Pré-visualização | Página 13 de 13
1978). C Three haptocysts at the tip of the tentacle of the suctorian Ephelota gemmipara (from Grell & Benwitz, 1984). D The trichocyst of the oligohymenophorean Paramecium tetraurelia (from Kersken et al., 1984). E A short toxicyst from the litostomatean Enchelydium polynucleatum (from Foissner & Foissner, 1985). F A longitudinal section through the contractile vacuole pore ( CVP ) of the oligohy- menophorean Colpidium campylum . Note that there is a set of helically disposed microtubules (arrows) supporting the pore canal and a set of radially disposed microtubules ( R) that position the contractile vacuole . (from Lynn & Didier, 1978.) 4.7 Other Conspicuous Structures 119 120 4. Phylum CILIOPHORA – Conjugating, Ciliated Protists with Nuclear Dualism reduced or absent (André & Fauré-Fremiet, 1984). These mitochondria-like organelles, which cannot accomplish oxidative phosphorylation, have inde- pendently evolved in these several ciliate classes to ferment pyruvate into acetate and H 2 , and hence are referred to as hydrogenosomes (Fenchel & Finlay, 1991a). With the isolation of a genome from a cili- ate hydrogenosome , there is now no doubt that these organelles are derived from mitochondria (Boxma et al., 2005; van Hoek, Akhmanova, Huynen, & Hackstein, 2000a). These anaerobic ciliates often have endosymbiotic and ectosymbiotic bacteria , typi- cally methanogens , associated with the hydrogeno- somes . This relationship, at least in the case of the endosymbiotic methanogens , provides the ciliate with increased efficiencies in growth (Fenchel & Finlay, 1991b). Finally, a variety of extrusomes are promi- nent features of the somatic cortex. The different orders and classes of ciliates have different types of extrusomes (see reviews by Dragesco, 1984a; Hausmann, 1978; Rosati & Modeo, 2003). All these organelles are membrane-bound, likely syn- thesized in the endoplasmic reticulum-Golgi sys- tem, transported to the cell cortex, and stimulated to fuse with the plasma membrane by ionic changes (Hausmann, 1978). Mucocysts , broadly distributed throughout the classes, function to provide a sur- face coat for the cell, sometimes during the process of encystment (Figs. 4.9D, 4.19B) (Lynn & Corliss, 1991). Upon ejection, both their length and diam- eter become much larger than those dimensions in the resting state (Hausmann, 1978). Possible modifications of the mucocysts are the scale-like structures or lepidosomes secreted on the surface of some haptorians (Foissner, Müller, & Weisse, 2005a; Nicholls & Lynn, 1984). Clathrocysts and lepidosomes may also be used to construct the cyst wall of the haptorian Didinium (Holt & Chapman, 1971) and the spirotrich Meseres (Foissner et al., 2005a). Trichocysts , restricted primarily to some nassophoreans and some peniculine oli- gohymenophoreans , are extrusomes that main- tain the diameter of the resting state but extend as thread-like filaments many times the resting length (Fig. 4.19D) (Hausmann, 1978). Trichocysts of the peniculine Paramecium appear to func- tion to protect the ciliate from predators, such as Climacostomum , Monodinium , and Dileptus (Harumoto, 1994; Miyake & Harumoto, 1996; Sugibayashi & Harumoto, 2000). While trichocysts may protect their ciliate bearer from predators, the last two common cate- gories of extrusomes – toxicysts and haptocysts – enable the ciliate to switch roles and become the predator. Toxicysts are typical of the Subclass Haptoria (Class LITOSTOMATEA ), and as the name suggests, are extrusomes with toxic poten- tial. Upon extrusion, their tube-within-a-tube structure everts, maintaining the same width as in the resting state, but rapidly increasing in length to deliver the poisonous material now at the tip to the prey (Fig. 4.19E) (Hausmann, 1978). The compounds within the toxicyst can enable attachment of the predator to its prey and also immobilize the prey, partly by causing lysis of the somatic cilia (Wessenberg & Antipa, 1969, 1970). Finally, haptocysts are typically found at the tips of tentacles of the Subclass Suctoria (Class PHYLLOPHARYNGEA ), and are small bottle- like organelles with a complex internal structure (Fig. 4.19C) (Hausmann). When prey contacts the suctorian tentacle , the haptocyst everts, cement- ing the two cells together and rapidly causing the prey to become immobile (Benwitz, 1982, 1984). Other extrusome types have been described as restricted to a particular group, and will be treated briefly in the appropriate chapter.