Fruiting body development in the mycetozoan Echinostelium bisporum

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Abstract
Fruiting was followed from the prespore cell stage through the appearance of synaptonemal complexes in the spores in the simple mycetozoan Echinostelium bisporum. The prespore cell differentiates from a uninucleate amoeboflagellate that rounds up on the surface of the substrate. Centrioles are present in the prespore cell. As the prespore cell continues to differentiate it secretes a sheath and becomes nearly spherical. The prespore cell begins to lay down the stalk within an invagination, then rises as a sporogen at the tip of the stalk. By the sporogen stage the centrioles disappear and a spherical microtubule organizing center (MTOC) appears between the nucleus and the stalk apex. At the end of stalk elongation, the nucleus undergoes an apparently closed acentric mitosis and the sporogen cleaves into two hemispheres. Each daughter cell then rounds up and begins to produce a sculptured spore wall. The nuclei of the spores enter presumed meiotic prophase with synaptonemal complexes after the spore wall is complete. The spherical MTOC is present in all stages after the sporogen. These characters suggest that E. bisporum is a reduced myxomycete that has lost the free living plasmoidal stage of the life cycle.