Archamoebae phylum Amoebozoa Conosa Pelobiontida

These common amoebae, such as Mastigella, Mastigina, Mastigamoeba and Pelomyxa, are found active in soils that are transiently anaerobic and rich in dissolved organic matter, and in peat or bogs (Fig. 1.12). They are secondarily amitochondriate, and possess an atypical cilium that appears rudimentary (Walker et al., 2001). The axoneme is reduced, always with a single and unpaired kinetosome. The amoeba in some species becomes rigid and loses its amoeboid motion when there is lack of food

Archamoebae

Fig. 1.11. (A) A common testate amoeba, Difflugia (110 ^m diameter). (B) An Astigmatid mite showing mouth parts (180 ^m wide). (C) A Pachygnatidae (Prostigmata) mite (280 ^m long). (D) A Zerconidsp. (Prostigmata) mite (650 ^m long). (E) A Cunaxa sp. (Prostigmata) mite (600 ^m long). (F) Mouth parts of the same Cunaxa specimen as in (E). (G) A Mesostigmata Rhodacarellus sp. (390 ^m long). (H) A Uropodidae (Mesostigmata) (body length 480 ^m). (I) Isotoma (Collembola) (420 ^m long). (J) Entomobryidae (Collembola) (1.3 mm long). (K) Onychiurus (Collembola) (470 ^m long) - continued overleaf.

Fig. 1.11. (A) A common testate amoeba, Difflugia (110 ^m diameter). (B) An Astigmatid mite showing mouth parts (180 ^m wide). (C) A Pachygnatidae (Prostigmata) mite (280 ^m long). (D) A Zerconidsp. (Prostigmata) mite (650 ^m long). (E) A Cunaxa sp. (Prostigmata) mite (600 ^m long). (F) Mouth parts of the same Cunaxa specimen as in (E). (G) A Mesostigmata Rhodacarellus sp. (390 ^m long). (H) A Uropodidae (Mesostigmata) (body length 480 ^m). (I) Isotoma (Collembola) (420 ^m long). (J) Entomobryidae (Collembola) (1.3 mm long). (K) Onychiurus (Collembola) (470 ^m long) - continued overleaf.

Prostigmata
Fig. 1.11. Continued.

Fig. 1.12. A Mastigamoeba (Pelobiontida, Archamoebae) with single anterior cilium and flexible body. Scale bar 25 ^m.

or conditions deteriorate. The nucleus is partially wrapped in a basket of microtubules emanating from the kinetosome. The storage material is glycogen, and mitosis involves an intranuclear spindle with persistent nucleoli. Species are osmotrophic on rich soil solution (and some can be grown axenically) supplemented with bacterivory.

Eumyxa (phylum Amoebozoa: Conosa, Mycetozoa)

The Eumyxa (or Myxogastria) are found in soil organic horizons, in surface litter including woody debris, in animal cellulosic dung and in tree bark. Specimens can spread across several centimetres and be 1-2 mm thick (e.g. Physarum). Species are brightly pigmented or clear. The cell is a multinucleate cytoplasm which continues to grow without cellular divisions and is referred to as a plasmodium (Fig. 1.13). Successive nuclear divisions are synchronous. The cytoplasm forms channels to increase diffusion, for moving cellular components, particularly food vacuoles and secretory vesicles. These channels appear like veins that branch across the plasmodium, and the movement of granules and cytoplasm is easily observed under the dissecting microscope. If desiccation occurs, the plasmodium secretes a siliceous material that hardens the dried cell and serves to protect the plasmodium, like a flat cyst. When food is scarce,

Amoebozoans Life Cycle

Fig. 1.13. Life cycle stages of a generalized Eumyxa (or Myxogastria). The sporangium (A) releases spores (B), from which an amoeba emerges (C). The amoeboid and ciliated morphologies (D) are feeding and dispersal haploid phases which lead to a diploid amoeboid cell (E) by fusion of complementary mating types. Nuclear divisions and growth of the diploid cells occur without cytokinesis, forming a large multinucleate cell (F and G) called a plasmodium. The plasmodium sporulates in numerous sporangia (A) where meiosis occurs. Scale bar (A) and (G) 1 mm, (B-F) 25 ^m.

Fig. 1.13. Life cycle stages of a generalized Eumyxa (or Myxogastria). The sporangium (A) releases spores (B), from which an amoeba emerges (C). The amoeboid and ciliated morphologies (D) are feeding and dispersal haploid phases which lead to a diploid amoeboid cell (E) by fusion of complementary mating types. Nuclear divisions and growth of the diploid cells occur without cytokinesis, forming a large multinucleate cell (F and G) called a plasmodium. The plasmodium sporulates in numerous sporangia (A) where meiosis occurs. Scale bar (A) and (G) 1 mm, (B-F) 25 ^m.

vertical stalks of cytoplasm extend upward. These become the site of mei-otic divisions that lead to haploid walled spores accumulating at the apex inside sporangia, with an external protective wall. The spores are 5-20 ^m and are released under suitable conditions. Light plays a role in initiating spore formation in certain species, so that some photoregulation exists. Once released, spores emerge as biciliated haploid cells for dispersal. The dispersal cells must fuse with another complementary mating type. The fusion forms a diploid cell which can grow into a new plasmodium. In Protostelids (e.g. Protostelium), the feeding phase consists of amoeboid cells with somewhat filose pseudopodia. These can fuse to form a plasmodium. Sporangia can form from single amoebae or fragments of plasmodium, but consist of one or a few spores only.

The activity of species is seasonal, with preference for wet or rainy periods, and species are restricted to preferred habitats. In temperate forests, different species are active during the spring, summer or autumn, but some may be active from ground thaw to frost. Substrate specificity is recognized in many species that prefer cellulosic dung or the bark of certain living plant species. There are five orders with nearly 600 species keyed from life history stages, habitat and sporangium morphology. However, accurate identification probably requires verification with gene sequences.

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