Pré-visualização17 páginas
to refine its taxonomic position. Corliss (1979) placed the dorsoventrally flat- tened spirotrichs with prominent ventral cirri and less conspicuous dorsal bristle cilia in the Order Hypotrichida . Characters that support the monophyly of this group include the pattern of development of the ventral cirri and the overall organization of the body plan, which are claimed to be too similar to have evolved convergently (Fleury, 1988; Martin, 1982; Tuffrau, 1987). At the same time, there is a recognition of considerable diversity within the group, both in respect to mor- phology, morphogenesis, and genetics. Fleury and coworkers (Fleury, 1988; Fleury, Iftode, Deroux, & Fryd-Versavel 1985a; Fleury, Iftode, Deroux, Fryd- Versavel, & Génermont, 1985b; Fleury, Iftode, Deroux, & Fryd-Versavel, 1986) have used these morphological and morphogenetic differences to establish the suborder Euhypotrichina (Fleury et al., 1985a) to include stichotrich-like forms and the suborder Pseudohypotrichina (Fleury et al., 1985b) to include euplotid-like forms. Euhypotrichs were so-named because they manifested truly derived \u201chypotrich\u201d characters: (1) the complete turn-over or replacement of the somatic ciliature, both ventral and dorsal, during all morphogenetic processes; (2) cysts that typically resorb all infraciliary con- stituents (i.e., kinetosome-resorbing cysts); and (3) dorsal bristles that lose kinetodesmal fibrils at the completion of development. In contrast, pseudo- hypotrichs were so-named because they appeared \u201chypotrich\u201d-like but lacked the derived characters noted above. Instead, they exhibit: (1) turn-over or replacement of only the ventral somatic infracili- ature during morphogenetic processes; (2) encyst- ment typically not accompanied by resorption of all kinetosomes; and (3) mature dorsal bristles that retain kinetodesmal fibrils . Small and Lynn (1981, 1985) had used these kinetid differences, along with differences in the organization of the somatic cortex, to separate the hypotrichs into two clades: the Subclass Stichotrichia Small and Lynn, 1985, which is equivalent to the euhypotrichs (Fleury et al., 1985b) and the Oxytrichia (Tuffrau & Fleury, 1994) of others; and the Subclass Hypotrichia Stein, 1859, which is equivalent to the pseudo- hypotrichs (Fleury et al., 1985b) and the Euplotia (Tuffrau & Fleury, 1994). Small and Lynn (1985) 7.1 Taxonomic Structure 145 146 7. Subphylum 2. INTRAMACRONUCLEATA: Class 1. SPIROTRICHEA placed the Subclass Hypotrichia within the Class NASSOPHOREA , based on similarities in the diki- netid of members of these two groups. However, molecular evidence (Lynn & Sogin, 1988) clearly refuted this relationship, demonstrating instead that stichotrichs and hypotrichs belong to the same major clade, a fact that has led Lynn and Corliss (1991) and Lynn and Small (1997) to place both groups in the Class SPIROTRICHEA . The stichotrichs and hypotrichs are separated deeply on small subunit ribosomal RNA (SSrRNA) gene trees with oligotrich and choreotrich genera diverging after the separation of the hypotrich lin- eage (Bernhard et al., 2001; Chen & Song, 2001; Petroni, Dini, Verni, & Rosati, 2002; Snoeyenbos- West, Salcedo, McManus, & Katz, 2002; Strüder- Kypke et al., 2003). Moreover, there are substantial and deep differences in the allele frequencies of the isoenzymes of hypotrichs and stichotrichs (Schlegel & Steinbrück, 1986). We have favored retaining separate subclasses for these four groups notwith- standing the fact that the apparently fast molecular clock of the hypotrich ribosomal RNA genes may be creating a \u201ctreeing artifact\u201d for these molecular sequences due to the long branch attraction artifact (Felsenstein, 1978; Morin, 2000; Philippe, Chenuil, & Adoutte, 1994). Molecular phylogenetic studies on a number of hypotrich genera, such as Euplotes and Diophrys , consistently demonstrate them as basal lineages in the spirotrich radiation (Fig. 7.2). Depending upon taxon sampling and variations in sequence align- ment, the hypotrichs appear to be monophyletic (Li & Song, 2006; Petroni et al., 2002) or paraphyletic (Chen & Song, 2001, 2002; Song, Wilbert, Chen, & Shi, 2004). Subdivision of the genus Euplotes is supported by some molecular analyses (Bernhard et al., 2001; Borror & Hill, 1995). We now rec- ognize two orders in the Subclass Hypotrichia : (1) Order Kiitrichida to include spirotrichs with uniformly small and multiple cirri arranged in curving files; and (2) Order Euplotida to include those with well-developed and sparse ventral cirri arranged in frontal, ventral, and transverse groups (see Chapter 17 ). Features of division morphogenesis have also been used to reevaluate systematic relation- ships among oligotrich ciliates and these have now been tested by sequences of the small subunit ribosomal RNA genes . Oligotrich and choreotrich SSUrRNA gene sequences support retention and separation of genera assigned to these two subclasses (Snoeyenbos-West et al., 2002; Strüder-Kypke et al., 2003). Corliss (1979) placed the Order Oligotrichida in the Class POLYHYMENOPHOREA , noting that its two suborders, the Oligotrichina and Tintinnina , were united by a reduced somatic ciliature and an anterior adoral zone of membranelles, which are primarily used in locomotion and feeding. Fauré- Fremiet (1970) had already drawn attention to the diversity in the oral structures of the oligotrichs , recognizing strombidiids , like Limnostrombidium and Laboea (Fig. 7.3) and halteriids , like Halteria (Fig. 7.4), to have an \u201copen\u201d adoral zone with what has been called a \u201ccollar\u201d and \u201clapel\u201d arrange- ment of the oral polykinetids. In contrast, there were the strombidinopsids , like Strombidinopsis , strobilidiids , like Strobilidium , and tintinnids , like Codonella and Tintinnopsis , with a \u201cclosed circle\u201d of oral polykinetids (Fig. 7.3). Fauré-Fremiet (1970) retained the classical sepa- ration of the tintinnids , based on their being loricate , even while noting the similarity in the \u201cclosed circle\u201d oral structures of strobilidiids and tintinnids . Small and Lynn (1985) established the Subclass Choreotrichia Small and Lynn, 1985 to include these latter two groups, placing the aloricate chore- otrichs in the Order Choreotrichida Small and Lynn, 1985 and retaining the Order Tintinnida for loricate choreotrichs. Division morphogenesis of oligotrichs was initially described by Fauré-Fremiet (1953), who described their division as enantiotropic and noted that the oral primordium of Strombidium developed in an invagination. Deroux (1974) con- firmed that oral development occurred in a pocket in Strombidium sulcatum , provided a new descrip- tion for stomatogenesis in Strobilidium gyrans (= Strobilidium caudatum ?), and drew attention to the similarity with the early stages of division mor- phogenesis in Euplotes , which also occurs in a cor- tical invagination (Wise, 1965). Petz and Foissner (1992) confirmed this \u201cpocket\u201d oral development in S. caudatum and also observed it in a Tintinnidium species while Dale and Lynn (1998) observed the same pattern of \u201cpocket\u201d oral development in the aloricate choreotrich Strombidinopsis . Petz and Foissner (1992) used this as an additional charac- ter to support the monophyly of the choreotrichs . Strombidium -like species may begin development of Fig. 7.3. Stylized drawings of representative genera from subclasses in the Class SPIROTRICHEA . Subclass Choreotrichia : the choreotrich Strombidinopsis ; the tintinnid Codonella ; the choreotrich Strobilidium ; the tintinnid Tintinnopsis ; the tintinnid Cymatocylis . Subclass Oligotrichia : Limnostrombidium ; Laboea 7.1 Taxonomic Structure 147 148 7. Subphylum 2. INTRAMACRONUCLEATA: Class 1. SPIROTRICHEA