Division Myxomycota: History, Characteristics and Classification

In this article we will discuss about:- 1. History of Division Myxomycota 2. Characteristics of Division Myxomycota 3. Classification.

History of Division Myxomycota:

The division Myxomycota includes the organisms like cellular slime molds, the plasmodial slime molds and the net slime molds. DeBary (1887), the Father of Plant Pathology and also the founder of modern mycology, termed the group Mycetozoa (Cr. mykes — mushroom, i.e., fungus and zoon — animal) i.e., fungus animal due to presence of their naked cell and phagotrophic i.e., holozoic mode of nutrition.

This was followed by different scientists for long time. Macbride (1899) was the first to name the group as Myxomycetes (Gr. myxa, slime and myketes, mushrooms or fungi). Martin (1960) also treated slime molds as Myxomycetes under fungi and which according to him has a protozoan ancestry.

Olive (1975) classified the group under the Kingdom Protista and divided them into seven classes: Protosteliomycetes, Ceratomyxo- mycetes, Dictyosteliomycetes, Acrasiomycetes, Myxomycetes, Plasmodiophoromycetes and Labrinthulomycetes.

Later, Hawksworth et al. (1983) kept the above seven classes under the division Myxomycota of the kingdom Fungi. Alexopoulos and Mims (1979) kept these orga­nisms under the division Gymnomycota along with other two fungal divisions namely, Mastigomycota and Amastigomycota. We have followed the classification of Ainsworth (1973) who placed them within the division Myxomycota.

The Myxomycota differs from true fungi because their vegetative body consists of only protoplast bounded by membrane and devoid of cell wall and also for their phagotrophic mode of nutrition. Economically, they have little impor­tance, but are very useful as experimental tools in the study of molecular biology, biochemistry, cytology, genetics etc.

Characteristics of Division Myxomycota:

The important characteristics of the division Myxomycota are:

1. Presence of trophic phase which may be of myxamoeba (simple uninucleate amoeboid cell), a pseudoplasmodium (aggregation of uninucleate amoeboid cells) or the plasmo­dium (a free-living multinucleate mass of protoplasm).

2. The mode of nutrition is phagotrophic.

3. The vegetative part consists of protoplast covered by plasma membrane and devoid of cell wall (cell wall is present on the spore if produced by them).

4. They resemble the primitive fungi in their mode of reproduction by producing spores within sporangia.

5. They are either holocarpic or eucarpic.

6. They exhibit variable colouration.

Classification of Division Myxomycota:

The classification of Myxomycota by Ainsworth (1973) is:

Key to Classes:

A. Assimilative phase tree-living amoebae which unites as a pseudoplasmodium before reproduction Acrasiomycetes.

AA. Assimilative phase a plasmodium.

B. Plasmodium forming a network (net-plasmodium) ……… Hydromyxomycetes.

BB. Plasmodium not forming a network.

C. Plasmodium saprophytic, tree- living Myxomycetes.

CC. Plasmodium parasitic within cells of the host plant………………………. Plasmodiophoromycetes.

A. Class: Acrasiomycetes:

Acrasiomycetes are commonly known as cellular or amoeboid slime molds, and are found profusely in the upper layer of humus in deci­duous forests and also in cultivated lands.

The characteristic features are:

1. Somatic phase commonly consists of amoe­boid cells or myxamoebae.

2. Myxamoebae aggregate to form a pseudo­plasmodium, which develops fruit body.

3. Lack of flagellated cells, except in Pocheina rosea.

4. Spore wall contains cellulose.

5. Fruit bodies may be sorocarps (in Dictyo­stelium) or sporocarp (in Protostelium).

Raper (1973) has given the outline of classi­fication of this group and expressed that the organisms may not represent natural group and it is also uncertain whether this group is related to fungi or to protozoa. He divided the class into 3 subclasses: Protostelidae, Acrasidae and Dictyostelidae.

Later, Olive (1975) raised the rank subclasses into classes as Protosteliomycetes, Acrasiomycetes and Dictyosteliomycetes.

B. Class: Hydromyxomycetes:

This group shows uncertain affinities. The thallus forms net plasmodium or filoplasmodium. They are commonly aquatic and saprobic, but rarely parasitic.

The characteristic features are:

1. The thallus consists of uninucleate spindle shaped cells, forming extensive filaments.

2. The filaments are tubular and form net-like structure, the net-plasmodium or filoplas- modium.

3. Reproduction by cyst formation, zoospore formation or by congregation.

The class consists of two orders Hydro- myxales and Labyrinthulales. But Hawksworth et al. (1983) excluded Hydromyxales from fungi and included it in Protozoans. The members of Labyrinthulales are parasitic on higher plants and also on marine algae. Labyrinthula macrocystis is a very common species, found as parasite on Zostera marina, the eel-grass.

C. Class: Myxomycetes:

This group is commonly known as true slime molds or plasmodial slime molds (Fig. 4.10). They are commonly found in damp places, espe­cially on old wood and other decomposing plant parts.

The characteristic features are:

1. The vegetative body is a free-living plasmo­dium.

2. They feed on yeast cells, protozoa, fungal spores and other substances.

3. Reproduction takes place by asexual and sexual means.

(a) Asexual reproduction takes place by fragmentation of plasmodium or binary fission in myxamoebae.

(b) Sexual reproduction takes place by fusion between flagellated zoospores or myxamoeba to form zygote, from which multinucleate plasmodium develops by mitotic divisions. They develop diffe­rent types of fructification. These are sporangium, aethalium and plasmodiocarp (Fig. 4.11D-G). Meiosis takes place during spore formation in the fructification.

Alexopoulos (1973) divided the class Myxomycetes into three subclasses: Ceratio- myxomycetidae, Myxogastromycetidae and Stemonitomycetidae.

D. Class: Plasmodiophoromycetes:

This group is commonly known as endo- parasitic slime molds. They are obligate para­sites, grow on algae, aquatic fungi and higher plants (commonly in the roots).

The characte­ristic features are:

1. Members of this class are obligate (i.e., biotrophic) parasites of fresh water algae, aquatic fungi and higher plants (commonly in the roots).

2. Somatic body consists of a naked holo­carpic plasmodium.

3. Plasmodia are of two types in their life cycle: sporangiogenous plasmodium (form sporangia) and cytogenous plas­modium (gives rise to cysts i.e., resting spores).

4. Zoospores biflagellate, having unequal flagella of whiplash type, situated in opposite direction, the shorter one in anterior and longer one in posterior side.

The class consists of a single order Plasmodiophorales with a single family Plasmo- diophoraceae. The class consists of 16 genera and 45 species. Some like Plasmodiophora brassicae causes club-root of crucifer; Spongospora subterranea causes pow­dery scab of potato.