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Classification:
The amoebae are a polyphyletic group of protists that have in common motility via sol-gel formation and destruction of ‘pseudopods’ in contrast to flagellates and ciliates. Colorless amoebae are currently placed in three major groups (Amoebozoa, Rhizaria, and Excavata), while several others remain ungrouped. Modern classification (SS rRNA) likely will reveal better lineages even though horizontal gene transfer is rife in th protists. Amoebozoa are mainly unicellular, although the slime moulds are multinucleate or multicellular. Most unicells are 10-20 μm while Amoeba proteus can be up to 80 μm long. Some multinucleate genera can be much larger (several mm long). Cytoplasm is divided into a granular endoplasm and a clear ectoplasm. The latter 'streams' or 'flow' forming finger-like, blunt pseudopods. The group are also called Opisthokonta, considered to be ancestral to kingdoms Fungi and Animalia. Rhizaria (Cavalier-Smith 2002) have reticulate pseudopods supported by microtubules. It is thought that they have descended from a biflagellate heterotroph, and are monophyletic. Some are testate with relatively complex 'shells' well known as microfossils. The group includes the Radiolaria common in marine plankton. Excavata include the Heterolobosea, a group of genera capable of transforming between amoeboid and flagellate forms. The Euglenozoa are closely related but not amoeboid at all (find them in PhycoKey). Excavata also include parasites such as Trypanosoma (Euglenozoa) and Giardia (in Phylum Fornicata).
Origins of Amoebae:
Because of their morphology the origin of amoebae is poorly known, and molecular studies are few. Views on their relatedness span polyphyletic to monophyletic (Fahrni et al. 2003). Examples of molecular studies include one on the genera Amoeba and Chaos considers the origin of the non-testate genera (gymnamoebae) (Bolivar et al. 2001), and one on lobose amoebae based on actin and ribosomal RNA genes (Fahrni et al. 2003). A study of small subunit ribosomal RNA (SSU rRNA) concludes that the 'heliozoans', as well as the radiolarians, which together are distinctly polyphyletic, are tentatively divided into at least three sections (Nikolaev et al. 2004).
Nutrition:
Colorless amoebae are phagocytes, ingesting particles via endocytosis into digestion vacuoles.
Habitat:
Amoebae inhabit most aquatic habitats, soils, and form symbiotic or parasitic relationships with other organisms. Some are pathogenic to a variety of organisms including humans.
References:
Bolivar, I., J.F. Fahrni, A. Smirnov, and J. Pawlowski 2001. SSU rRNA-based phylogenetic position of the genera Amoeba and Chaos (Lobosea, Gymnamoebia): The origin of gymnamoebae revisited. Mol. Biol. Evol. 18(12):2306–2314. Cavalier-Smith, T. 2002. The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa. Internat. J. of Systematic and Evolutionary Microbiology 52:297-354. Fahrni, J.F., I. Bolivar, C. Berney, E. Nassonova, A. Smirnov and J. Pawlowski 2003. Phylogeny of lobose amoebae based on actin and small-subunit ribosomal RNA genes. Mol. Biol. Evol 20(11):1881-1886. Nikolaev, S.K., C. Berney, J.F. Fahrni, I. Bolivar, S. Polet, A.P. Mylnikov, V.V. Aleshin, N.B. Petrov, and J. Pawlowski. 2004. The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes. Proc. National. Acad. Sci. 101:8066-8071. (online)