In this article we will discuss about:- 1. Introduction to Myxomycetes 2. Characteristics of Myxomycetes 3. Somatic Phase 4. Reproductive Phase 5. Classification.
Introduction to Myxomycetes:
It is a group of organisms of great scientific interest. Some of these are of remarkable beauty, have delicate structure and brilliant colours. They exist in non-green slimy masses of protoplasm sending out pseudopodia. This has earned for them the name slime molds or slime fungi.
They are found in cold, moist shady places in dead organic matter such as decaying logs and fallen dead leaves in the woods. The class includes the true plasmodial acellular slime molds most of which produce pigmented spores in small, delicate sporangia.
Characteristics of Myxomycetes:
The chief characteristics of this class are:
1. The somatic phase is represented by a multinucleate apparently naked acellular slimy protoplasmic mass called the Plasmodium.
2. The Plasmodium is the product of syngamy, hence a diploid structure.
3. The diploid Plasmodium is holocarpic, free living and active. It contains and secretes slime.
4. Normally at the fruiting time, the entire Plasmodium is organised into one or more plantlike reproductive structures, the sporangia (sporophores) or under conditions of stress and strains it becomes converted into an irregular hard structure, the sclerotium.
5. The reproductive phase is stationary.
6. With the exception of three species which are exosporous, all the others are endosporous and produce spores within sporangia.
7. The sporangium generally develops a tough non-cellular layer or wall called the peridium which is often studded with tiny crystals of calcium salts.
8. Within the peridium is usually an intricate network of fine tube-like structures constituting the capillitium.
9. The numerous spores are differentiated from the diploid protoplast of the sporangium by meiosis.
10. The encapsulated haploid spores or meiospores are close packed between the fine tubes of the capillitium but are free from them when mature.
11. The spore wall is differentiated into two layers, the outer of which is sculptured or spiny.
12. On germination, the haploid spores or meiospore give rise either to myxamoebae or biflagellate swarm cells which function as gametes.
13. The swarm cells or myxamoebae do not produce slime.
14. The sporangium in Physarum thus functions as on organ of sexual reproduction.
15. Sexual reproduction is of isogamous type.
16. The diploid zygote, by repeated mitoses but no cytokinesis, directly gives rise to multinucleate Plasmodium.
Somatic Phase of Myxomycetes:
The creeping multinucleate apparently naked Plasmodium which represents the somatic or assimilative phase in the life cycle of Myxomycetes, varies in structure in the different species. Alexopoulos described three types namely protoplasmodium, aphanoplasmodium and phaneroplasmodium.
The Plasmodium in some Myxomycetes (order Echinosteliales) is simple and of a primitive type. It is a uninucleate tiny mass of nearly homogeneous slimy protoplasm which form pseudopode but shows no distinction into veins.
It is the smallest among the Myxomycetes and remains microscopic as long as it exists. The cytoplasmic stream is indistinct, slow and irregular. At the fruiting time it gets converted into a single sporangium (Fig. 2.1 A).
This type of Plasmodium is characteristic of the order Stemonitomycetales. Early in its development the aphanoplasmodium looks very much like a protoplasmodium. During further growth it elongates and branches, finally resulting in a network of delicate strands.
The aphanoplasmodium lacks the slimy sheath. The plasmodial protoplasm is less granular and thus transparent and not easily visible. The distinction into ectoplasm and endoplasm in not conspicuous. The cytoplasmic streaming is, however, rapid and confined by a fine membrane (Fig. 2.1 B).
It is the most common type and characteristic of the order Physarales. The mature phaneroplasmodium is a massive structure. In its initial stages of development it is very much like the proloplasmodium. The multinucleate slimy protoplasm of the phaneroplasmodium is highly granular.
It is differentiated into ectoplasm and endoplasm. At maturity it is divisible into an anterior fan- shaped perforated sheet of protoplasm and posterior zone consisting of a reticulate network of tubular veins or strands in which flows the rapid endoplasmic stream (Fig. 2.1 C).
Under conditions of stress and strain, the phaneroplasmodium becomes converted into an irregular hardened mass of thick-walled cellular units. It is termed the sclerotium.
The polynucleate thick-walled units constituting the sclerotium are termed spherules. The sclerotia and spherules remain dormant under conditions unfavourable for vegetative growth. With the return of conditions suitable for growth, the sclerodium germinates to give rise to a new Plasmodium.
Reproductive Phase of Myxomycetes:
Normally on reaching a certain stage of maturity, the Myxomycete Plasmodium passes into the reproductive stage. During this stage the entire Plasmodium becomes converted into one or more fruit-like bodies, the sporophores or spororigia which bear the spores. This process is termed sporulation.
With the exception of three species belonging to the order Ceratiomyxales which bear spores externally and are termed exosporous, all other Myxomycetes are endosporous. The latter bear spores within sporophores. The sporophores in the endosporous Myxomycetes chiefly are of three types namely sporangia, aethalia and plasmodiocarps.
Most of the endosporous Myxomycetes (order Physarales) produce fruit bodies of this type. At the fruiting time the planeroplasmodium becomes converted into a group of several stalked, sometimes sessile sac-like structures, the sporangia. The sporangia in the group remain separate from one another.
However, they arise in close proximity from a common thin transparent cellophane like base called the hypothallus. The latter is secreted and deposited during conversion of the plasmodium on the portion of the substratum formerly occupied by it. In some cases the hypothallus is absent (Fig. 2.2). Examples are Arcyria, Physarum, Trichia, Didymium and others.
Aethalium type of fruit body is characteristic of Lycogala and Fuligo. The Plasmodium, at the fruiting time is fairly large structure. It becomes converted into a group of saclike sporangia that do not separate from one another.
The entire fructification which is termed aethalium, is enclosed in a common peridium and share a common hypothallus. The sporangial walls in the aggregation may be distinct, hardly visible or not present at all (Fig. 2.3A).
In forms like Hemitrichia the fruit body is very much like a sessile sporangium that retains the shape of the plasmodial venation. This type of sporophore is called a plasmodiocarp. It is formed by the concentration of the plasmodial protoplasm around some of the main veins followed by the development of peridium around each (Fig. 2.3B).
A typical sporophore or shorangium consists of six parts namely hypothallus, stalk, peridium, columella, capillitium and spores. All the sporophores produce spores but may or may not have all the other five parts. Generally the spores are globose in form and have a thick cell wall which is usually sculptured and rarely smooth. The mature spores are uninucleate and haploid. Meiosis occurs in the spores when young.
Classification of the Myxomycetes:
The class Myxomycetes is often divided into the following three subclasses:
It contains a single order Ceratiomyxales represented by a single family Ceratiomyxaceae. Ceratiomyxa with three exosporous species is the only genus included in this family.
It includes a single order Stemonitales which comprises only one family Stemonitaceae. It is endosporous. The somatic phase is of aphano-plasmodium type.
It comprises four orders namely Physarales, Siceales, Echinosteliales and Trichiales. All are endosporous. The somatic phase is usually of protoplasmodial or phaneroplasmodial type. The class Myxomycetes thus contains six orders. Of these Physarales belonging to the subclass Myxogastromycetidae is the most important.