Dictyostelia phylum Amoebozoa Conosa Mycetozoa

The dictyostelid groups families which may be unrelated but that are the result of convergent evolution, towards aggregation of amoeboid cells into a sorocarp. There are two orders, Dictyosteliida (~70 species) and the poorly studied Guttulinida. The better known Dictyosteliida include Acytostelium, Coenonia, Dictyostelium and Polysphondylium (Fig. 1.14). A review of the most studied genus Dictyostelium can be consulted for its general biology (Kessin, 2001). The amoeboid feeding cells are bacterivorous with filose or lobose pseudopods, and are common in soils. When bacteria are reduced in numbers in Dictysteliida, secretion of cAMP or other short signal molecules initiates an aggregation response. Cells move by chemotaxis towards the source of the signal, and begin to release the signal themselves. The amplified response causes the aggregation of hundreds or thousands of cells. The aggregated cells can be seen with the naked eye on agar plates. Within the aggregate, cells differentiate into base cells, stalk cells which climb on

Fig. 1.14. Life cycle stages of a generalized dictyostelid (Amoebozoa, Mycetozoa). (A) Cyst and emerging amoeba. (B) Amoebae attracted to a large diploid cell which engulfs them and becomes a large macrocyte. (C) A stream of starving amoebae aggregating. (D) Typical sporangium morphology of aggregated amoebae. Scale bar (A-C) 25 ^m, (D) 1 mm.

Fig. 1.14. Life cycle stages of a generalized dictyostelid (Amoebozoa, Mycetozoa). (A) Cyst and emerging amoeba. (B) Amoebae attracted to a large diploid cell which engulfs them and becomes a large macrocyte. (C) A stream of starving amoebae aggregating. (D) Typical sporangium morphology of aggregated amoebae. Scale bar (A-C) 25 ^m, (D) 1 mm.

top of each other, and cells at the apex which produce cysts. The amoebae develop a cellulosic cell wall for support. The apex cells continue to divide while the stalk and base cells die. Release of the cysts under suitable conditions leads to amoeboid cells emerging from the cysts. Complementary mating types will pair under reduced food conditions. Under certain conditions, this leads to a macrocyst. The macrocyst secretes cAMP to attract other responsive amoebae, and feeds on them. The diploid cells can grow very large in some species. Encysted macro-cysts undergo meiosis and multiple mitotic divisions to release haploid feeding amoebae.

In the order Guttulinida (e.g. Acrasis, Copromyxa, Copromyxella, Fonticula, Guttilinopsis and Pocheina), the amoeboid cells produce eruptive lobopodia reminiscent of the schizopyrenid amoebae (Percolozoa, Vahlkampfamoebae), and without ciliated dispersal cells (except Pocheina flagellata). There are species with flat cristae, and others with tubular cristae. These genera are poorly sampled and probably not so rare.

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