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In this article we will discuss about the life cycle of slime mold with the help of a suitable diagram.
The life cycle of an endogenous slime mold such as Physarum starts with the germination of a meiospore under favourable conditions. One to four uninucleate haploid biflagellate swarm cells or myxamoebae are released through a slit or pore; the former under wet conditions and the latter under drier conditions.
Fusion between two swarm cells or myxamoebae followed by karyogamy results in the formation of a zygote which is uninucleate, diploid and amoeboid in form. It is acellular.
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It creeps over the substratum feeding on bacteria and organic matter synthesizing more protoplasm resulting in growth. The growth in size is accompanied by repeated and successive divisions of the diploid parent nucleus.
The divisions are mitotic. As a result of subsequent growth, repeated karyokinesis but no cytokinesis, the zygote gradually becomes changed into a multinucleate amoeboid mass of protoplasm, called the Plasmodium.
The numerous nuclei embedded in its protoplasm are diploid in nature. Favourable temperature, abundant moisture and food favour its growth, movement and reproduction. In many cases the young diploid plasmodia may combine with zygotes or other plasmodia of the same species or a number of zygotes may coalesce to form a single larger plasmodium.
In all these cases, the union involves the fusion of their cytoplasm only. There is no fusion between the nuclei. Eventually the plasmodium in many cases becomes a massive structure called the macroplasmodium or phaneroplasmodium.
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Normally the plasmodium after attaining a certain size and stage of maturity enters the reproductive phase. The amoeboid life ceases. The slime layer dries. The quiescent plasmodium thickens. The diploid protoplast concentrates at a few points forming a mound like structure.
The latter grows into a stalked sporangium. The diploid protoplasm or of the sporangium cleaves into numerous young spores each of which has a diploid nucleus.
The diploid nuclei of the young spores undergo meiosis to form meiospores. When mature the meiospores are released and dispersed by wind. On falling on a suitable substratum and under favourable conditions, the meiospore germinates to release swarm cells or myxamoebae which fuse in pairs to form the zygote in which the diploid condition is re-established.
Under conditions of stress and strain mature plasmodium by differentiation and cleavage becomes transformed into an irregular hard structure consisting of thick-walled cellular units. The former is called the sclerotium and the latter spherules. The sclerotium remains dormant when the conditions are unfavourable for growth. With the return of suitable conditions it grows into a new Plasmodium.
Alternation of Generations of Slime Mold:
Strictly speaking, there is no alternation of two distinct generations in the life cycle of true slime molds. The diploid Plasmodium is the sporophyte. Along with other diploid structures such as the zygote, sporangia and the young diploid spores, it constitutes the sporophyte generation or diplophase.
There is, however, no gametophyte plant. Meiosis occurs in the young diploid spores. The mature haploid spores (meiospore) and the gametes (swarm cells or myxamoebae) they give rise to on germination represent the extremely reduced gametophyte or haplophase limited only to a few haploid cells.
Truly speaking there is alternation of sporophyte generation with a few haploid cells in the life cycle. Such a life cycle is called diploid or diplontic. The sclerotia and spherules do not play any role in the phenomenon of alternation of generations. They primarily serve as means of perennation and serve to prolong the diplophase in the life cycle.
