THE BALLISTOSPORIC PROTOSTELID GENUS SCHIZOPLASMODIUM

The genus Schizoplasmodium is a member of the Protostelia, a primitive subclass of mycetozoans. It has previously been known only from the commonly occurring type species, S. cavostelioides Olive and Stoianovitch. Two additional species of rare occurrence are described here-S. obovatum, found on bark of living trees in Florida and Uganda, and S. sechellarum, found in the Seychelles Islands and Papua New Guinea on old corn and leguminous capsules. All three members of the genus produce reticular plasmodia but lack flagellate cells. The most unusual feature of the group is the production of ballistospores that are discharged from their stalks by the development and explosion of gas bubbles. SCHIZOPLASMODIUM is a genus of the subclass Protostelia of the Eumycetozoa (Olive, 1975). It is unique in having the spores of its single-spored fruiting bodies discharged by the force of a bursting gas bubble mechanism (Fig. 1-5, 10). A single valid species, S. cavosteliolides Olive and Stoianovitch (1966a), has been recognized. The present paper describes two new species of rare occurrence and provides a key to the genus. In addition to having ballistospores, Schizoplasmodium species also produce thin reticular plasmodia that feed on yeast or bacterial cells or both. Food vacuoles containing the ingested cells may be observed within the plasmodia. However, plasmodia often appear to crawl over the food cells, partially enveloping them in groups between plasmodium and agar medium but not completely surrounding them. It is possible that some digestion may occur without complete ingestion of the cells into food vacuoles. Spores and cysts are typically plurinucleate, and each on germinating produces a single protoplast with slender filopodia that, by anastomosis, may begin to produce a reticular configuration before germination is complete. Plasmodia enlarge by fusion with each other and by growth and nuclear division unaccompanied by division of the protoplast. The reticular nature continues to be enhanced by further proliferation and anastomosis of filopodia (Fig. 9, 15). Also, smaller plasmodia may develop from larger ones by plasmotomy. In S. sechellarum single plasmodia may reach a breadth of 2.5 cm. Feeding plasmodia migrate slowly through colonies of food organisms on agar media. They usually have a distinctly reticular posterior region 'Received for publication 20 January 1975. Supported by National Science Foundation grants GB35964X and BMS-72-02392 and a grant from the BrownHazen Fund of the Research Corporation. The senior author is grateful to Dr. Ian Frost, Head of the Veterinary Division, Department of Agriculture, Mahe, Seychelles for the use of laboratory facilities. and a nearly continuous anterior area from which numerous fine filose pseudopodia extend into the food supply (Fig. 15). As the food decreases, the plasmodia may become entirely reticular. Particularly at the edge of a colony or in parts of it where the food organisms have become more or less depleted, rhizopodia, which often anastomose into reticulopodia, extend into the agar medium (Fig. 16). Protoplasmic movement within the plasmodium is generally quite active but not coordinated into the shuttle movement characteristic of most myxomycetes. Movement in a single relatively narrow strand often occurs in opposite directions simultaneously. Numerous contractile vacuoles and nuclei, each with a single centrally located nucleolus, are present (Fig. 9, 17). The nuclei constantly shift in position in the moving cytoplasm. The onset of sporulation is recognizable by the concentration of the plasmodial protoplasm into a number of small masses, which are at first connected by narrow strands that later snap apart and leave separate prespore cells on the substrate surface (Fig. 18). These develop directly into stalked fruiting bodies in the manner characteristic of the protostelids (Olive and Stoianovitch, 1966b; Olive, 1975). The entire process of sporogenesis from prespore cell to mature sporocarp requires only about one hour. Since prespore cells and mature spores contain about the same range of nuclear numbers, it is unlikely that mitosis occurs in the process. The sporocarp consists of a single spore on a slender tubular stalk and is enveloped entirely by a distinct sheath that spreads out at the base into a wrinkled supportive disc (Fig. 20, 21). The frequent irregular appearance of the stalk results from irregularities in the stalk tube and sheath. When sporocarps are placed in water or pushed over onto the agar, the stalk tube tends to become less distinct, possibly deliquescing. The sheath in