Amoebozoa
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Mycetozoa (slime moulds) |
The Amoebozoa are a major group of amoeboid protozoa, comprising the majority of the amoebae that move using blunt or lobose pseudopods. Most members are unicellular, for instance the famous species Amoeba proteus, which is often studied in laboratories. They include the most common protists in soils and aquatic habitats. Some are symbiotic on other organisms, including some pathogens, responsible for amoebiasis (dysentery) and other diseases. They also include most slime moulds (Mycetozoa), multinucleate or multicellular forms that produce spores, and are often visible to the unaided eye.
Amoebozoans rely on pseudopods for both locomotion and feeding. The majority lack flagella, and more generally do not form microtubule-supported structures except during mitosis. The cell is typically divided into an outer layer of clear ectoplasm and an inner layer of granular endoplasm. In the active form there is usually one or more pseudopods along the anterior margin and sometimes a posterior bulb called a uroid, which may support slender projections. A number of amoebae produce shells from secreted materials, as in Arcella, or by cementing together collected materials, as in Difflugia.
Locomotion usually takes place by the cell mass flowing into one of the leading pseudopods, as in Amoeba, but there is notable variation. For instance, some amoebae crawl using relatively permanent pseudopods as limbs. Unlike the superficially similar but unrelated percolozoans, which pass through both amoeboid and flagellate stages, pseudopod formation is typically smooth and non-eruptive. The primary mode of nutrition is phagocytosis: the cell surrounds potential food particles, sealing them into vacuoles where they may be digested and absorbed. When food is scarce, most amoebae can form cysts, which may be carried aerially and introduce them to new environments. The spores of slime molds serve a similar purpose.
Most amoebozoans have mitochondria, which characteristically have tubular, branching cristae. However, a few have secondarily lost them, collectively referred to as archamoebae from an earlier assumption that the condition was primitive. These comprise the giant amoeba Pelomyxa, the parasitic entamoebae, and the uniflagellate mastigamoebae. Flagellate cells are also found among certain slime moulds. They generally have a cone of microtubules anchoring the flagellum, suggesting a close relationship to the opisthokonts.
Although many amoebae are only around 10-20 μm in diameter, they also include many of the larger protozoa. For instance A. proteus may reach 800 μm in length, and Pelomyxa are often several millimetres long. Most slime moulds take the form of giant multinucleate amoebae called plasmodia, and the largest may cover several square feet. In contrast, cellular slime moulds survive as separate amoebae most of the time but under appropriate conditions aggregate to form fruiting bodies.
Classification
Traditionally all amoebae with lobose pseudopods were treated together as the Lobosea, placed with other amoeboids in the phylum Sarcodina or Rhizopoda, but these were considered to be unnatural groups. Structural and genetic studies identified several independent groups: the percolozoans, pelobionts, and entamoebae. In phylogenies based on rRNA their representatives were separate from other amoebae, and appeared to diverge near the base of eukaryotic evolution, as did most slime molds.
However, revised trees by Cavalier-Smith and Chao in 1996 suggested that the remaining lobosans do form a monophyletic group, and that the archamoebae and Mycetozoa are closely related to it, although the percolozoans are not. Subsequently they emended the older phylum Amoebozoa to refer to this supergroup. Studies based on other genes have provided strong support for the unity of this group. Patterson treated most with the testate filose amoebae as the ramicristates, based on mitochondrial similarities, but the latter are now removed to the Cercozoa.
Amoebae do not have many characteristics they can be classified from, and the relationships within the phylum remain confused. Originally it was divided into the subphyla Lobosa and Conosa, the latter comprising the archamoebae and Mycetozoa, but more recent phylogenies do not support either. They also suggest the existence of a third major subgroup including Amoeba, Chaos, and several other naked amoebae. However, many amoebae have not yet been studied in detail, including all those that produce shells.
References
- Cavalier-Smith, T. & Chao, E. E. (1996). Molecular phylogeny of the free-living archezoan Trepomonas agilis and the nature of the first eukaryote. Journal of Molecular Evolution 43: 551-562.
- Cavalier-Smith, T. (1998). A revised six-kingdom system of life. Biological Reviews of the Cambridge Philosophical Society 73: 203-266.
- Baldauf, S. L. et al. (2000). A kingdom-level phylogeny of eukaryotes based on combined protein data. Science 290: 972-977.
- José F. Fahrni et al. (2003). Phylogeny of Lobose Amoebae Based on Actin and Small-Subunit Ribosomal RNA Genes. Molecular Biology and Evolution 20(11): 1881-1886.
External links
- Molecular Phylogeny of Amoeboid Protists - Tree of Amoebozoa (http://www.unige.ch/sciences/biologie/biani/msg/Amoeboids/Amoebozoa.html)