THE EUEXCAVATAE
| SYSTEMATIC BIOLOGY |
THE EUEXCAVATAE |
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| HOME | SYLLABUS | WEEKLY ASSIGNMENTS | J. SYSTEMATIC BIOLOGY | TAXA OF LIFE | |
| PHYLUM PARABASALA | |||||
INTRODUCTION TO THE PARABASALA
Parabasala (pa-ra-BA-sa-la) comes from two Greek roots that mean beside (para -παρα) and base (bas-e -βάση). The reference is to a specialized organelle that is intimately associated with the basal bodies in this phylum.
The parabasalids are large multiflagellated unicells that inhabit the guts of insects. They seem to be united by the synapomorphy of a large modified golgi called a parabasal body, a structure that gives the group its name. Some taxa are associated with mammals and birds. Trichomonas vaginalis is an important agent of human disease. Others are important as commensals that allow termites and wood-eating cockroaches to digest the cellulose of wood.
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A. Trichomonas cells stained to show nuclei. Recurrent flagellum forms an undulating membrane which is evident on the lower cell. |
B. A drawing of Lophomonas to show the nucleus encapsulated by flagellar roots, which then form an elongate axostyle. |
C. A DIC micrograph of Spirotrichonympha showing the spiral of karyomastigonts down the cell. |
D. A DIC micrograph of Trichonympha. |
| Images taken from: A: http://www.medicine.mcgill.ca/tropmed/ B&C: http://microscope.mbl.edu/ D: The Systematic Biology Biodiversity Collection |
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SYNOPTIC DESCRIPTION OF THE PARABASALA
| The following description comes from Grell (1976), Dyer (1990c), Lee et al. (1985), Margulis et al. (1990), and Cavalier-Smith (2003). |
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I. SYNONYMS: Trichomonads, Hypermastigids, Polymonads. II. NUMBER: >2,000 species known. III. PHYLUM CHARACTERISTICS: A. Structure and Physiology Cell Form: Unicellular. Flagella: Several or many flagella borne on the anterior end of the cell; in clusters of 4 or multiples of 4 (?) with 3 anterior and one posterior (recurrent) flagellum. Basal Bodies: At least some of the basal bodies are associated with parabasal structures composed of striated filamentous fibers with closely allied golgi complexes. Cytoplasmic microtubular elements also arise from the basal bodies. Basal bodies appear to be parallel. Cell Covering: Naked; with specialized cytostome-like region on posterior end of cell. Chloroplasts: Not present. Food Reserves: Not reported. Mitochondria: Not present, but hydrogenosomes, anaerobic metabolic organelles, in some. Golgi: Present and modified to parabasal body, a golgi complex that is attached by a striated root, the parabasal root, to the nucleus. Nucleus: In karyomastigonts; or the axostyle associated with nucleus (sometimes encapsulating it). Chromosomes remain condensed during interphase. Centrioles: Not reported but certain basal bodies with elaborate "atractophores" behave as centrioles. Inclusions and Ejectile Organelles: Food vacuoles; B. Mitosis, Meiosis and Life History Mitosis: The nuclear membrane remains intact (closed) during mitosis; the spindle is extranuclear and extends across one side of the nucleus with chromosomes connected to microtubules at kinetochores situated in the nuclear envelope; the spindle poles are associated with "atractophores" (fibrous extensions from certain basal bodies). Meiosis: Presumed. Sexual Reproduction and Life History: Reported in some. C. Ecology: Mostly endobiotic and phagotrophic. A few taxa are free-living. |
Margulis and Schwartz (1988) and Sleigh et al. (1984) considered the parabasalids to be a somewhat coherent group, unified by the presence of a modified golgi body called a parabasal body. Grell (1976) and Lee et al. (1985) lumped these organisms together with the metamonads (and other flagellated unicells). Likewise, Margulis and Schwartz (1988) and Margulis et al. (1990) treated this group as part of a large, heterogeneous collection of flagellated organisms called the "Zoomastigina" (Pr-8). Later, Margulis and Schwartz (1998) grouped these with other amitochondriates in a phylum called Archaeprotista (Pr-1), greatly divergent in form and presumably primitive in the eukaryotic tree. Earlier, Sleigh et al. (1984) raised the classes of the "Zoomastigina" to the phylum level in recognition of their disparity in structure. Prior to that, Taylor (1976) suggested that the Hypermastigid-Trichomonad-Diplomonad-Retortamonad complex was not primitive and had affinities with the chrysophytes. More recently, Taylor (1999) and Tudge (2000) by using both molecular and ultrastructural evidences supported the assumption that these were primitive taxa and a natural grouping. Thus their chaotic history seemed to be resolved by the Roger (1999) modification of the Archezoa Hypothesis. Then, revelations since 2000 (summarized in Cavalier-Smith 2003) showed that the excavates were not primitive, and their apparent primitive state was due to the loss of mitochondria and an artifact of phylogenies based on single gene sequences. Baldauf (2003) presents a consensus view based on molecular and ultrastructural characters in which this group is no longer considered primitive. In this system, the parabasalids are considered part of a natural group called the excavates which includes the diplomonads, retortomonads, and oxymonads.
HIERARCHICAL CLASSIFICATION OF THE PARABASALA
| TAXONOMY OF THE PHYLUM PARABASALA. This system is a modification of Cavalier-Smith (2003). Details are informed by many earlier works including Grell (1976), Kudo (1966), Lee et al. (1985), and Margulis et al. (1990). |
CLASS TRICHOMONADEA
CLASS TRICHONYMPHEA
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This page is maintained by Jack R. Holt. Last revised 02/14/2008.
