Amoebozoa

2 Key excerpts on "Amoebozoa"

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  eBook - ePub

  The Biology of Parasites

  • Richard Lucius, Brigitte Loos-Frank, Richard P. Lane, Robert Poulin, Craig Roberts, Richard K. Grencis, Ron Shankland, Renate FitzRoy(Authors)
  • 2017(Publication Date)
  • Wiley-VCH

    (Publisher)

  T. foetus in cows?

  2.4 Amoebozoa

  • Mostly saprophytes, some parasitic taxa
  • Variable shape, locomotion through pseudopodia
  • No flagella, no tubulin structures except in centrioles
  • Monoxenous life cycle, propagation mostly through cysts
  • Reproduction through division, sexual reproduction not known

  Amoebae (Greek amoibe = change, modification) are a polyphyletic group defined by their way of locomotion and the lack of highly developed characteristics. They colonize all aquatic and terrestrial habitats. Amoebae can be free-living or endoparasitic endoparasites. The feeding proliferative stages (trophozoites) are enclosed in a membrane with an underlying layer of viscous ectoplasm. The inside contains more fluid, granular endoplasm embedding various food vacuoles, endocytic vesicles, ribosomes, glycogen granules, and the nucleus. Amoebae form pseudopodia for directed movement. These are ectoplasmatic protrusions at the anterior pole of the amoeba, into which endoplasm flows, while at the posterior pole, an equivalent amount of matter is retracted. The pseudopodia flow around nutrient particles such as bacteria that are then taken up into food vacuoles. According to their different shapes, pseudopodia have been described as lobopodia, filopodia, or acanthopodia (lobe, thread, or spine-shaped feet, respectively). The trophozoite stages reproduce by fission, while sexual reproduction has been not observed. Many amoebae have the ability to form environmentally resistant cysts in response to adverse conditions.

  The best-known amoeba is probably the free-living Amoeba proteus, which can be easily visualized in the film residue of hay infusions. The saprophytic life in a decaying environment, where many free-living amoebae thrive, is only a small evolutionary step away from endoparasitism in the gut. The medically important Entamoeba species that live in an anaerobic milieu in the gut lack conventional mitochondria. However, these parasites possess an extremely reduced endosymbiotic organelle, a mitosome, which is the likely relict of a mitochondrion. Some nuclear-located genes encode proteins that are evolutionary related to mitochondria-derived genes. The corresponding proteins are targeted to the mitosome where they function in activities such as Fe–S cluster formation. It is likely that the predecessors of existing entamoebae exhibited an oxidative metabolism associated with the endosymbiotic organelle that was lost as they adapted to their low-oxygen environment. The cytoskeleton of entamoebae is unusual in that it does not feature microtubules apart from the centrioles and there are neither flagella nor cilia. Entamoebae were once thought to lack Golgi apparatus and rough endoplasmic reticulum, but are now known to have these in a reduced form. Thus, rather than branching from other eukaryotes prior to the acquisition of key organelles, entamoebae are believed to have lost these characteristics as they adapted to a parasitic life style. Notably, other amoebazoae, including Acanthamoeba

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  eBook - ePub

  Principles of Veterinary Parasitology

  • Dennis Jacobs, Mark Fox, Lynda Gibbons, Carlos Hermosilla(Authors)
  • 2015(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Figure 4.2 ) can be recognised by the thousands of tiny hairs (‘cilia’) covering the body surface. To propel the organism along, these bend in coordinated ripples (like Mexican waves round a sports stadium).

  Figure 4.2

  Balantidium trophozoite. Redrawn after Cheng, 1986 with permission of Elsevier.

  Amoebae are amorphous jelly-like blobs (see Figure 4.3 ) that progress by pushing out a finger-like protrusion (‘pseudopodium’) which enlarges as cytoplasm flows into it. Amoebae need a suitable substrate for active movement.

  Figure 4.3

  Entamoeba trophozoite. Redrawn after Jeanne Robertson from Roberts and Janovy Jr, 1996 with permission of McGraw-Hill Education.

  The flagellates, in contrast, are strong swimmers employing one or more long contractile fibres (‘flagellae’ – see Figure 4.4 ) that flex in a whiplike fashion. They are well suited for life in blood or other body fluids.

  Figure 4.4

  Giardia trophozoite. Redrawn after William Ober from Roberts and Janovy Jr., 1996 with permission of McGraw-Hill Education.

  The intracellular life-cycle forms of apicomplexans are immobile but the forms that leave one host cell to find another, such as the Toxoplasma merozoites illustrated in Figure 4.5 , have sleek, crescent-shaped bodies that appear to glide along a spiral trajectory. This effect is achieved by intracellular contractile microfilaments that induce subtle alterations along the body surface. These propel the organism along an excreted slime trail in a manner analogous to the progress of a snail. Once a suitable host cell is located, penetration is achieved by means of a specialised structure, the ‘apical complex’.

  Figure 4.5

  Schizont: SEM (with artificial colouring) showing merozoites (pink) within host cells (blue). Reproduced with permission of D.J. Ferguson.

  4.2.3 Nutrition

  Protozoa feed mainly on particulate material. The cell membrane indents and folds slowly over, thereby entrapping a small quantity of food and drawing it into the cell. This process is known as pinocytosis or phagocytosis, depending on the size of the particle (in ascending order). In ciliates, food particles are directed by the action of cilia towards the base of a funnel-like structure (the ‘cytostome’). When sufficient food has accumulated in this, a vacuole forms which is engulfed into the cytoplasm. Many parasitic protozoa are also able to absorb liquid nutrients and in some cases this may be their main source of nourishment.

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