda magna , Bursaria truncatella ) (Foissner, 1994b). The macronucleus varies from globular to ellipsoid in smaller species to an elongate band-shape in larger forms (Figs. 12.1, 12.2). The micronucleus is globular to ellipsoid. Colpodids especially have a prominent nucleolus, which takes a vari- ety of forms (Burt, Kidder, & Claff, 1941). The nucleolus is actually an aggregation of a number of nucleolar organizing regions that assume various, often species specific arrangements. Although giving the appearance of a heteromer- ous nucleus, distribution of nucleoli in other colpodeans confirms that the colpodean macro- nucleus is homomerous (Raikov, 1969). Another unusual feature of the colpodean nuclear apparatus is the fusion of macronuclear and micronu- clear envelopes, especially in cyrtolophosidids (Detcheva, 1976; Dragesco et al., 1977; Golder, 1976). This has been used as a defining charac- ter for the Order Cyrtolophosidida (Foissner, 1994b; Lynn & Small, 2002; Small & Lynn, 1985). However, there are now reported excep- tions within the genus Cyrtolophosis (Díaz et al., 2000) and in forms related by stomatogenesis, such as Bryometopus (Wirnsberger et al., 1985b). Thus, we have abandoned this feature as diag- nostic of the Order Cyrtolophosidida . Division of colpodean macronuclei occurs through participation of intramacronuclear micro- tubules (Kuck & Ruthmann, 1985). Micronuclear chromosomes are attached by kinetochores to single microtubules that apparently shorten as the interpolar microtubules elongate during micro- nuclear division (Kuck & Ruthmann, 1983). Micronuclei in multimicronucleate species prob- ably all undergo fission but, due to the large size of the cells that they are found in, may not separate their products to different poles (e.g., see Beers, 1946a). Occasional amicronucleate clones may arise but they apparently have limited viability (Beers, 1946a, 1946b; Piekarski, 1939). The divi- sion of the macronucleus of small colpodids , like Colpoda steinii , involves the formation and separa- tion of a small number of discrete chromatin aggre- gates (Burt et al., 1941; Piekarski, 1939; Frenkel, 1978). This was confirmed by electron microscopy in both small and larger species (Frenkel, 1980, 1982). Quantitation of the DNA in these nuclei led to the hypothesis that these were diploid subnuclei undergoing separation (Frenkel, 1978; Frenkel, Kudryavtsev, & Kudryavtseva, 1973). Although this may be the case in colpodeans with low macronu- clear ploidy, such as Colpoda steinii , it seems less likely for colpodeans like Bursaria whose ploidy can exceed 5,000 n (Raikov, 1969). In Bursaria , composite chromosomes and polynemic structures have been observed, suggesting that there is a higher order organization to its DNA structure, even if it is not organized as diploid subnuclei (Raikov; Sergejeva & Bobyleva, 1995). Popenko et al. (1998b) demonstrated that the macronuclear DNA molecules of Bursaria range from 50\u2013360 kbp, probably packed in chromatin aggregates that are themselves formed into higher-order structures. DNA from Bursaria is about 50% single copy sequence with much of the remainder being highly repetitive (Borchsenius & Sergejeva, 1979). Other 12.6 Nuclei, Sexuality and Life Cycle 255 256 12. Subphylum 2. INTRAMACRONUCLEATA: Class 6. COLPODEA colpodeans have macronuclear chromosomes that range from 90\u20132,000 kbp and show karyotypic variation among species in the chromosome size on which rDNA genes reside (Martín et al., 1997). Conjugation has only been described in Bursaria and has never been reported in any colpodid (Raikov, 1972). Poljansky (1934) reported conjugation to occur in the fall in Bursaria , but the environmental cues are not known. Micronuclear meiosis is typical and demonstrates a \u201c parachute stage \u201d as is commonly observed in ciliates. Although many micronuclei undergo meiosis , only the one nearest the zone of contact and its denser cytoplasm undergoes a third division to produce the gametic nuclei . Raikov (1972) established a unique subtype for the develop- ment of macronuclear anlagen and micronuclei in Bursaria , which is characterized by the development of four macronuclear anlagen and four micronuclei. Poljansky and Sergejeva (1981) described oligo- tenic-like chromosomes during macronuclear anla- gen development and likened the process to anlagen development in spirotrichs and phyllopharyngeans , but this observation has yet to be corroborated. Sexual processes are known to \u201crestart\u201d the life cycle clock of ciliate populations that are becoming senescent (Sonneborn, 1954; Dini & Nyberg, 1993). Under this model, it is remarkable that some lines of colpodids , which never undergo conjugation (e.g., Colpoda magna ), show no signs of senescence after over 500 generations (Beers, 1944), although senescence may begin at double that number of cell generations (Crippa Franceschi, Schieti Cavazza, & Boccardo Rinesi, 1967). Extrusion bodies have been consistently observed during division of the macronucleus