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Jerka-Dziadosz, 1980). In fact, the ventral kinetids of some stichotrichs , such as Engelmanniella , may complete development as dikinetids, which are characterized as follows: an anterior ciliated kinetosome with a single post- ciliary microtubule and a tangential transverse ribbon at triplets 4, 5; and a posterior ciliated kinetosome with a divergent postciliary ribbon and a kinetodesmal fibril near triplets 6-8 (Fig. 7.5) (Wirnsberger-Aescht, Foissner, & Foissner, 1989). The ultrastructure of hypotrich cirri is quite variable, but typically kinetosomes are hexagonally packed and joined by a basal plate and a distal plate of dense or filamentous material. Microtubules originate from the lateral edges of the distal plate and extend out into the cortex. These microtubules are joined by those of the transverse and postciliary ribbons, which originate from the basal plate adja- cent to kinetosomal bases. Kinetodesmal fibrils may be associated with kinetosomes along the right edge of the polykinetid (Lynn, 1991; Tuffrau & Fleury, 1994). The ultrastructure of stichotrich cirri is quite consistent: kinetosomes are typically hexagonally packed and joined at the basal level and at mid-height. Microtubules originate from the mid-height connective material and extend out into the cortex. As in the hypotrichs , these microtubules are joined by the microtubules of transverse and postciliary ribbons, which arise from the basal plate adjacent to kinetosome bases. Kinetodesmal fibrils may be associated with kinetosomes along the right edge of the polykinetid (Fig. 7.6) (Fleury et al., 1985a; Grain, 1984; Lynn; Matsusaka et al., 1984; Tuffrau & Fleury, 1994). However, kineto- desmal fibrils or ciliary rootlets may be resorbed in a domain-specific fashion in some stichotrichs (Jerka-Dziadosz, 1990). Therefore, it is difficult to generalize about the presence or absence of these structures unless a thorough analysis has been done of the entire infraciliature. Even the somatic polykinetids of Plagiotoma , which was assigned by Corliss (1979) to the heterotrichs , demonstrate fea- tures of the stichotrich cirrus (Fig. 7.6) (Albaret & Grain, 1973). This supports transfer of Plagiotoma to the Subclass Stichotrichia (see also Tuffrau & Fleury), a fact that is also corroborated by SSUrRNA gene sequences (Affa\u2019a et al., 2004). Members of the Family Halteriidae are now placed in the Subclass Stichotrichia (see above and Chapter 17 ) (Fig. 7.4). Meseres has somatic dikinetids while Halteria and Pelagohalteria have fused, bristle-like cilia arising from dikinetids (Petz & Foissner, 1992; Song, 1993). The somatic dikinetids of Halteria are highly unusual. They apparently lack the classical fibrillar associates of the somatic kinetid, but are surrounded by dense material from which cortical microtubules originate, a feature shared with the cirri of other stichotrichs (Grain, 1972). We have placed the Family Reichenowellidae incertae sedis in the Subclass Hypotrichia . The ultrastructural study ofBalantidioides (= Transitella ?) Fig. 7.6. Schematics of the somatic polykinetids or cirri of representatives of the Class SPIROTRICHEA . ( a ) Somatic polykinetid or cirrus of the stichotrich Plagiotoma (based on an electron micrograph of Albaret & Grain, 1973). ( b ) Somatic polykinetid or cirrus of the stichotrich Histriculus . (Based on an electron micrograph of Matsusaka et al., 1984.) 7.3 Somatic Structures 159 160 7. Subphylum 2. INTRAMACRONUCLEATA: Class 1. SPIROTRICHEA shows its somatic kinetids to be assembled into groups of typically 2-6 dikinetids whose structure is characterized as follows: a ciliated anterior kine- tosome with a tangential transverse ribbon near triplets 3-5, a single postciliary microtubule, and possibly two microtubules near triplet 1, which could be transverse microtubules for the posterior kinetosome; and a ciliated post erior kinetosome with a large divergent postciliary ribbon and a pos- teriorly-directed kinetodesmal fibril . The postciliary microtubules originate in a large dense structure that is the base of an interiorly directed nematodesma (Fig. 7.5) (Iftode et al., 1983; Lynn, 1991). The structural variation among spirotrich somatic structures contradicts the hypothesis of \u201c structural conservatism \u201d of the somatic cortex (Lynn, 1976a, 1981) (see Chapter 4 ), and begs the question \u201cWhy?\u201d. In his discussion of the unusual nature of the somatic kinetids of choreotrichs , Grim (1987) suggested one explanation: when somatic kinetids are no longer used in locomotion, selection may be relaxed on the structure since it no longer performs a critical locomotor function. Although this is helpful in explaining variations in somatic kinetids of oligotrichs and choreotrichs and the dorsal dikinetids of hypotrichs and stichotrichs , it is not helpful in explaining the structural diversity of locomotory somatic kinetids among spirotrichs (Fig. 7.5). We currently have no explanation for this deviation from structural conservatism, except to suggest that these lineages could be extremely ancient, as suggested by the branch lengths in molecular phylogenetic analyses (e.g., Bernhard et al., 2001; Snoeyenbos-West et al., 2002; Strüder- Kypke et al., 2002). The cirri of hypotrichs and stichotrichs can per- form complex \u201cwalking\u201d or \u201crunning\u201d movements (Erra et al., 2001; Sleigh, 1989). These movements in Euplotes and Stylonychia are controlled by membrane hyperpolarizations that produce forward movement (i.e., rearward beating) and membrane depolarizations that produce rearward movement (i.e., forward beating) (Epstein & Eckert, 1973; Deitmer, Machemer, & Martinac, 1984). Rhythmic depolarizations determine the rhythm of walking in Euplotes (Lueken, Ricci, & Krüppel, 1996). Similar to the model for Paramecium , Ca 2+ ions probably interact with ciliary axonemal components, serv- ing as the intracellular messengers for membrane potential changes. The cirri of Stylonychia also respond to slow changes in membrane potential by \u201cinclining\u201d or bending at the base without beating, which adds a further degree of sophistication to their movement (Machemer & Sugino, 1989). Spirotrichs can be quite flexible or contractile. However, in choreotrichs , this contractility appar- ently does not rely on filamentous structures but on highly unusual membranous elements, for example in the tail of Tontonia (Greuet, Gayol, Salvano, & Laval-Peuto, 1986) and the posterior end of tintin- nids (Laval-Peuto, 1994). We do not know how these structures work. Contractile vacuoles are common throughout the group, especially in freshwater forms. Marine tin- tinnids do not have contractile vacuoles while their freshwater relatives do. A wide variety of extrusomes has been described in the group. Mucocyst -like extrusomes are found in this class ( Phacodinium , Didier & Dragesco, 1979; Stylonychia , Görtz, 1982b; Transitella , Iftode et al., 1983). Cortical ampules found around the dorsal dikinetids of hypotrichs may be a special type of mucocyst ( Aspidisca , Rosati et al., 1987; Certesia , Wicklow, 1983; Euplotes , Görtz). Other extrusomes with a distinctly lamellar nature appear to be involved in cyst formation (e.g., Grim & Monganaro, 1985; Walker et al., 1980; Verni, Rosati, & Ricci, 1990), while the highly unusual lepidosomes , mentioned above, form a surface coat on the cysts of Meseres (Foissner et al., 2005a). The trichites of oligotrichs have been demonstrated to be extrusomes , but their function remains unknown (Modeo, Petroni, Bonaldi, & Rosati, 2001). Finally, to round out this brief treatment of other somatic structures, lithosomes or calculi have been observed in both hypotrichs (Lenzi & Rosati, 1993; Rosati et al., 1987;