In this article we will discuss about:- 1. Features of Myxomycota 2. Phylogeny of Myxomycota 3. Class Myxomycetes Introduction 4. Economic Importance 5. Occurrence 6. Sexual Reproduction 7. Life Cycle Pattern.
Features of Myxomycota:
The Myxomycota or slime molds, are fungus-like organisms. They are characterised by the absence of cell wall from their amoeboid,’ animal-like vegetative or assimilatory phase.
The amoeboid assimilatory phase may consist of a free-living multinucleate mass of protoplasm called a plasmodium or simple uninucleate amoeboid cells, the myxamoebae (sing, myxamoeba) often aggregating into a pseudoplasmodium (Fig. 4F) or amoeboid cells interconnected by slime filaments giving rise to a structure known as net plasmodium or filoplasmodium (Fig. 4G).
Plasmodial slime molds are the true slime molds, pseudoplasmodial slime molds the cellular slime molds and the net plasmodial slime molds are known as the net slime molds.
The true slime molds are mostly saprophytic with a few parasitic ones. In the former the entire plasmodium is consumed in the formation of fructifications. Whereas, in the latter there is absence of development of fructifications, the plasmodium is converted into masses of spores. All the true slime molds produce flagellated cells, the swarm cells.
But the formation swarm cells is absent in the cellular slime molds. In the cellular slime molds fructifications are developed from the pseudoplasmodia. But in the net slime molds each cell is converted into a mucilaginous sporocyte in which spores are developed.
The spores escape as biflagellate zoospores which after swimming for some time, lose their flagella become surrounded by delicate envelope and give rise to new net plasmodium.
Following is a simplified key to the classes of the Myxomycota:
A. Free-having plasmodium bearing haploid or diploid nuclei
B. Plasmodium non-parasitic, fructification present.
BB. Plasmodium parasitic, fructification lacking
AA. Amoeboid cells aggregating into a pseudoplasmodium
AAA. Amoeboid cells spindle-shaped to oval interconnected by slime filaments forming a net commonly known as ‘net plasmodium’ or ‘filo-plasmodium’
Phylogeny of Myxomycota:
Although the members of the Myxomycota have some characteristics in common, certain differences are of major magnitude, with the result that a single evolutionary series seems quite unlikely. Macbride (1899) believed that the slime molds are fungi. But Karling (1944) suggested that the slime molds are closely related to Protozoa than to fungi.
Martin (1932, 1960-61) has pointed out that if the algal origin of fungi is accepted, then the slime molds are not fungi; but if the fungi have been descended from colourless flagellates, the slime molds can well be regarded as fungi, derived from the same general ancestral types as other fungi, but independent and highly specialized to constitute a group of their own—the Myxomycota.
Sparrow (1958) proposed that the parasitic slime molds be regarded as a separate Class the Plasmodiophoromycetes. The Plasmodiophoromycetes are again grouped with zocsporic fungi by some taxonomists though the Plasmodiophoromycetes have controversial affinities.
Both the Myxomycetes and the Plasmodiophoromycetes are apparently of flagellate origin. However, the cytological details are so different that the two groups are related only to the extent of a distant flagellate ancestry. It is possible that the Plasmodiophoromycetes arose very early from a primitive fungus of the Phycomycetes, most likely the Ghytridiales.
Talbot (1971), Webster (1983), and others grouped all the four classes of slime molds under the Division Myxomycota close to fungi.
Bonner considers the Acrasiomycetes to have had their origin from the free-living amoebae of the soil. This opinion is based largely on the absence of a flagellated stage. It is generally assumed that the free-living amoebae themselves came from flagellates. This assumption deserves careful consideration.
The points of divergence were apparently quite early with the result that all members of the Myxomycota show separate and independent origins along parallel lines of descent.
The Myxomycetes, or true slime molds often designated as slime molds, or slime fungi, or Mycetozoa as they are called, are a unique group of fungus-like organisms concerning whose origin and relationships there is no common agreement. They exhibit characteristic of both animals and plants.
Their somatic phase of multinucleate mass of protoplasm without cell wall exhibiting creeping movement, known as plasmodium, is definitely animal-like, resembling a giant amoeba in its structure and in its physiology. The reproductive process of the Myxomycetes, however, is plantlike producing spores with definite cell walls.
T. Panckow in 1654 was the first to discover the Myxomycetes, but they were first described in detail by Micheli in 1729. The Myxomycetes were first recognized as a natural group in 1829 by E. M. Fries. Later in 1833 Wallroth introduced the name Myxomycetes for this group of organisms. De Bary (1887) designated them as Mycetozoa or Fungus animals.
The greatest contribution to the morphology and taxonomy of the group was made by J. Rostafinski (1875-1928), a student of De Bary.
Economic Importance of Myxomycota:
The Myxomycetes are of relatively little economic importance, but they have been the subject of intensive laboratory studies. They contribute to the carbon and nitrogen cycles by using various organic matter including bacteria as food.
They provide a large amount of protoplasm free from cell walls which has been used as an ideal medium to solve variety of fundamental problems of biochemists, biophysicists, mycologists and even the geneticists.
Some of the areas of studies are: the structure and chemical composition of protoplasm, the velocity of the protoplasmic movement, the chemical changes governing the production of sporangia and spores, the behaviour of nuclei and chromosomal changes during plasmodial growth, various aspects of plasmodial compatibility, etc.
Occurrence of Myxomycota:
The Myxomycetes are common but inconspicuous inhabitants of moist dead wood, rotting logs, damp soil, leaf mold, moist sawdust, bark of trees, decaying fleshy fungi, or other organic matter. They often spend most of their lives within the substrate and emerge only when about to produce sporangia. Following periods of rainy weather they may occur on leaves of grasses or other plants on lawns.
Sexual Reproduction of Myxomycota:
The myxamoebae or swarm cells as the case may be, behave as gametes and as such take part in the sexual reproduction. Fusion may take place between the two swarm cells or two myxamoebae or between a swarm cell and a myxamoeba resulting in the formation of a zygote, when plasmogamy is followed by karyogamy.
Fusion between the two swarm cells or between a swarm cell and a myxamoeba leads to the development of a flagellated zygote which sheds its flagella and becomes amoeboid. The zygote formed in any one of the processes grows, along with which the diploid nucleus also divides mitotically giving rise to a multinucleate amoeboid structure, the plasmodium.
According to some, karyogamy takes place during zygote formation. To them, the plasmodium represents the diplophase in the life history of a myxomycete and meiosis takes place during the formation of spores in the fructification (fruit body).
Whereas, others are of opinion that karyogamy takes place just before meiosis during the development of spores in the fructification and as such the plasmodium is in the dikaryophasic condition.
Life Cycle Pattern of Myxomycota:
The general life cycle pattern of the Myxomycetes is more or less clear. The spores on germination produce one to four swarm cells or myxamoebae. The swarm cells or myxamoebae behave as gametes. They fuse in pairs, plasmogamy is followed by karyogamy. The zygote so formed grows accompanied with repeated mitotic division of the diploid zygotic nucleus resulting in the development of a plasmodium.
Many zygotes may also coalesce to produce a plasmodium. A mature plasmodium thickens and gives rise to the fructification of varied nature. During the development of the fructification (fruiting body) and spore formation the diploid nuclei divide reductionally. Each haploid nucleus with a portion of cytoplasm enveloped by a thick wall develops into a spore.
The spores vary greatly in colour and characteristics of the wall which are of taxonomic importance. Life cycle pattern of a myxomycete is indicated in Figure 327.