INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

SARCODINA

PHYLUM AMOEBOZOA

CLASS MYCETOZOA

SUBCLASS DICTYOSTELIA

Members of this Class differ from other myxomycetes by the absence of flagellated stages in their life cycle. The vegetative stage is represented by uninucleate amoeboid Cells that move by means of lobopodia (Fig. 23). They actively feed on Bacteria, Fungi, and myxomycetes, and reproduce by fission.

Fig. 23. Life Cycle of Dictyostelium discoideum (after Hausmann):

1 - pseudoplasmodium; 2-6 - formation of sporangium;

7 - sporangium; 8 - spores; 9 - amoebae

Upon exhaustion of food reserves, the amoeboid cells cease phagocytosis and reproduction, expel their digestive vacuoles, and decrease in size. Aggregation centers—groups of cells that attract other cells—arise within The Cell cluster. Free amoeboid cells begin to crawl toward the aggregation centers. It has been found that aggregation is driven by the release of the attractant acrasin by individual cells, a substance that attracts amoebae. The process culminates in The formation of a cone-shaped cell mass surrounded by a secreted common mucous sheath. Such a formation is called a pseudoplasmodium because, unlike a multinucleate plasmodium, it consists of individual, tightly grouped cells.

The pseudoplasmodium behaves as a single Organism: it assumes a horizontal position and begins to move across the substrate, responding to stimuli (light, heat, chemicals). Subsequently, it stops moving and starts building a fruiting body: it assumes a vertical position, some of its cells form a disk-shaped base, while other cells migrate to form a tubular stalk with spores at the tip, which rises up to 5 mm above the disk. Amoeboid vegetative cells emerge from the spores.

Under unfavorable conditions, a sexual process occurs in The life cycle. The amoeboid cells behave as Gametes, fusing in pairs to form binucleate cells that increase in size, and their nuclei eventually fuse as well, meaning copulation takes place. The zygotes formed in this way are gigantic compared to vegetative cells. The zygote attracts uninucleate amoebae by secreting an attractant. It then phagocytizes the surrounding cells until it has consumed them all. Next, the zygote becomes covered with a triple membrane, transforming into a macrocyst. The Nucleus of the macrocyst divides repeatedly—first meiotically and then mitotically—becoming multinucleate. Under favorable conditions, uninucleate cells emerge from the macrocyst and also divide mitotically, giving rise to numerous amoeboid cells.

Dictyostelids are widely distributed wherever animal droppings, fallen leaves, and decaying PLANT AND ANIMAL remains are found. The most common species is Dictyostelium mucoroides.