Pré-visualização25 páginas
(Fig. 15.1; also Fig. 4.4). This phenotype is typically characterized by a more elongate cell shape than the trophont or \u201cfeeding\u201d phenotype and by more rapid swim- ming with fewer tumbles and turns (Fenchel, 1990; Nelsen & Debault, 1978). Depending upon the species, after a certain period of starvation , during which autophagy occurs (Nilsson, 1984), the second typical response to starvation \u2013 encyst- ment \u2013 occurs. Encystment has been reported in all major groups of oligohymenophoreans , except the astomes . The cyst wall, derived in part from extrusomes, is often multilayered, composed of an ectocyst , mesocyst , and endocyst . The cyst wall is composed of chitin , other complex carbohydrates, and some proteins. Cysts have been analyzed in the peniculines Furgasonia (= Cyclogramma ) and Pseudomicrothorax (Bussers, 1976), the scu- ticociliate Pseudocohnilembus (Olendzenski, 1999), the hymenostomes Tetrahymena (McArdle, Bergquist, & Ehret, 1980) and Ichthyophthirius (Ewing, Kocan, & Ewing, 1983), the peritrichs Telotrochidium (Walker, Edwards, & Suchard, 1989) and Opisthonecta (Calvo, Fernandez- Aliseda, Garrido, & Torres, 2003), and the apos- tome Hyalophysa (Bradbury, 1974; Landers, 1991a, 1991b). Excystment is stimulated by a number of factors (for review see Corliss & Esser, 1974). Ultimately and in all probability, a small set of \u201csignal\u201d molecules may be responsible: for exam- ple, glycogen is very effective in inducing excyst- ment of the phoront stage of apostomes (Bradbury & Trager, 1967). Paramecium species, which are not known to encyst , can survive for more than a month without food (Jackson & Berger, 1985b). 15.3 Somatic Structures While oligohymenophoreans are an assemblage more speciose than the spirotrichs , the range of variation in body form is less dramatic. The \u201ctypical\u201d oligohymenophorean is ovoid in shape, ranging in size from about 10 µm in small scutico- ciliates , to almost 1 mm in the infective stage of the hymenostome Ichthyophthirius , and up to 3 mm in some astomes (Figs. 15.2\u201315.5). As a group, the peritrichs demonstrate considerable diversity of body form: zooids can be borne on stalks, attach directly to the substrate on a modified scopulary 15.3 Somatic Structures 297 298 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA Fig. 15.2. Stylized drawings of representatives of the Class OLIGOHYMENOPHOREA . Members of the Subclass Peniculia \u2013 Frontonia , Paramecium , and Lembadion . Members of the Subclass Apostomatia \u2013 Hyalophysa and the adult of Conidophrys \u201cimpaled\u201d on the seta of a crustacean and its ciliated dispersive bud Fig. 15.3. Stylized drawings of representatives of the Class OLIGOHYMENOPHOREA . Members of the Subclass Hymenostomatia \u2013 the tetrahymenid Tetrahymena and the ophryoglenid Ichthyophthirius with its small theront and gigantic trophont , which causes \u201c Ich \u201d. Members of the Subclass Peritrichia \u2013 two sessilids, the stalked and sessile Vorticella and its telotroch or swarmer and the permanently mobile and stalkless Opisthonecta ; and the mobilid Trichodina , which causes trichodinosis 15.3 Somatic Structures 299 300 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA Fig. 15.4. Stylized drawings of representatives of the Class OLIGOHYMENOPHOREA . Members of the Subclass Scuticociliatia \u2013 the philasterids Dexiotricha , Anophryoides , Uronema , Philaster , Pseudocohnilembus , and Cohnilembus region or attach and migrate along substrates with a complex adhesive disk , as does the mobiline peri- trich Trichodina (Fig. 15.3). Oligohymenophoreans are typically holotrichous with somatic kinetids as either monokinetids or dikinetids (Figs. 15.2\u201315.5). Dikinetids are often Fig. 15.5 Stylized drawings of representatives of the Class OLIGOHYMENOPHOREA . Members of the Subclass Scuticociliatia \u2013 the pleuronematid Pleuronema and the thigmotrichids Boveria and Hemispeira . Members of the Subclass Astomatia \u2013 Anoplophrya , Radiophrya , and Maupasella 15.3 Somatic Structures 301 302 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA more common in the anterior half of the body and can be organized as paratenes , which are seem- ingly transverse rows of kinetids (Ehret, 1967). The somatic kineties can converge forming a preoral suture or anterior secant system at the anterior end and a postoral suture or posterior secant system at the posterior end. The shape, kind, and arrange- ment of these convergence regions is system- atically important, especially in the astomes . The posterior end of oligohymenophoreans , especially scuticociliates , often bears kinetids whose cilia are significantly longer than the general body ciliature (Fig. 15.4). These so-called caudal cilia are typi- cally not active, but can be used for attachment to the substrate. The oligohymenophorean cortex, especially those of Tetrahymena and Paramecium , has been and continues to be exhaustively studied by cell and molecular biologists. The literature on this is extensive, and we provide here a few selected ref- erences as an introduction to the literature and to demonstrate the structural attributes of the ciliates in this class. The oligohymenophorean cell is covered by a plasmalemma , which is underlain by cortical alveoli (Allen, 1967, 1971, 1978). Intramembranous particles are distributed over the surface of the plasmalemma , joining this surface membrane with the subsurface alveoli and trichocysts and inte- grating them with the cilia (Allen, 1978; Plattner, Miller, & Bachmann, 1973). The lipid content of these surface membranes can be varied so that the ciliate can maintain a particular membrane fluidity over large ranges of environmental temperatures: at lower temperatures, membranes have more unsatu- rated fats and at higher temperatures, membranes have more saturated fats (Kitajima & Thompson, 1977; Nozawa, Iida, Fukushima, Ohki, & Ohnishi, 1974; Wunderlich, Speth, Batz, & Kleinig, 1973). Variations in thermotolerance among species of Paramecium may be due to genetic variation in the fatty acid profiles of their cell membranes (Sasaki et al., 2006). The plasma membrane is covered by a sur- face coat, which is about 20 nm thick, and com- posed of glyco- and other proteins (Allen, 1978). Prominent among the proteins are the immobili- zation antigens or i-antigens , so named because antibodies to them bind the cilia together and prevent ciliary locomotion (Beale, 1954). These proteins are GPI-anchored proteins (Capdeville, Cardoso de Almeida, & Deregnaucourt, 1987; Clark, Gao, Gaertig, Wang, & Cheng, 2001; Ko & Thompson, 1992). They show considerable vari- ation in both laboratory and natural populations (Lin et al., 2002; Saad & Doerder, 1995). There has long been speculation that these antigens pro- tect the plasma membrane from the environment. Clear seasonal differences in the appearance of particular antigens suggests an important, but undetermined, ecological role for them (Doerder et al., 1996; Gerber, Lopez, Shook, & Doerder, 2002; Saad & Doerder, 1995). The plasma membrane is underlain, as in other ciliates, by cortical alveoli , which are conspicuous in the oligohymenophoreans . The alveoli were likened to the membrane cisternae of muscle cells (Allen, 1971; Satir & Wissig, 1982), a specula- tion that predicted they would sequester calcium. Calcium , which was later directly visualized in the alveoli of Paramecium using secondary ion mass spectrometry, was shown to decrease on extrusion of trichocysts \u2013 a calcium-induced process. Calcium refilled the alveoli over several hours following depletion (Mohamed et al., 2003; Stelly, Halpern, Nicolas, Fragu, & Adoutte, 1995). In addition to calcium, alveoli