Pré-visualização11 páginas
which appears either as isolated granules in vesibuliferid endo- plasm or as skeletal plates in the entodiniomor- phids (Grain, 1994; Nakai & Imai, 1989; Wakita & Hoshino, 1980). Starch-like storage products have also been observed in the macropodiniids (Cameron & O\u2019Donoghue, 2002a, 2003a). Contractile vauoles are a typical feature of the litostomes with some endosymbionts having up to 20. The cytoproct is a typical feature of tri- chostomes , but often not reported in haptorians . Entodiniomorphids have a distinct cytoproct sur- rounded by a conspicuous filamentous sphincter. 9.4 Oral Structures The litostome oral region in its simplest state is on the cell surface at the anterior end of the body as in haptorids and archistomatines (i.e., buetschliids ). The cytostome is on a dome-like elevation sur- rounded by circumoral cilia , which are slightly longer than the adjacent somatic cilia. In pleu- rostome haptorians , the oral region has become slit-like, extending along the ventral edge of the laterally flattened body. In vestibuliferids , blepha- rocorythids , and entodiniomorphids , the oral region has become invaginated to varying degrees and the oral ciliature can become functionally differenti- ated, although still arising essentially from the anterior ends of somatic kineties. Prior to electron microscopy, the \u201cmembranelle-like\u201d behavior of the oral cilia of entodiniomorphids suggested that they actually had an adoral zone of membranelles (Corliss, 1961). This functionality has now been redescribed by scanning electron microscopy of Entodinium and Cycloposthium (Imai & Yamazaki, 1988; Imai, Tashiro, & Ishii, 1983). However, there is no corresponding structural subdivision at the level of the kinetosomes where a \u201cmembranellar\u201d ultrastructure as in the spirotrichs might be expected (Furness & Butler, 1983, 1985). At the ultrastructural level, the circumoral kinetids of haptorians are typically dikinetids characterized as follows: a ciliated posterior or right kinetosome with a postciliary ribbon and sometimes a transverse ribbon; and a non-ciliated anterior or left kinetosome that bears a tangential transverse ribbon, sometimes a single postciliary 9.4 Oral Structures 203 204 9. Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA microtubule, and a nematodesma (see Grain, 1994; Lynn, 1991). These oral dikinetids are often linked up by a filamentous annulus that is continuous with the filaments of the ecto-endoplasmic layer . The transverse ribbons of the anterior kinetosomes extend anteriorly and then bend posteriorly to support the cytopharnynx. The nematodesmata extend posteriorly to form the rhabdos or litostome cytopharyngeal apparatus . Dileptus species may have two rings of nematodesmata, the inner one of which is not associated with kinetosomes (Grain & Goli\u144ska, 1969). A set of bulge microtubules may be found interior to the nematodesmata and extending as a \u201cpallisade\u201d deep into the cytoplasm surrounding the cytopharynx from the point where the transverse ribbons bend interiorly (Foissner & Foissner, 1988; Grain; Kuhlmann, Patterson, & Hausmann, 1980; Lipscomb & Riordan, 1991; Lynn, 1991). The oral dikinetids can be rotated in some haptorians (e.g., Didinium \u2013 Lipscomb & Riordan, 1992; Foissner & Foissner, 1988) so that it may be preferable to refer to right and left kinetosomes rather than anterior and posterior ones (Grain, 1994). Finally, the slit-like oral region of pleurostomes is also bordered by oral dikinetids whose transverse ribbons are also associated with bulge microtubules (Foissner & Leipe, 1995). Oral monokinetids are only found in some hap- torians (Foissner & Foissner, 1985). Lipscomb and Riordan (1990) used a parsimony analysis and cladistics to conclude that the haptorian oral monokinetids are homologous to the anterior or left kinetosome of the oral dikinetid (i.e., the posterior right kinetosome has not differentiated). The trichostomes have what are termed \u201coral- ized\u201d somatic kinetids since Lipscomb and Riordan (1990) concluded that these ciliates have completely lost the oral dikinetid . This is correlated with the loss of toxicysts in these ciliates. Transverse ribbons of several of the more anterior oral monokinetids of trichostomes may still extend to support the cyto- pharynx, while nematodesmata and bulge micro- tubules support the oral cavity or vestibulum and cytopharynx of some of these ciliates (Grain, 1994; Lynn, 1991). Typically, these \u201coralized\u201d kineto- somes have kinetodesmal fibrils that are consider- ably reduced or absent (Grain, 1966a, 1994; Grim, 1993a; Guinea et al., 1992; Paul et al., 1989). Feeding and ingestion by litostomes involves intimate contact with their prey since they are not suspension or filter-feeding ciliates. Clavate cilia may be involved in sensing prey or prey metabo- lites, while the girdle ciliature of the planktonic cyclotrichiids may detect prey by hydromechanical signals (Jakobsen et al., 2006). In the rumen ciliate Ophryscolex , the paralabial organelle is situated on the ventral side of the body adjacent to the oral region. This organelle, which has clavate cilia arising from files of dikinetids that border a central kinety of monokinetids, has been implicated as a sensory structure involved in feeding (Bretschneider, 1962; Schrenk & Bardele, 1987). Treatment of Dileptus with proteolytic enzymes has demonstrated that sur- face proteins are essential for ingestion, while inges- tion itself is a calcium-dependant process (Estève, 1984b). Toxicysts are crucial to prey capture in haptorians and their discharge is also calcium- dependant (Iwadate, Katoh, Kikuyama, & Asai, 1999). Microtubules are also essential to the ingestive process as the anti-microtubule drug, colchi- cine , decreases food vacuole formation in Dileptus (To\u142\u142oczko, 1980). The food vacuole is formed by the fusion of vesicles with the plasma membrane of the cytostome (Kuhlmann & Hausmann, 1983). The digestive process is similar to that in Paramecium (see Chapter 15 ) with the fusion of acid vesicles with the food vacuole membrane during the initial stages of digestion in Litonotus (Verni & Gualtieri, 1997). In the entodiniomorphids , the complexity of the cytopharyngeal apparatus is directly related to the diet of the ciliates: species that ingest large plant fragments have larger and more fibrous oral structures than those that ingest bacteria and starch grains (Furness & Butler, 1988). Extrusomes with toxic capacities are typical of haptorians but have apparently been lost by trichos- tomes , possibly coincident with the loss of oral dik- inetids . Several categories of toxicysts have been described and the number and their types are char- acteristic of certain genera. Toxicysts can be both proteolytic and paralytic. Didinium typically has three types: (1) the longer toxicysts that extend out tube-like to paralyze prey and begin proteolysis of the prey; (2) cyrtocysts that may be prototype toxi- cysts; and (3) pexicysts that are shorter and appear to fasten the predator to its prey (Hausmann, 1978; Wessenberg & Antipa, 1969, 1970). Homalozoon also has both long and short toxicysts . Even shorter extrusomes , called conocysts , are trichocyst-like in that their contents are rod-like, bearing a pin-like element at the tip when extruded (Hausmann, 1977; Kuhlmann & Hausmann, 1980). Toxicysts develop in the endoplasm and are probably assembled by the endoplasmic reticulum through the classical cellular secretory pathway (Dragesco, Auderset, & Baumann, 1965; Hausmann, 1978). Finally, brief mention must be made of what we know about the regulation of oral structures in litostomes . Kink (1976) demonstrated a zone of kinetosome proliferation near