Pré-visualização11 páginas
187 Abstract The ciliates in this class are divided into two major assemblages, represented by the Subclass Haptoria and Subclass Trichostomatia. The hapto- rians are predatory ciliates that are commonly found in a variety of habitats, feeding on \ufb02 agellates and other ciliates, which they immobilize and kill with extrusomes called toxicysts. An exception to this rule is the marine planktonic haptorian Myrionecta rubra , which harbors a cryptophyte endosymbiont and which produce red tides that contribute up to 70% of the primary production. The trichostomes have lost toxicysts and are all endosymbionts in a variety of metazoans, ranging from \ufb01 sh to humans. In fact, the only ciliate known to be pathogenic to humankind is the trichostome Balantidium , which can be an intestinal parasite. The somatic monokinetid of litostomes is unique in possessing two transverse microtubular ribbons, T1 and T2 \u2013 a strong synapo- morphy for the class. Oral structures are typically simple, hence the name litos (Gr.) for simple. Hap- torians typically have either circumoral dikinetids or oralized monokinetids. Trichostomes show more diversity: some forms, like Balantidium , have a vesti- bulum with extensions of densely packed somatic kineties lining it, while the entodinio morphids have polybrachykineties, more complex assemblages of short kinetofragments. Stomato genesis is character- ized as telokinetal, but there is a range of types from holotelokinetal to crypto telokinetal. Keywords Karyoklepty, pexicyst, bulge micro- tubules, conocyst The ciliates included in this class are divided into two subclasses, the Subclass Haptoria and the Subclass Trichostomatia . The former includes free- living and the latter endosymbiotic forms. These cili- ates are extremely variable in size and form. Small endosymbiotic trichostomes can be around 50 µm in length while some free-swimming haptorians , such as Homalozoon species, can be 2,500 µm long! Form is no less variable: small forms are typically ovoid while elongate forms are ribbon- like, flexible, and contractile. Endosymbiotic forms can have lobes, spines, and unusual cell processes, while some free-swimming forms have extensible \u201cnecks\u201d, flexible proboscises, and toxicyst-bearing tentacular processes. Body ciliation is also vari- able, ranging from isolated tufts and bands in entodiniomorphid trichostomes to holotrichous ciliation in most haptorians . Free-living hapto- rians are distributed world-wide in freshwater and marine habitats and are characterized as voracious predators of flagellates, other ciliates, and even small metazoans. The endosymbiotic trichostomes are found in a variety of vertebrates, ranging from fish to rep- tiles and mammals where they typically consume bacteria and plant material. Trichostomes inhabit two major groups of mammals : (1) they are found in ruminants or foregut fermenters, such as cattle , sheep , hippopotamus , and kangaroos ; and (2) they are found in various hindgut fermenters, such as horses , tapirs , and some anthropoid apes . The only ciliate known to be pathogenic to human beings is the trichostome Balantidium coli , which damages the intestinal mucosa of humans. Balantidium has also been found in some invertebrates and other verte- brates. In the vast majority of human infections, there has been a history of human contact with pigs Chapter 9 Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA \u2013 Simple Ciliates but Highly Derived 188 9. Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA (Ferry et al., 2004; Zaman, 1978), although rare human cases occur apparently with no reported contact with pigs (Anargyrou et al., 2003). Culture methods have been developed for some of the endosymbiotic forms. Entodiniomorphids can be cultured short-term or long-term on several different media that model the rumen microenvironment (Bonhomme, Fonty, & Senaud, 1982; Coleman, Laurie, & Bailey, 1977; Hillman, Williams, & Lloyd, 1991; Michalowski, Muszy\u148ski, & Landa, 1991). Essential lipids have extended cultivation of ento- dinia for more than 3 months (Hino, Kametaka, & Kandatsu, 1973). These cultivation methods have enabled exploration of the biochemistry and physi- ology of rumen ciliates. Methods for cultivation of Balantidium have been in use since the early 1900s, modeled on media that support the growth of Entamoeba histolytica (Zaman, 1978). Klaas (1974) has reported on media that enable cultiva- tion of some isolates for well over 2 years. Cox (1963) has argued that initiation of cultures is the only reliable method for diagnosis of Balantidium coli infections. The name of the Class LITOSTOMATEA is derived from the Greek litos meaning simple and the Greek stoma meaning mouth. Small and Lynn (1981) sug gested it as a replacement name for the classical Gymnostomata (i.e., Greek gymnos mean- ing naked) to which a number of the included taxa were assigned by Corliss (1979) and others. Electron microscopic research on the litostomes had revealed that the mouth was not naked, but encircled by a spe- cialized, but simple, circumoral infraciliature (Grain, de Puytorac, & Bohatier, 1973). The litostomes have long been regarded as little-modified descendants of the most primitive ciliates. However, ultrastructural research led several workers (Bardele, 1989; Small & Lynn, 1981) to argue that the simplified nature of litostomes was likely secondarily derived. This has been confirmed by rRNA gene sequences that show litostomes to be several branches removed from the base of the ciliate \u201ctree\u201d (Baroin-Tourancheau, Delgado, Perasso, & Adoutte, 1992; Baroin- Tourancheau, Villalobo, Tsao, Torres, & Pearlman, 1998; Leipe, Bernhard, Schlegel, & Sogin, 1994; Wright, Dehority, & Lynn, 1997). While \u3b1-tubulin sequences suggested that litostomes were indeed basal in the ciliate tree (Baroin-Tourancheau et al., 1998), it is thought that this may be due to the poor resolving power of this gene. Small and Lynn (1981) established the mono- phyly of the class based on the ultrastructural pattern of the somatic kinetids . These are monoki- netids that were ultimately shown to have a con- vergent postciliary ribbon, whose microtubules are arranged in a double-row configuration, and two transverse ribbons (Leipe & Hausmann, 1989; Williams, Williams, & Hogan, 1981). This kinetid pattern is the primary synapomorphy for the class. Three other features unite the ciliates in this class. First, the oral kinetids are either monokinetids or dikinetids whose transverse ribbons extend to sup- port the cytopharyngeal apparatus , which is called a rhabdos . In all other ciliate classes, cytopharyn- geal ribbons are derived from postciliary ribbons. Second, there are regions of at least several somatic kineties in holotrichous species whose ciliature is differentiated as clavate cilia , forming a clavate field or brush (Foissner, 1996b). Third, conjugation in litostomes is often preceded by a preconjuga- tion cell division during which the first meiotic reduction division occurs leaving characteristically swollen nuclear division products. Although a preconjugation division has a scattered distribution among other classes (Raikov, 1972), we regard it as convergently evolved in each group since only lito- stomes show the micronuclear swelling, although there may be exceptions (see Xu & Foissner, 2004). Finally, McEwan et al. (2000) demonstrated a bias against the use of G in the third position of the codons for lysine, glutamine, and glutamic acid in entodiniomorphid ciliates (e.g., Entodinium , Epidinium , Polyplastron ) and one haptorian (i.e., Spathidium ). Could this be a molecular synapo- morphy for the Class LITOSTOMATEA? 9.1 Taxonomic Structure Corliss (1979) placed the major groups now included in the Class