Life Cycle of Physarum polycephalum


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As is the case with many other acellular slime molds, Physarum polycephalum feeds on bacteria and fungi as well as bits of decaying organic material (i.e. it is holozoic). The most commonly-observed form is the plasmodium.

1. The plasmodium is the main vegetative phase of the life cycle. Usually diploid, it is a large syncytium (multiple nuclei in a common cytoplasm) that can grow to very large sizes (under laboratory conditions it can be many centimeters in extent). The plasmodium diagrammed here is in the actively migrating stage, "searching" for additional food. Such plasmodia either cease to migrate when they encounter a fresh source or enter one of two other stages.

2. Under certain conditions of starvation and dessication, plasmodia assume a dormant stage called sclerotia. Properly-prepared and -stored sclerotia can be stored for many years and then reactivated by placing small fragments on a moist food source; a favorite such food (for biologists who study plasmodia) is oatmeal flakes.

3. Sporulation, which is an example of cellular differentiation, is induced if starved plasmodia sense visible light, heat shock or other environmental stress (such as flooding, high or low pH, etc.). Cellular commitment to sporulation is followed by the sequential biosynthesis of many new proteins that are required for the formation of fruiting bodies. About eleven hours after induction, the plasmodial mass dissociates into cytoplasmic nodules, each of which culminates to form a fruiting body suspended by a millimeter-sized stalk. The cytoplasmic mass enclosed by the fruiting body divides up into smaller clumps, within which meiotic divisions occur, producing haploid nuclei that become packed as spores. Sporulation is of great practical use for the geneticist since it allows the genetic analysis of all kinds of mutants in Physarum.

4. The sporulation process ends with the rupture of the sporangial mass and the release of spores into the surround. Mechanisms for dispersing such spores are not yet well-studied.

5. Spores are induced to open in environments that have "proper" levels of moisture and nutrients, releasing haploid amoebae.

6. The amoebae that are released from the spore coat are, in most cases, haploid cells that form the gametes of the system. Amoebae can be cultured on solid substrates, with bacteria (live or formalin-killed) as a food source or in suspension culture, with a semi-defined nutrient medium. Amoebae can undergo at least four distinctive stage conversions.

7. Under unfavorable circumstances, such as limited nutrients, dessication, too many neighboring amoebae, etc. the amoebae can form cysts, each of which is a dormant form that is resistant to adverse conditions but can excyst when conditions become more favorable. Encysted amoebae can be stored, at low temperatures, for extended periods of time.

8. When amoebae growing (in the laboratory) on "lawns" of bacteria are immersed in any of a variety of aqueous solutions, they transform into flagellate swimming cells called myxoflagellates or "swarm cells". This amoeboflagellate transformation is rapid and reversible, does not require gene activation or protein synthesis, and involves extensive rearrangement of cytoskeletal elements such as actin filaments and microtubules.

9. Amoebae can also mate (fuse with) other amoebae with complementary mating alleles (6a), thus forming a diploid cell from which a new plasmodium grows up. Certain strains of amoebae have the ability to "self" and create haploid plasmodia.

10. The diploid (or haploid) cells thus formed can be considered uninucleate plasmodia that, upon being cultured, become multinucleate (syncytial) plasmodia.

11. Small plasmodia can be grown on solid substrata with a suitable food source to yield the large plasmodia discussed above (1).

12. Small plasmodia growing on filter paper wet with a semi-defined liquid growth medium can be vigorously shaken and fragmented into microplasmodia, which can be subcultured repeatedly to yield large quantities of microplasmodia grown in suspension.

13. If cultured in liquid medium that is depleted of nutrients (starvation), microplasmodia form another dormant phase, spherules which can be dried by streaking on dry filter paper, stored indefinitely and used to start new shaker cultures of microplasmodia. Microplasmodia can also be fused to form macroplasmodia and then cultured on solid substrata.


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Last modified: Thursday, October 16, 2003