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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Hyperamoeba

Hyperamoeba Alexeieff, 1923 (ref. ID; 4923) or Alexeieff, 1925 emend. Lepsi (ref. ID; 3687)

[ref. ID; 4923]
Hyperamoeba is a genus of free-living amoeboid, flagellated protists that has been reported from micro-aerobic and aerobic freshwater environments. Some reports describe facultative parasitism (Mascaro et al. 1986; Niszl et al. 1995) and apparent endocommensalism, and the genus was originally described from an infusion of horse faeces (Alexeieff 1923). The genus contains two species, H. flagellata Alexeieff, 1923 and H. dachnaya n. sp. (ref. ID; 4923)

[ref. ID; 5694]
Circumscription; Heterotrophic protist with amoeboid and gliding (uni)flagellated stages, flagellated stage uniflagellated, and reminiscent of pelobionts but with tubulocristate mitochondria. Cysts also produced. Flagellated stage reminiscent of protostelids and relatedness to eumycetozoa is probable. (ref. ID; 5694)
Ultrastructural identity; Mitochondria with tubular cristae, some with central core, dictyosome present cristate. Two apical basal bodies give rise to four microtubular rootlets, two conical arrays of microtubules extend toward nucleus and center of cell. (ref. ID; 5694)
Synapomorphy; Tubulocristate protist with double cone of microtubules arising from anterior pair of basal bodies. (ref. ID; 5694)
Composition; Probably one species. (ref. ID; 5694)
References; Patterson and Zolffel 1991; Karpov and Mylnikov 1997. (ref. ID; 5694)
  1. Hyperamoeba dachnaya Walker et al., 2003 (ref. ID; 4923 original paper)
  2. Hyperamoeba flagellata Alexeieff, 1925 (ref. ID; 3687) reported year? (ref. ID; 3493)
  3. Hyperamoeba metachromatica (Alexeieff, 1917) (ref. ID; 3687)
  4. Hyperamoeba refringens Alexeieff, 1917 (ref. ID; 3687)

Hyperamoeba dachnaya Walker et al., 2003 (ref. ID; 4923 original paper)

Diagnosis

Hyperamoeba with flagellated cells from 12 to 25 um long; paraflagellar finger-shaped pseudopodium, and spinous appearance to cysts. (ref. ID; 4923)

Descriptions

Flagellated cells are round to elongate, up to 25 um long. A single, apically inserted flagellum up to 40 um long is present in gliding, and swimming cells. In sessile cells the flagellum is about 10 um long. In gliding cells there is usually a single filose pseudopodium up to 15 um long, arising just next to the base of the flagellum. The nucleus is pyriform and flexible, about 4 um long. In gliding, swimming and sessile flagellates its anterior, pointed end is loosely associated with the base of the flagellum. The anterior end of the cell usually appears hyaline. Food vacuoles and other inclusions are present in the posterior region of moving cells. The contractile vacuole is located posteriorly in swimming cells, and fills by fusion of smaller vesicles. Rounded, finger-shaped or filose (sometimes branching) pseudopodia may form from the sides or the posterior end of the cell. Filose pseudopodia are most common in sessile flagellates. A posterior uroid, composed of very fine pseudopodia up to 5 um long, is usually present in gliding cells. Amoeboid cells are rounded to elongate, up to 15 um long. The nucleus is rounded and about 4 um in diameter. Food vacuoles and a contractile vacuole are present (in the posterior region of moving cells). In moving cells, eruptive, hyaline pseudopodia form at the anterior end, while a uroid of fine filose pseudopodia forms at the posterior end. Sessile amoebae exhibit eruptive, finger-shaped and fine, branching pseudopodia. Some sessile cells do not form pseudopodia, and may correspond to the apparently encysting cells observed by electron-microscopy. Cysts are spherical, 6-12 um in diameter. The cyst wall is covered in short spines that appear T-shaped under differential interference contrast (Nomarski) optics. The nucleus is visible near the centre of the cysts. (ref. ID; 4923)

Comments

The organisms isolated from Lake Osinovoye mud were referred to Hyperamoeba because of the common presence in H. dachnaya and H. flagellata of life cycle stages including cysts, amoebae and amoeboid flagellates with the nucleus attached to the base of the flagellum and with eruptive, conical, finger-shaped and filose pseudopodia (Alexeieff 1923; Karpov and Mylnikov 1997). These stages also occur in pelobionts, and the nucleus of Mastigamoeba is attached to the base of the flagellum. However the ultrastructural identity of Hyperamoeba is distinct from that of pelobionts, which have only a single basal body with radiating cone (rather than a curtain) of microtubules, a single microtubular rootlet, and no mitochondria (Walker et al. 2001). The identity of H. dachnaya was therefore confirmed by electron microscopic features: mitochondria with a filamentous nucleoid; and the arrangement of the two basal bodies, their associated rootlets R1-R5, and fibrillar rootlet with MTOC. Most aspects of the ultrastructure, such as the four purely microtubular rootlets (R2-R5), fibrillar rootlet with an MTOC (R1), and the mitochondria, are as previously reported for Hyperamoeba flagellata (Karpov and Mylnikov 1997). Features not previously observed in Hyperamoeba include cysts with a multilamellate wall with connectives between the layers, the long pseudopodium arising next to the flagellum, the 'dent' in the nucleus, and the long striated fibre. On the basis of these differences, we conclude we are not dealing with H. flagellata. For practical purposes, H. dachnaya can be distinguished from H. flagellata at LM level by its flagellates being about twice the size of those in H. flagellata; the presence of a much more prominent flagellar pseudopodium in H. dachnaya than in H. flagellata; and cysts with 'T-shaped' connective spines between the layers in H. dachnaya rather than a smooth appearance in H. flagellata. Another nominal Hyperamoeba sp. (ATCC isolate 50750, from human faeces) has been reported (Zaman et al. 1999), which has the same cell dimensions and smooth cyst wall as H. flagellata (Karpov and Mylnikov 1997). However Zaman et al.'s identification of their isolate as Hyperamoeba was based only on the presence of R1 and R3- each of which is present in all myxogastrids and some protostelids. The SSU rRNA sequence of H. dachnaya is most similar to those of the physariid myxogastrids (Didymium nigripes, Physarum polycephalum and Protophysarum phloiogenum) while Hyperamoeba sp. ATCC 50750's sequence is most similar to the stemonitids. We conclude that the Zaman isolate has been under-described and hence assigned to the wrong genus: we reject a sister-group relationship between it and H. dachnaya. Niszl et al. (1995) reported an organism isolated from contact lens solutions, which was identified as Mastigina sp. It is morphologically indistinguishable from H. dachnaya at the light microscopical level. In subsequent RFLP analyses of SSU rRNA sequences (T. Nerad, personal communication) this organism was indistinguishable from Hyperamoeba dachnaya. Since the variation within Hyperamoeba does not separate it from the myxogastrids, we recommend that the genus be placed within the myxogastrids but regarded as incertae sedis therein, until further molecular and morphological information is obtained. Unless further study reveals spore-forming structures in the life cycle of either species of Hyperamoeba, or molecular analyses involving H. flagellata show Hyperamoeba to be polyphyletic, we recommend that the genus be retained, rather than rendered into junior synonymy with a genus of myxogastrids. (ref. ID; 4923)

Etymology

The name "dachnaya" comes from the Russian word "dacha" (summer house): the mud sample was taken from A.F's dacha. (ref. ID; 4923)