Excavata

3 Key excerpts on "Excavata"

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  eBook - ePub

  The Princeton Guide to Evolution

  • David A. Baum, Douglas J. Futuyma, Hopi E. Hoekstra, Richard E. Lenski, Allen J. Moore, Catherine L. Peichel, Dolph Schluter, Michael C. Whitlock, David A. Baum, Douglas J. Futuyma, Hopi E. Hoekstra, Richard E. Lenski, Allen J. Moore, Catherine L. Peichel, Dolph Schluter, Michael C. Whitlock(Authors)
  • 2013(Publication Date)
  • Princeton University Press

    (Publisher)

  The most familiar Excavata are the causative agents of parasitic diseases. These include Giardia, a major cause of diarrhea worldwide that exacerbates malnutrition in children in the developing world. Other parasitic excavates include trypanosomes that cause sleeping sickness and Trichomonas vaginalis, which causes the sexually transmitted disease trichomoniasis. Members of Excavata are heterotrophic with the exception of one lineage of euglenids (a close relative of the trypanosomes) that acquired photosynthetic ability by engulfing a green alga endosymbiont hundreds of millions of years ago (see section 5). Many taxa, and all the parasites, within the Excavata are anaerobic and have highly reduced mitochondria, either hydrogenosomes or mitosomes. Some of these parasites also have rapid rates of evolution that artificially pulled them to the base of early ribosomal DNA trees (see chapter II.2 for a discussion of long-branch attraction artifacts). These two observations led to the now-disproven hypothesis that members of the Excavata represented early diverging eukaryotes that had branched off the eukaryotic lineage prior to the acquisition of mitochondria. The final major clade of eukaryotes that is moderately well supported, SAR, is the amalgamation of the three other monophyletic clades, the stramenopiles, alveolates, and rhizarians. The stramenopiles contain diatoms (algae with beautiful silica shells), kelps, and the causative agent of the Irish potato famine (Phytophthora). The alveolates include the morphologically diverse ciliates, the dinoflagellates critical to the survival of coral reefs, and the apicomplexa, which include the malaria parasite. Both the stramenopiles and alveolates are defined by ultrastructural synapomorphies. The stramenopiles have specific hairs on one of their flagella and the alveolates have sacs (alveoli) underlying their cell membrane that lend rigidity

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  eBook - ePub

  Georgis' Parasitology for Veterinarians E-Book

  • Dwight D. Bowman(Author)
  • 2020(Publication Date)
  • Saunders

    (Publisher)

  3: Protista

  The eukaryotes have been divided now into four large groups: the Archaeplastida (the plants); the Unikonts (animals, fungi, and amoebae); the Excavata (many of the better known flagellate parasites); and the SAR group, that is, the Alveolates, including ciliates, coccidia, the organisms causing toxoplasmosis and equine protozoal myeloencephalitis, and the agents of babesiosis and malaria, and the Rhizaria with no significant parasites of domestic animals (Hampl et al, 2009 ). This chapter will discuss members of the Excavata, the Alveolates, a Stramenopile, and a few Unikonts (true amoebae). This classification scheme makes it difficult to talk in terms of the classical “protozoa” because the protozoa actually were representatives of many unrelated groups, often related to various multicellular forms. However, after one gets past the larger groupings and their new names, information about individual parasites remains unchanged.

  A vast majority of the eukaryotes are unicellular organisms that are related to other multicellular forms (e.g., malaria being related to the brown algae that compose the giant kelp forests of northern oceans). However, in veterinary medicine, interest is focused on agents that cause disease, and these tend to be unicellular forms. The popular term protist is applied to these eukaryotes with a unicellular level of organization, and it is understood that when cell differentiation occurs in these groups, it is restricted to the purposes of sexual reproduction, motility, alternate vegetative morphology for different habitats (e.g., intestinal lumen vs. liver tissue), and quiescent and resistant transmission stages, such as cysts (Adl et al, 2005

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  eBook - ePub

  Taxonomic Guide to Infectious Diseases

  Understanding the Biologic Classes of Pathogenic Organisms

  • Jules J. Berman(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Trypanosoma brucei rhodesiense (African trypanosomiasis, sleeping sickness)

  Passage contains an image

  Chapter 18

  Percolozoa

  “The cell is basically an historical document.”

  Carl R. Woese [21]

  Eukaryota Bikonta (2-flagella) Excavata

  Metamonada (Chapter 16 )

  Discoba

  Euglenozoa (Chapter 17 )

  Percolozoa (Chapter 18) Heterolobosea Schizopyrenida Vahlkampfiidae *Naegleria

  Archaeplastida (Chapter 24 )

  Chromalveolata (Chapters 19 –21 )

  Alveolata

  Apicomplexa (Chapter 19 )

  Ciliophora (ciliates) (Chapter 20 )

  Heterokontophyta (Chapter 21 )

  Unikonta (1-flagellum)

  Amoebozoa (Chapter 22 )

  Opisthokonta

  Choanozoa (Chapter 23 )

  Animalia (Chapters 25 –32 )

  Fungi (Chapters 33 –37 )

  Below Class Bikonta (two flagella) is Class Excavata (from Latin excavare, to make hollow, or, in this case, grooved). All subclasses of Class Excavata descend from an organism with a ventral feeding groove. Under Class Excavata is Class Discoba.

  Class Discoba is a newly invented class, and its taxonomic origin is instructive. Recently acquired molecular data suggest that three subclasses of Class Excavata share a common direct ancestor: Class Percoloza (Chapter 18), Class Euglenozoa (Chapter 17 ), and Class Jakobid (which happens to contain no infectious organisms). The common ancestor of these three classes is apparently not shared with another subclass of Excavata: Class Metamonada (Chapter 16 ). To preserve monophyly within Class Excavata, a newly named class needed to be inserted under Class Excavata. This class would contain Class Percolozoa, Class Euglenozoa, and Class Jakobid and would exclude Class Metamonada. The newly named class is Discoba [75] .

  Under Class Discoba is Class Percolozoa, single-celled organisms containing mitochondria with discoid cristae, and the ability to shift between three morphologic forms: amoeboid, flagellate, and cyst. The amoeboid form consists of a non-flagellated feeding cell. Like all amoeboid forms, it moves slowly, by extending a section of its cytoplasm (the pseudopod). Under adverse conditions, the amoeboid form can develop flagella, which presumably enhance its ability to move to a more hospitable location. As you would expect from organisms in a subclass of Bikonta, two flagella are observed in the flagellate form. Under conditions that are severely unsuitable for growth, the organism converts to a cyst form.

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