Pré-visualização5 páginas
233 Abstract Ciliates in the Class NASSOPHOREA have played a pivotal role in phylogenetic schemes of the evolution of diversity of ciliates. Their simplified oral structures were thought to represent the ancestral condition of the more well-developed oral polykinetids of oligohymenophoreans, hetero- trichs, and spirotrichs. They are united by two ultrastructural features: alveolocysts are a presumed synapomorphy of all representatives, although they have not been observed yet in synhymeniids; and the nematodesmata of the nasse bear nematodesmal or X-lamellae, which are not found in the phyllo- pharyngean cytopharyngeal basket. The highly developed nasse is used to ingest various \u201calgae\u201d, typically cyanobacteria such as Anabaena and Oscillatoria , whose natural populations in rare instances nassophoreans may control. The somatic cortex has a highly developed epiplasm. In addition to the nasse, there is a set of \u201coral\u201d polykinetids that extends often around the body circumference as a linear assemblage called a frange or synhy- menium. This is why stomatogenesis in these forms is considered mixokinetal because both somatic and oral kinetal elements are involved. The genetics of these ciliates is virtually unexplored so details of conjugation, mating type system, and nuclear development remain to be discovered. Keywords Cyrtos, articulins, B-cartwheel, pavés, blue-green algae The ancestors of Pseudomicrothorax , a ciliate now assigned to the Class NASSOPHOREA , were argued to have played a pivotal role in the evolu- tion of the oligohymenophoreans (Corliss, 1958a, 1958b; Thompson & Corliss, 1958). This was based on both the revelation by silver staining of three adoral polykinetids , similar in position to those of the Class OLIGOHYMENOPHOREA , and in the mode of stomatogenesis. The \u201coral\u201d ciliature of nassophoreans is typically arranged as a hyposto- mial \u201cfrange\u201d , an extensive ventral band of more complex kinetids that courses slightly posterior to the cytostome and may extend onto the dorsal surface (Fig. 11.1). Fauré-Fremiet (1967a, 1967b) analyzed this ciliary \u201cfrange\u201d and the adoral struc- tures of other nassulid -like ciliates, Chilodontopsis , Nassulopsis , Nassula , Cyclogramma , Paranassula , and Pseudomicrothorax , and argued that, despite their diversity, these oral structures could all be considered homologues, justifying the recognition of a clade of nassulid ciliates. De Puytorac, Grain, Legendre, and Devaux (1984) demonstrated that cortical ultrastructural features related peniculines (e.g., Paramecium , Frontonia ) and nassulids , separating them from the hymenostomes (e.g., Glaucoma , Tetrahymena ). This analysis expanded on the previous, more restricted analysis of Lynn (1979a) who had shown that nassulids , peniculines , and hymenostomes were all related using phyl- lopharyngeans as the outgroup taxon: nassulids were the basal clade of the three (Lynn, 1979a). Sequence analyses of the large and small subunit rRNA genes have confirmed a close relationship between nassulids , peniculines , and hymenostomes (Baroin-Tourancheau, Villalobo, Tsao, Torres, & Pearlman, 1998; Bernhard, Leipe, Sogin, & Schlegel, 1995; Strüder-Kypke, Wright, Fokin, & Lynn, 2000b). Histone gene sequence similarities Chapter 11 Subphylum 2. INTRAMACRONUCLEATA: Class 5. NASSOPHOREA \u2013 Diverse, Yet Still Possibly Pivotal Fig. 11.1. Stylized drawings of representative genera from the orders in the Class NASSOPHOREA . The synhyme- niids Nassulopsis , Chilodontopsis , and Scaphidiodon . The nassulid Obertrumia related nassulids and hymenostomes (Bernhard & Schlegel, 1998) although the \u3b1-tubulin gene sequence of Zosterodasys does not support this rela- tionship (Baroin-Tourancheau et al., 1998). Overall, the earlier conception that nassulid -like ciliates were ancestors for the oligohymenophoreans still seems a reasonable view (see below Division and Morphogenesis ). Ciliates in this class are typically holotrichous. Larger nassulids , which can be >200 µm in length, are densely ciliated. However, some of the smaller microthoracids , which may be about 10 µm in length, can exhibit regions of the cortex that are barren of cilia, including the dorsal surface in discotrichids . Scaphidiodon is tentatively placed in this class, although it has three features that relate it to the cyrtophorian phyllopharyngeans : (1) a non- ciliated dorsal surface; (2) right somatic kineties that arch over the anterior end onto the left ventral surface and terminate on the anterior suture; and (3) a podite -like appendage at the posterior end (Dragesco, 1965). The pattern of the somatic cili- ation of other nassophoreans is also similar to that of cyrtophorians as the right somatic kineties may arch over the oral region onto the left ventral surface (Deroux, 1994b). Small and Lynn (1981) were the first to elevate this group to the class level, establishing the Class NASSOPHOREA . The class derives its name from the French \u201cnasse\u201d meaning basket and the Greek phoros meaning to bear. This refers to the com- plex cytopharyngeal basket of nematodesmata that are used in feeding. Original descriptions of the ultrastructure of the nasse (Fauré-Fremiet, 1962a) stimulated later research on the structure, function, and development of this complex microtubular apparatus in Nassula (Tucker, 1968, 1970a, 1970b). Earlier demonstration of the thick epiplasm in Pseudomicrothorax (Fauré-Fremiet & André, 1967) has led to the discovery of a novel class of pro- teins, the articulins , which are found in ciliates and euglenoid flagellates (Huttenlauch & Stick, 2003; Huttenlauch, Peck, & Stick, 1998a). Cellular and biochemical research has been possible because these ciliates can be easily grown on filamentous cyanobacteria (Peck, 1977b; Tucker, 1968). Members of the class are united by two synapomor- phies: (1) the presence of alveolocysts , extensions of the cortical alveoli into the cytoplasm; and (2) the presence of nematodesmal or X lamellae , accompa- nying the nematodesmata of the nasse (Eisler, 1989; Eisler & Bardele, 1983). These two features are presumed to be present in synhymeniids , although ultrastructural analysis of their nasse is needed to confirm this (see Taxonomic Structure ). 11.1 Taxonomic Structure Corliss (1979) placed nassophorean ciliates in the Subclass Hypostomata of the Class KINETOFRAGMINOPHORA based on the pres- ence of a hypostomial \u201cfrange\u201d that extends to varying degrees across the ventral surface of the cell and that may ultimately be restricted to the oral region. Small and Lynn (1981, 1985) were led by similarities in the somatic kinetids and extrusomes to include synhymeniids , nassulids , microthorac- ids , peniculines , and hypotrichs in their newly conceived Class NASSOPHOREA . Gene sequence data have now refuted a close relationship of hypotrichs with these taxa and demonstrated that peniculines are a basal clade in the oligohy- menophorean radiation (e.g., Baroin-Tourancheau, Delgado, Perasso, & Adoutte, 1992; Lynn & Sogin, 1988; Strüder-Kypke et al., 2000b). Fauré-Fremiet (1967a) set the conceptual perspec- tive for phylogeny within this class by proposing a phylogenetic transformation series for the ciliary \u201cfrange\u201d , the French for fringe. Some synhymeniids are considered to represent its ancestral state: a transverse line of dikinetids, not well differentiated from the adjacent somatic monokinetids, extend- ing completely across the ventral surface and onto the dorsal surface (Fig. 11.1) (e.g., Zosterodasys , formerly Chilodontopsis ). It is imagined that these dikinetids became polymerized into the \u201cpavés\u201d , French meaning paving-stone or tile, or small polykinetids (e.g., some Nassulopsis species). These polykinetids