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an elongation phase and a constriction phase (Raikov). The elongation phase is likely driven by both the assembly and slid- ing of microtubules, which may assemble inside the macronuclear envelope (e.g., Tucker, Beisson, Roche, & Cohen, 1980; Williams & Williams, 1976) or outside the macronuclear envelope (e.g., Diener, Burchill, & Burton, 1983). While duplication of all cell constituents occurs during the cell cycle, developmental biologists and systematists have been particularly fasci- nated by the duplication of the cortical compo- nents. Lynn and Corliss (1991) have separated this development into cortical somatogenesis and cortical stomatogenesis : the replication of the com- ponents of the somatic cortex and the oral cortex, respectively, which are often highly co-ordinated processes. Frankel (1989) has provided a detailed review of these processes from the perspective of a developmental biologist. In particular, ciliate systematists have long been fascinated with the ontogeny of the oral apparatus (see Chapter 1 ; Corliss, 1968; Fauré-Fremiet, 1950a, 1950b). Foissner (1996b) has provided a detailed discus- sion of the comparative stomatogenesis of ciliates, but see also Tuffrau (1984). Briefly, the conspicuous elements of cortical somatogenesis that have attracted attention are the kinetosomes , contractile vacuole pores , and cytoproct . Ciliates were one of the first groups of organisms to be investigated for replication of kinetosomes, demonstrating that the \u201cdaughter\u201d kinetosome developed in close proximity to and in a well-defined relationship with the parental kine- tosome (Allen, 1969; Dippell, 1968). This proc- ess, now called cytotaxis or structural guidance , is responsible for the precise positioning of new cortical units (Aufderheide, Frankel, & Williams, 1980; Frankel, 1989). Kinetosomal replication can occur throughout the cell cycle or be confined to a period close to the time of cytokinesis and be highly correlated with cortical stomatogenesis. Initiation of kinetosomal replication undoubtedly involves participation of gene products that diffuse through the cytoplasm: for example, the product of one such gene, sm19+ , appears to be involved in kinetosomal replication in Paramecium (Ruiz, Garreau de Loubresse, & Beisson, 1987). New contractile vacuole pores (CVPs) are typically replicated at cell division, although in some cili- ates with large numbers of contractile vacuoles the replication process may be uncoupled from cell division. In Tetrahymena , the proter develops new CVPs adjacent to somatic kineties in a predictable location in its posterior right quadrant, defined by the \u201c central angle \u201d. This angle is a manifesta- tion of a mechanism that places the new pores in a roughly proportional fashion in relation to the total number of somatic kineties (Frankel; Nanney, 1980; Nanney, Nyberg, Chen, & Meyer, 1980b). In Chilodonella species, a proportioning mechanism may also exist, but in this case the many contrac- tile vacuole pores , which are distributed over the ventral surface, are newly placed in both proter and opisthe, apparently in relation to major features of the cortex, such as somatic kineties, the oral region, and the boundaries of the ventral surface. During this somatogenesis in Chilodonella , the old contractile vacuoles and their pores dedifferentiate and disappear (Kaczanowska, 1981; Kaczanowska, Wychowaniec, & Ostrowski, 1982). The old cyto- proct , since it is typically in the posterior end of the cell, is inherited by the opisthe, and a new cytoproct develops in the appropriate position in the proter, presumably positioned by mechanisms similar to those specifying the position of CVPs. In addition to these conspicuous cortical ele- ments, we should remember that all other organelles are typically duplicated during each interfission period \u2013 mitochondria , extrusomes , Golgi apparati , ribosomes , lysosomes , and all the smaller molecu- lar constituents not visible as discrete entities by the microscopist. Cortical stomatogenesis is literally the forma- tion of a mouth. This process is usually the most conspicuous cortical ontogenetic event, since the oral region is generally the most obvious corti- cal differentiation. Since the oral apparatus was historically considered highly significant as a taxo- nomic feature, its development in different taxa has preoccupied ciliate systematists . In the chapters that follow, stomatogenesis of each of the classes is briefly characterized, based on the primary lit- erature and the comprehensive review of Foissner (1996b). Stomatogenic patterns are now divided into five major types with subtypes \u2013 apokinetal , parakinetal , buccokinetal , telokinetal , and mixoki- netal (Corliss, 1979; Foissner, 1996b). However, all subtypes within a pattern of stomatogenesis should not be regarded as diversifying from an ancestral type: they should not be considered as homologous . Rather, the several kinds of telokinetal stomatogen- esis probably have evolved independently in dif- ferent classes as the morphology of these ciliates diversified. For example, cyrtophorids , prostomate- ans , colpodeans , and litostomateans all exhibit dif- ferent kinds of telokinetal stomatogenesis (Foissner, 1996b), but molecular phylogenetic analyses clearly demonstrate that these classes are not closely related. Thus, typifying stomatogenesis using this classification system should be viewed only as a descriptive approach, enabling a systematic charac- terization of the existing diversity. It may be of phy- logenetic significance in relating groups within the classes. More complete definitions of these kinds of stomatogenesis can be found in the Glossary (see Chapter 2 ) and in Foissner (1996b). The conspicuousness of stomatogenesis has also attracted the attention of developmental biologists who have investigated a variety of its aspects. The primordium or anlage for the new oral apparatus may be positioned by mechanisms that are influ- enced by the global properties of the cell, ensuring that the new oral apparatus is placed in some pro- portional manner in relation to the whole (Frankel, 1989; Lynn, 1977b). However, the assembly of the oral apparatus adds a level complexity to cortical developmental processes as it encompasses at least three levels of biological organization \u2013 organelles (e.g., kinetosomes ), organellar complexes (e.g., membranelles , polykinetids ), and organellar systems (e.g., the entire apparatus itself). There is a complex interplay of controls at these levels and different 4.5 Division and Morphogenesis 113 114 4. Phylum CILIOPHORA \u2013 Conjugating, Ciliated Protists with Nuclear Dualism processes that coordinate assembly at each level (Frankel, 1989). Furthermore, this development takes place in the context of the cell, so that the entire apparatus, both in terms of the size of each oral polykinetid, for example, and sometimes the numbers of oral polykinetids are strongly related, for example, to cell size (Bakowska & Jerka- Dziadosz, 1980; Bakowska, Nelsen, & Frankel, 1982a; Jacobson & Lynn, 1992). Thus, systema- tists must be aware of all of these potential con- straints on oral development when they consider which aspects of the process and which features of the differentiated oral apparatus are significant from a systematic perspective. For example, are the differences in number and size of oral polykinetids in two isolates of a genus evidence of different species or of the phenotypic plasticity of these components in a single species as it varies in cell size? Even somatic structures, such as numbers of somatic kineties, can be strongly correlated with cell size (Lynn & Berger, 1972, 1973). Finally, once the cells have separated, there are often significant morphogenetic processes