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some detailed reinvestigations of armophoreans before any generalizations can be made about their somatic dikinetids . A further intrigu- ing physiological observation is that Nyctotherus ovalis switches swimming direction in response to voltage changes rather than showing a ciliary reversal. Moreover, this behavior appears to be influenced by host-dependent factors (van Hoek et al., 1999). Contractile vacuoles are present in armopho- reans . The cytoproct is often conspicuous, and in clevelandellids may open to the outside by a cilia- lined channel. Mucocysts appear to be present in the cortex of clevelandellids (Paulin, 1967; de Puytorac & Grain, 1969) and armophorids (Esteban et al., 1995). Finally, mention must be made of the apparent absence of mitochondria with tubular cristae in all armophoreans . The mitochondria in these ciliates have evolved into hydrogenosomes (van Hoek, Akhmanova, Huynen, & Hackstein, 2000a; Boxma et al., 2005). These hydrogenosomes have a hydro- genase that uses electrons derived from pyruvate oxidation to reduce protons and generate hydrogen (Fenchel & Finlay, 1991a; Müller, 1993; Voncken et al., 2002). The hydrogen is typically used in armophoreans by endosymbiotic methanogens (see Life History and Ecology ). 8.4 Oral Structures The armophoreans were placed until recently with the heterotrichs because of their holotrichous somatic ciliation and the presence of multiple oral polykinetids forming an adoral zone . The two or 182 8. Subphylum 2. INTRAMACRONUCLEATA: Class 2. ARMOPHOREA three rows of kinetosomes of the oral polykinetids are hexagonally packed. In armophorids , a third or fourth row of kinetosomes is added continuing the hexagonal packing (Esteban et al., 1995; Foissner & Agatha, 1999). Armophorid oral polykinetids have been called paramembranelles . Clevelandellids typically have three rows of kinetosomes hexago- nally packed, but a fourth, shorter row lies directly opposite to, rather than hexagonally packed with, the kinetosomes of the third row, leading to their designation as heteromembranelles because of the different packing of these kinetosomes of the fourth row (de Puytorac & Grain, 1976). This dif- ferent packing leads to a different orientation and beating of the cilia that was nicely revealed in some published micrographs (Paulin, 1967; Takahashi & Imai, 1989). The adoral zones of armophorids and clevelan- dellids may be quite extensive, spiralling around the body one or more times in some armophorids Fig. 8.3. Somatic cortex of Metopus whose postciliary ribbons extend alongside each other into the cortical ridges. This schema was constructed based on the brief descriptions provided in reports by Schrenk and Bardele (1991) and Esteban et al. (1995) (Fig. 8.1). The clevelandellids have a deeper oral cavity called an infundibulum where the heteromembranelles typically occur (Tuffrau & de Puytorac, 1994). Postciliary ribbons are associ- ated with the kinetosomes of the posterior row in both armophorids and clevelandellids (Tuffrau & de Puytorac). Paroral structures are quite variable in the class. Armophorids appear to have a single file of cilia, which may be derived from linearly arranged oral dikinetids (Esteban et al., 1995; Foissner & Agatha, 1999; Sola, Serrano, Guinea, & Longás, 1992). Clevelandellids have a paroral with two sets of cilia deriving from two files of kinetosomes separated by a ridge (Grim, 1998; Paulin, 1967; de Puytorac & Grain, 1969; Takahashi & Imai, 1989), termed a diplostichomonad by de Puytorac and Grain (1976). The oral structures of armopho- reans are underlain by complex fibrillar structures and microtubules. The filamentous components are implicated in the movement of vesicles to the food vacuole forming region (Eichenlaub-Ritter & Ruthmann, 1983). 8.5 Division and Morphogenesis There have been only a few papers on cell divi- sion and division morphogenesis of armophoreans since Wichterman (1936) described division in Nyctotheroides (= Nyctotherus ). He observed the oral primordium to develop subequatorially. Since silver-staining was not used, kinetosomal replica- tion was not detailed. As far as we know, armo- phoreans divide while swimming freely. Foissner (1996b) has characterized stomatogenesis as pleu- rotelokinetal (i.e., occurring within or at the end of several somatic kineties). Two studies on the armophorids , Metopus and Caenomorpha , demonstrated pleurotelokinetal stomatogenesis . Martín-González, Serrano, and Fernández-Galiano (1987) showed that the oral primordium in Caenomorpha develops by prolif- eration from the posterior ends of many perizonal somatic kineties . The primordial field splits later in development with an anterior portion devel- oping into the paroral and the posterior portion developing into the oral polykinetids . In Metopus , a number of posterior dorsolateral somatic kineties begin to proliferate kinetosomes (Foissner & Agatha, 1999). These differentiate as the oral polykinetids (Fig. 8.4). The paroral differenti- ates later. Foissner and Agatha (1999) interpreted it to develop from kinetosomes derived from perizonal kineties . However, it is just as pos- sible from the evidence presented that paroral dikinetids could derive from \u201canterior\u201d or \u201cright- side\u201d kinetosomes in a fashion very similar to that reported for Caenomorpha . If this were the case, there would be strong similarities in Fig. 8.4. Division morphogenesis of Metopus , a representative of the Class ARMOPHOREA . ( a ) Kinetosomal replication begins at the \u201cequatorial ends\u201d of a number of somatic kineties. ( b ) Oral polykinetids assemble through side-by-side alignment of dikinetids units. ( c ) The posterior ends of several somatic kineties adjacent to the develop- ing oral region disassemble, and it may be that the paroral ( d, e ) is assembled from these as division proceeds. (from Foissner & Agatha, 1999.) 8.5 Division and Morphogenesis 183 184 8. Subphylum 2. INTRAMACRONUCLEATA: Class 2. ARMOPHOREA stomato genesis between these two genera. Caenomorpha undergoes a complicated post-sto- matogenesis morphogenesis, reminiscent of the enantiotropic division of some oligotrichous spiro- trichs (Martín-González et al., 1987). Considering the current evidence, we are not convinced that the differences between metopids and caenomorphids are sufficient to justify ordinal status for these two groups, as suggested by Foissner and Agatha (1999). Santos, Guinea, and Fernández-Galiano (1986) have provided a preliminary account of stoma- togenesis in the clevelandellid Nyctotherus . Breaks occur in somatic kineties posterior to the oral region and kinetosomal proliferation occurs at the anterior ends of these breaks. A lateral groove develops as proliferation proceeds and primordium elements on the posterior wall of the groove differentiate as oral polykinetids while those on the anterior wall develop as paroral dikinetids, eventually forming the two files of the diplostichomonad (Santos et al., 1986). This is clearly a pleurotelokinetal stomato- genesis , showing significant similarities to that of the armophorids. 8.6 Nuclei, Sexuality and Life Cycle Armophoreans have the typical complement of macronucleus and one or more micronuclei . The macronuclei can also be variable in number in caenomorphids , sometimes numbering more than four (Fig. 8.1). In smaller forms, the macronucleus is typically globular to ellipsoid, but in larger clevelandellids it can become elongated and quite irregular in shape. The macronucleus of some clevelandellids is \u201csuspended\u201d from the cortex by microfibrillar strands that collectively are called the karyophore (Fig. 8.1). Eichenlaub-Ritter and collaborators have under- taken some detailed