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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Postgaardi

Postgaardi Fenchel et al., (1995) (ref. ID; 4885)

[ref. ID; 4885]
Remarks; [Similarities between Postgaardi and Euglenozoa]: Members of the Euglenozoa show a common pattern of organisation of flagellar roots that has been one of the most compelling synapomorphies for the group (Kivic & Walne 1984). The system of three microtubular roots, with one closely associated with the outside of each basal body while a third root originates between them has been confirmed in several studies of kinetoplastids (Brugerolle 1985; Brugerolle et al. 1979; Nohynkova 1984), euglenids (Dawson & Walne 1991; Farmer & Triemer 1988; Hilenski & Walne 1985; Owens et al. 1988; Solomon et al. 1987, 1991; Surek & Melkonian 1986; Triemer & Farmer 1991; Willey & Wibel 1985, 1987) and Diplonema (Montegut-Felkner & Triemer 1994). Although it has not been extensively recognized, a reinforced, spaced, immediately sub-membranous structure, very similar to the MTR of bodonids (Brugerolle et al. 1979), is present in Diplonema and in at least one euglenid (Montegut-Felkner & Triemer 1994): The MTR has been shown to be an extension of the ventral root in several kinetoplastids (Brugerolle et al. 1979) and in one diplonemid (Simpson et al., unpubl.). The patterns of roots observed in Postgaardi are extremely similar to those described previously for Euglenozoa in terms of their number, origin, orientation and in the presence of an MTR. The paraxial rods of kinetoplastids and euglenids share a distinctive substructure unlike that of similarly located structures in other well known protists taxa such as dinoflagellates (Kivic & Walne 1984; Walne & Dawson 1993). In euglenids and kinetoplastids the paraxial rod other dorsal/anterior flagellum has a cylindrical cross-sectional appearance, while the structure in the ventral/recurrent flagellum is squarer in cross-section with a three dimensional lattice-work substructure (Brugerolle et al. 1979; Dawson & Walne 1991; Walne & Dawson 1993). Similar paraxial rods are also present (intermittently) in at least one diplonemid (Simpson, in press). The paraxial rods of Postgaardi are consistent with the pattern found in Euglnozoa. The extrusomes of Postgaardi are virtually identical to the organelle described as trichocysts by Brugerolle (1985). These tubular, parallel-sided extrusomes with a thick coat enclosing a faint cruciform structure have only been observed in a few kinetoplastids (Brugerolle 1985; Elbrachter et al. 1996; Mylnikov 1986; Vors 1992) and a single species of Diplonema (Schuster et al. 1968). Similar structures differing only in (possibly) lacking the cruciform internal structure are found in the eugenid Entosiphon sulcatum (Mignot 1963; Mignot & Hovasse 1973). Despite the often used name "trichocysts" they are not homologous to the trichocysts found in some ciliates. We prefer the term "tubular extrusome". Given their distribution across euglenids, kinetoplastids and Diplonema (and absence from any other taxa of eukaryotes), it is likely that these extrusomes are synapomorphic for the Euglenozoa, or at least, a major clade within the Euglenozoa. Other unusual similarities between Postgaardi and at least some members of all three established groups of Euglenozoa include a system of parallel, cell-supporting microtubules, a flagellar pocket, and permanently condensed chromosomes (Kivic & Walne 1984). The hydrogenosomes-like organelle of Postgaardi, which is bounded by two membranes and is presumably derived from a mitochondrion (see Fenchel & Finlay 1995) shows similarities to the mitochondria of euglenozoa in that its profiles tend to be located peripherally (Brugerolle et al. 1979; Pellegrini 1980; Vickerman 1991). No character was observed that suggested closer affinities with any other robust group of eukaryotes. All of the shared and shared-derived features mentioned above convincingly suggest that Postgaardi is a member of the Euglenozoa. (ref. ID; 4885)
Position of Postgaardi within the Euglenozoa: The euglenids as a group are identified by the presence of a pellicle -a system of strips of glycoprotein that subtend the plasma membrane and are supported by sub-pellicular microtubules (Leedale 1967; Larsen & Patterson 1990; Triemer & Farmer 1991). There is convincing evidence that these microtubules are homologous to the "dorsal band" microtubules which forms part of all of the cortical cytoskeleton in kinetoplastids and diplonemids (Brugerolle et al. 1979; Kivic & Walne 1984; Simpson et al., unpubl.) because they originate from the DR of the flagellar apparatus (Owen et al. 1988; Solomon et al. 1987; Triemer & Farmer 1991; Wiley & Wibel 1987). However, in both kinetoplastids and diplonemids, the cortical microtubules are very evenly spaced around the cell and are linked together by short arms of cytoskeletal material. In euglenids the microtubules have more complicated patterns of spacing related to overlying pellicular strips and are not as obviously connected to each other outside the reservoir (Kivic & Walne 1984). This study demonstrates that Postgaardi mariagerensis does not have a pellicle. The electron dense layer seen in Postgaardi differs from all euglenid pellicles in that it is positioned under the cortical microtubules, rather than being subtended by them and there is no evidence that it is composed of strips. In addition, the arrangement and connections of the cortical microtubules do not follow the "euglenid pattern" and are instead indistinguishable from pattern seen in large kinetoplastids and in diplonemids. We did no detect any other characteristic of Postgaardi that is otherwise restricted to some or all of the euglenids. Therefore, while there is strong evidence that Postgaardi is a euglenozoon, there is no evidence to support the placement of this genus in (or as the unambiguous sister group to) the euglenids, as was originally suggested (Fenchel et al. 1995). The cortical cytoskeletal similarities suggest that Postgaardi is more likely to be closely related to either the kinetoplastids genus Rhynchobodo, the enigmatic kinetoplastid Hemistasia. Species from these taxa share features which are otherwise very rare or completely lacking among other non-euglenid Euglenozoa. They are: 1) possession of euglenozoan trichocysts (Brugerolle 1985; Schuster et al. 1968) and ii) having basal bodies that insert almost parallel to each other (rather than at an angle of 30 degrees or more) and are separated by more than 100 nm. Comparatively few well defined features distinguish Rhynchobodo and the diplonemids. These include morphology of the mitochondrial cristae, presence or absence of a kinetoplast, feeding apparatus structure and its path through the cell. The unusual nature of the mitochondria (i.e. having a hydrogenosome-like appearance) in Postgaardi makes assessment of characters such as kinetoplastidy difficult, while the lack of cristae in Postgaardi is also presumably apomorphic. The shortness of the feeding apparatus in Postgaardi contrasts both with the long, straight structure in Rhynchobodo and the long, but looped, state seen in Diplonema (Montegut-Felkner & Triemer 1996). The arrangement of the mouth structures and gutters in Postgaardi appears to be highly apomorphic. These features tend to add uncertainty to the relationships among Postgaardi, diplonemids and Rhynchobodo. All well studied diplonemids have a distinctive feeding apparatus which includes vanes with fuzzy coats (Montegut-Felkner & Triemer 1996; Porter 1973; Schuster et al. 1968; Schnepf 1994; Simpson et al. unpubl.; Triemer & Ott 1990). Unlike all other Euglenozoa, except a few derived parasitic kinetoplastids, all known diplonemids lack paraxial rods for part or all of their life cycle. Although the cristae of Diplonema are often sparse, where present they form large flattened parallel sheets, rather than the rigid discs observed in kinetoplastids and euglenids. None of them are shared by Postgaardi (although mitochondrial cristae character is difficult to assess). There is no good evidence to suggest that Postgaardi belongs within, or is the sister group to the diplonemids. Rhynchobodo, as redescribed by Vors (1992), is distinguished from other genera of kinetoplastids and other Euglenozoa by the presence of a long, but not bulbous, rostrum which contains the tubular ingestion apparatus. The species that have been studied with electron microscopy (Brugerolle 1985; Mylnikov 1986; Simpson et al., unpubl.; Vors 1992) display a combination of features that are distinctive for Rhynchobodo, although no unique ultrastructural character has been identified (Simpson et al., unpubl.). Apart from a complete corset of linked cortical microtubules and, with one known exception, euglenozoan trichocysts, these features are a polykinetoplast, a long electron dense spur, and a microtubular prism associated with the ingestion apparatus (Brugerolle 1985; Simpson et al., unpubl.; Vors 1992). None of these latter features, or a prominent rostrum are present in Postgaardi. We therefore cannot regard Postgaardi as referable to Rhynchobodo. There is also no special similarity with Hemistasia, which has a mixture of Rhynchobodo-like and diplonemid-like ultrastructural features (Elbrachter et al, 1996; Simpson, in press). While it appears that Postgaardi is most similar to Rhynchobodo, we are unable to identify any potential synapomorphy to support the idea that these genera form a clade: Almost all of the similarities between Rhynchobodo and Postgaardi are also shared by Diplonema. The kinetoplast and microtubular prism of Rhynchobodo, lacking in Postgaardi, are formed (together) in some species of Bodo (Burzell 1975; Eyden 1977) and Cephalothamnium (Hitchen 1974): At present there is less evidence for Postgaardi being the closest relative to Rhynchobodo than for that position to be held by (a species of) Bodo. Postgaardi does not possess the synapomorphic/distinctive features of any of the three "established" groups of Euglenozoa and no sister group within any of these can be identified. Consequently Postgaardi can currently only be placed as Euglenozoa incertae sedis. Further ultrastructural studies of this genus and other poorly understood Euglenozoa-like organisms are now required to resolve the affinities among the groups that comprise this clade within eukaryotes. (ref. ID; 4885)
  1. Postgaardi mariagerensis Fenchel et al., 1995 (ref. ID; 4885) reported author and year? (ref. ID; 4898, 4980)

Postgaardi mariagerensis Fenchel et al., 1995 (ref. ID; 4885) reported author and year? (ref. ID; 4898, 4980)

Descriptions

The original description of Postgaardi (Fenchel et al. 1995) refers to a rigid cell covered in rod-shaped epibiotic bacteria, flagella inserting in an anterior pocket (one flagellum being acronematic) and an ovoid nucleus in the anterior half of the cell. The cells in Fenchel et al.'s (1995) account were about 20 um long and occurred only below the chemocline. In all of these respects, the Antarctic flagellate is indistinguishable from P. mariagerensis. The only discrepancy between our observations and the original description is that we usually observed the ends of the cell as pointed, rather than the cell being ovoid, s indicated by the text and drawing of Fenchel et al. (1995). W note that at least one of the micrographs published with the type description (Fig.16b of Fenchel et al. 1995) shows a cell with a pointed posterior. In the absence of nay clear distinguishable feature, we regard the Antarctic flagellate as Postgaardi mariagerensis. (ref. ID; 4885)
  • Light microscopy: Fixed cells examined in detail by light microscopy were 16-25 um long (n=15) and oval shaped with somewhat pointed ends. Two thick flagella, each about 2+1/2 times the length of the cell body insert in a large anterior depression. The cell is covered by longitudinally arranged rod-shaped epibiotic bacteria. In fixed material some of these bacteria can be seen peeling off from the flagellate. The nucleus is positioned somewhat anterior to the middle of the cell. The nuclear material has a lumpy appearance when viewed with Nomarski optics, suggestive of condensed chromatin. There is a prominent central nucleolus. Cells were mostly colourless, however some contained small green inclusions presumed to be ingested remnants of the phototrophic bacteria, Chlorobium sp. which occurs throughout the anoxic region of Burton Lake (Burke & Burton 1988). Live cells showed no red autofluorescence from chlorophyll, though a faint golden fluorescence of the cytoplasm was observed. Observation on living cells was difficult as the organism is extremely sensitive to light and temperature; cells survived longer in dark than illuminated refrigerated cabinets. Living cells swim slowly, spiralling around their long axis. No dividing cells were encountered. (ref. ID; 4885)
  • SEM: SEM demonstrates that the epibiotic bacteria have somewhat pointed ends and cover the whole flagellate, except for the inside of the anterior depression. The bacteria arch over the depression to give it a lipped appearance. A single longitudinal groove, indistinct under the light microscope shows clearly when SEM is used. In one cell a collection of closely packed domes was observed within the array of epibiotic bacteria, near the lip of the anterior depression. We interpret these domes as the tops of extrusomes. (ref. ID; 4885)

    This species is a recently described organism with a covering of rod-shaped bacteria and with two thickened flagella inserting into anterior pocket (Fenchel et al. 1995). On the basis of light microscopy it was suggested that P. mariagerensis might be a euglenid (Fenchel et al. 1995). Electron microscopical examination demonstrated that Postgaardi lacks a euglenoid pellicle, nonetheless a strong case for inclusion within the Euglenozoa could be made based on ultrastructural similarities with the euglenids, kinetoplastids and Diplonema (Simpson et al. 1997). (ref. ID; 4898)

    Remarks

    During a survey of the protists of the anoxic region of a Danish fjord, Fenchel et al. (1995) observed a new species of protist with a dense covering of rod-shaped bacteria and two thick flagella emerging from a deep flagellar pocket. They regarded this organism as sufficiently different from all known flagellates to justfy the creation of a new genus and named the organism Postgaardi mariagerensis. Based on their light microscopical (including epifluorescence) observations Fenchel et al. (1995) tentatively suggested that Postgaardi was a euglenid, especially nothing similarities with the genus Anisonema. (ref. ID; 4885)