8.18F: Amoebozoa and Opisthokonta
- Describe characteristics of Amoebozoa
Amoebozoa
Protists are eukaryotic organisms that are classified as unicellular, colonial, or multicellular organisms that do not have specialized tissues. This identifying property sets protists apart from other organisms within the Eukarya domain. The amoebozoans are classified as protists with pseudopodia which are used in locomotion and feeding. Amoebozoans live in marine environments, fresh water, or in soil. In addition to the defining pseudopodia, they also lack a shell and do not have a fixed body. The pseudopodia which are characteristically exhibited include extensions which can be tube-like or flat lobes, rather than the hair-like pseudopodia of rhizarian amoeba. Rhizarian amoeba are amoeboids with filose, reticulose, or microtubule-supported pseudopods and include the groups: Cercozoa, Foraminifera, and Radiolaria and are classified as bikonts. The Amoebozoa include several groups of unicellular amoeba-like organisms that are free-living or parasites that are classified as unikonts. The best known and most well-studied member of this group is the slime mold. Additional members include the Archamoebae, Tubulinea, and Flabellinea.
Slime Molds
A subset of the amoebozoans, the slime molds, has several morphological similarities to fungi that are thought to be the result of convergent evolution. For instance, during times of stress, some slime molds develop into spore -generating fruiting bodies, similar to fungi.
The slime molds are categorized on the basis of their life cycles into plasmodial or cellular types. Plasmodial slime molds are composed of large, multinucleate cells that move along surfaces like an amorphous blob of slime during their feeding stage. Food particles are lifted and engulfed into the slime mold as it glides along. Upon maturation, the plasmodium takes on a net-like appearance with the ability to form fruiting bodies, or sporangia, during times of stress. Haploid spores are produced by meiosis within the sporangia. These spores can be disseminated through the air or water to potentially land in more favorable environments. If this occurs, the spores germinate to form ameboid or flagellate haploid cells that can combine with each other and produce a diploid zygotic slime mold to complete the life cycle.
The cellular slime molds function as independent amoeboid cells when nutrients are abundant. When food is depleted, cellular slime molds pile onto each other into a mass of cells that behaves as a single unit called a slug. Some cells in the slug contribute to a 2–3-millimeter stalk, drying up and dying in the process. Cells atop the stalk form an asexual fruiting body that contains haploid spores. As with plasmodial slime molds, the spores are disseminated and can germinate if they land in a moist environment. One representative genus of the cellular slime molds is Dictyostelium, which commonly exists in the damp soil of forests.
Archamoebae, Flabellinea, and Tubulinea
The Archamoebae are a group of Amoebozoa distinguished by the absence of mitochondria. They include genera that are internal parasites or commensals of animals ( Entamoeba and Endolimax ). A few species are human pathogens, causing diseases such as amoebic dysentery. The other genera of archamoebae live in freshwater habitats and are unusual among amoebae in possessing flagella. Most have a single nucleus and flagellum, but the giant amoeba, Pelomyxa , has many of each.
The Tubulinea are a major grouping of Amoebozoa, including most of the larger and more familiar amoebae like Amoeba , Arcella , and Difflugia . During locomotion, most Tubulinea have a roughly cylindrical form or produce numerous cylindrical pseudopods. Each cylinder advances by a single central stream of cytoplasm, granular in appearance, and has no subpseudopodia. This distinguishes them from other amoeboid groups, although in some members this is not the normal type of locomotion.
Key Points
- Amoebozoa (amoebas) can live in either marine and fresh water or in soil.
- Amoebozoa are characterized by the presence of pseudopodia, which are extensions that can be either tube-like or flat lobes and are used for locomotion and feeding.
- Amooebozoa can be further divided into subclassifications that include slime molds; these can be found as both plasmodial and cellular types.
- Plasmodial slime molds are characterized by the presence of large, multinucleate cells that have the ability to glide along the surface and engulf food particles as they move.
- Cellular molds are characterized by the presence of independent amoeboid cells during times of nutrient abundancy and the development of a cellular mass, called a slug, during times of nutrient depletion.
- Archamoebae, Flabellinea, and Tubulinea are also groups of Amoebozoa; their defining characteristics include: Archamoebae lack mitochondria; Flabellinea flatten during locomotion and lack a shell and flagella; Tubulinea have a rough cylindrical form during locomotion with cylindrical pseudopodia.
Key Terms
- rhizaria : a species-rich supergroup of mostly unicellular eukaryotes that for the most part are amoeboids with filose, reticulose, or microtubule-supported pseudopods
- plasmodium : a mass of cytoplasm, containing many nuclei, created by the aggregation of amoeboid cells of slime molds during their vegetative phase
- sporangia : an enclosure in which spores are formed (also called a fruiting body)