Pré-visualização13 páginas
89 Abstract The ciliated protozoa are a distinct group of protists characterized by (1) the presence of cilia de- rived from kinetosomes with three \ufb01 brillar associates; (2) nuclear dimorphism; and (3) conjugation as a sexual process. They are exceedingly diverse in shape and size, and may have evolved over 2 billion years ago. The ciliate body form likely evolved from a \ufb02 agellate that proliferated kinetids near its oral apparatus, and these kinetids became \ufb01 rst the ciliate paroral, which itself replicated to provide somatic kineties and \ufb01 nally the adoral kinetids. Ciliates are now divided into two subphyla and 11 classes based on features of nuclear division and the pattern of \ufb01 brillar associates in their somatic kinetids. They are found in a diversity of microhabitats, with the majority of species likely cosmopolitan. However, endemism appears to be characteristic of perhaps as many as 30% of the species. Ciliates are important components of the microbial loop, often responsible for consuming the majority of primary production and bacterial production in certain habitats. Their development is complex since the kinetid patterns of the somatic and oral cortex are them- selves complex. This complexity has made them useful models for developmental biologists while the systematists have used these patterns to relate different taxa to each other. The life cycles of cili- ates are also complex and clearly separate sexual processes from asexual reproduction. Sexual proc- ess, conjugation, occurs in the context of a breeding strategy, ranging from an inbreeding one to an outbreeding one. A new macronucleus typically develops at each conjugation cycle through a com- plicated set of processes of DNA fragmentation, diminution, and replication. However, this process likely occurs at each cell cycle in karyorelictean ciliates whose macronuclei do not divide. Keywords Cytotaxis, structural guidance, extru- some, metachrony, alveoli The ciliates, without doubt, are a most homo- geneous group, which have long been separated from other protists. They are briefly distinguished by three major features: (1) by their cilia, variable in number and arrangement, distributed over the body surface and derived from kinetosomes with three typical fibrillar associates \u2013 the kinetodesmal fibril , the postciliary microtubular ribbon , and the transverse microtubular ribbon ; (2) by their two kinds of nuclei \u2013 a macronucleus and a micro- nucleus , the former controlling the physiological and biochemical functions of the cell and the latter as a germ-line reserve; and (3) by conjugation , a sexual process in which partners typically fuse temporarily to exchange gametic nuclei. All cili- ates are heterotrophic and most possess a mouth, although some are mouthless (e.g., the astomes ) and others could be called polystomic (e.g., sucto- rians with their tentacles). Ciliates vary in shape and size. There are stalked and colonial species that have unusual forms, but generally the shapes are simple geo- metric ones \u2013 spheres, cones, oblate spheroids, and cylinders, which may be flattened dorsoven- trally in substrate-oriented species. Body form is relatively permanent since the cortex is supported by a complex microtubular and microfilamentous cytoskeleton. Ciliates range in size from 10 µm in very small spheroid forms to 4,500 µm in highly Chapter 4 Phylum CILIOPHORA \u2013 Conjugating, Ciliated Protists with Nuclear Dualism 90 4. Phylum CILIOPHORA \u2013 Conjugating, Ciliated Protists with Nuclear Dualism elongate and contractile ones. Biovolume ranges give another impression of size: in the Class COLPODEA , cell volume ranges from 10 2 µm 3 for Nivaliella to 10 8 µm 3 for Bursaria ; and in the Class OLIGOHYMENOPHOREA , cell volumes range from 10 3 µm 3 for some Cyclidium species to >10 9 µm3 for the trophonts of Ichthyophthirius (Lynn & Corliss, 1991). Ciliates were first observed microscopically by van Leeuwenhoek (1674), the founder of pro- tozoology (Corliss, 1975b). Ciliates were visible as blooms or colored waters in both marine and freshwater habitats, probably thousands of years before van Leeuwenhoek\u2019s discoveries. Their for- mal nomenclatural history begins in 1767 with the establishment of Vorticella by Linnaeus (Aescht, 2001). They were named the INFUSORIA through- out the 19th century, a name that was replaced in the early 20th century by the present name of the phylum \u2013 CILIOPHORA . The ciliates were an isolated group until 1991 when it was proposed that the membrane-bound sacs underlying their plasma membrane, the alveoli , were a synapo- morphy or shared-derived character for a major clade of protists, the ALVEOLATA , which initially included dinoflagellates , apicomplexans , and cili- ates (Cavalier-Smith, 1991; Wolters, 1991). This clade was reaffirmed in the revised classification of the protists proposed by Adl et al. (2005). Based on strong molecular evidence, this clade now includes these three major groups, along with the aber- rant \u201c dinoflagellate \u201d Oxyrrhis and the perkinsids associated with the dinoflagellate clade and several flagellates, such as Colpodella , associated with the base of the apicomplexan clade. Dinoflagellates and apicomplexans are strongly supported as sister taxa, leaving the ciliates as a separate lineage (Leander & Keeling, 2003). When and how the cil- iates diverged from the alveolate common ancestor are still open questions, but we have some ideas. With regard to when, ciliates probably arose in the Precambrian era . While there are no fos- sils from this time, Wright and Lynn (1997c) argued that the phylum could be over 2 billion years old, based on the rate of evolution of the small subunit rRNA molecular clock (Fig. 4.1). While this estimate can be challenged as it makes some strong assumptions on the constancy of the molecular clock , there is no dispute over fossils of the loricate tintinnines , which stretch from the Ordovician period of the Paleozoic era , some 400\u2013450 million years before present (myBP) into the Pleistocene , about 1 myBP. Tintinnine fossils are most abundant in the sediments of the Jurassic and Cretaceous periods of the Mesozoic era (see Bonet, 1956; Colom, 1965; Tappan & Loeblich, 1968, 1973). Fossils of other loricate forms have also been found: Deflandre and Deunff (1957) reported a fossil folliculinid heterotrich while Weitschat and Guhl (1994) described several fossil loricate peritrichs from the Lower Triassic period . Even more recently, extremely well-preserved ciliates, a Paramecium species and several colpodeans, have been described from amber derived from southern Germany in amber-bearing sandstone deposits (Schönborn, Dörfelt, Foissner, Krientiz, & Schäfer, 1999), which have now been assigned to the Late Cretaceous , some 90 million years ago (Schmidt, von Eynatten, & Wagreich, 2001). However, these fossil forms can generally be placed in contempo- rary families and even genera, so they provide little insight into the origin of the ciliates. Nevertheless, this has not deterred some speculation on how ciliates diverged. Given the molecular phylogenies, there is no doubt that the ciliates evolved from a flagellate ancestor. Two major features of ciliates \u2013 their complex cortex and their nuclear dualism \u2013 have preoccupied those who have speculated on how a flagellate ancestor might have evolved into the ancestral ciliate. The complex ciliate cortex undoubtedly evolved from a simpler flagellate dikinetid. Earlier evolu- tionary schemes of Orias (1976) and Small (1984) argued that the dikinetids of the flagellate ances- tor replicated to form first a \u201ctorsionless chain\u201d, similar to the polynergid dinoflagellate Polykrikos , and then a ribbon-like form with multiple somatic kineties . Orias imagined that the ribbon-like