Physarum
Physarum:-
1. Classification:-
Kingdom:- Myceteae
Division:- Gymnomycota
Class:- Myxomycetes
Sub-class:- Myxogastromycetidae
Order:- Physarales
Family:- Physaraceae
Genus:- Physarum
2. Important Species:- Largest genus of Myxomycetes with over 100 species.
i. P. polycephalum (Many headed slime mould)
ii. P. cinerium
iii. P. vicomum
iv. P. gyrosum
v. P. stellatum
vi. P. dictyosporum
3. Habit and Habitat:-
> It is found in cool and shady places where organic matter hlods abundant moisture.
> It appears abundantly in rainy and winter season.
> P. polycephalum is a common slime mold in North America used as a research tool in the McArdle laboratory for cancer research, University of Wisconsin.
> P. cinereum form colonies several feet in diameter on city lawns in the form of bluish patches.
5. Thallus structure:-
> Plasmodium:- It is the naked (without cell wall) and multinucleated mass of protoplasm which moving and feeding in amoeboid fashion.. It is delimited by a thin plasma membrane and a gelatinous sheath.
> Shuttle streaming (Reversible streaming):- Protoplasm of plasmodium moves rapidly at one time in one direction and at other time in opposite direction. This phenomenon is called as shuttle streaming.
In P. polycephalum speed of shuttle streaming is 1.35mm / second. It is the greatest velocity of protoplasmic flow.
> True plasmodium:- Aggregation is completely absent so called as true plasmodium.
> Phaneroplasmodium:- Gr.- phaneros = visible
plasma = a moulded object
- Most advanced plasmodium.
- Elongate and branched.
- Very thick and visible strands form network inside protoplasm.
- Protoplasm is granular.
- Protoplasm is differentiated into 2 zones: Ectoplasm and Endoplasm.
- Slime sheath present.
- Rhythmic and reversible streaming.
> The pseudopodia of the assimilative stages are always filopodia.
> Sclerotium:-
- During unfevourable conditions a phaneroplasmodium becomes converted into an irregular hard mass that remains dormant for a long time. It is known as sclerotium.
- It grows out into a plasmodium again when fevourable conditions for growth retuns.
- Sclerotium of P. polycephalum composed of small cells which are known as spherules.
- Spherules vary in size from 10 to 25μm and each contain 0 - 14 nuclei.
- Three major activities during sclerotization:
i. Condensation of cytoplasm.
ii. Cleavage of protoplasm into portions.
iii. Formation of walls consisting two different fibrillar layers.
6. Reproduction:- Asexual reproduction is entirely absent. Only sexual reproduction is found which includes following points:
a. Structure of sporophyte:-
> Peridium is a proteinaceous outer covering.
> CaCO3 (lime) is present in whole of the sporophore.
> Stalk is always stuffed, i.e. filled with strands.
> Columella is true and spherical in shape.
Exceptions:-
i. P. stellatum has spherical pseudocolumella.
ii. P. dictyosporum has rod shaped pseudocolumella.
Psudocolumella:- If columella is free from peridium and located in center of the spore sac. It consisting fused lime nodes. It is spherical or rod shaped.
> True capillitium is found which is formed by the deposition of material in preformed vacuoles.
> Granular lime is found on both peridium and capillitium.
> Light is necessary for the fruiting of P. polycephalum and other yellow plasmodial species.
Note:- P. polycephalum consist of both homothallic and heterothallic strains, whereas other slime moulds consist only homothallic or heterothallic strains.
b. Type of sporophore:-
> It is a simple sporangium.
> It is and individual sporophore.
> Each sporangium has its own peridium.
> Its hypothallus is transparent and cellophane like.
c. Development of sporophore:- It is of subhypothallic type.
d. Formation of Meiospores:-
> Meiospores are of endogenous origin.
> These spores are dark black in colour and in the form of spore masses.
> During the time if capillitium development, meiospore formation starts.
> Spore wall of P. polycephalum consist chemicals:
i. 81% Galactosamine
ii. 15% Melanin
iii. 3% Amino acids
iv. 1% Phosphate
e. Germination of meiospores:-
> Wet conditions stimulate the germination of meiospores.
> Meiospores germinate by slit germination.
> On germination, meiospore gives either myxamoebae or flagellated swarm cells, that depends upon environment.
> Both myxamoebae and swarm cells are interchangeable with each other.
Note:- In P. polycephalum and P. vicomum, myxamoebaa are completely absent in their life cycle.
f. Sexual fusion:-
> It takes place between either two myxamoebae on two swarm cells.
> Before sexual fusion when food is abundant and environmental conditions are fevourable, myxamoebae or swarm cells divide rapidly, giving rise to a large population of haploid cells.
> These divisions are always:
i. Centric i.e. centriole present
ii. Open i.e. nuclear membrane disappear during karyokinesis.
Note:- This type of division is characteristics feature of all haploid myxomycetes nuclei.
> During the time of fusion posterior ends of swarm cells make contact.
> Motile zygote swims for a time and after that detach its flagella and convert into myxamoebic zygote.
> In P. gyrosum fusion takes place between both flagellated and Non - flagellated cells.
g. Plasmodium formation:-
> Zygote undergoes mitosis to form diploid plasmodium.
> Mitosishave following characteristics:
i. Synchronous division:- All nuclei divide at the same time.
ii. Non - centric mitosis:- Centriole absent.
iii. Closed (Intranuclear):- Nuclear membrane does not disppear during the time of karyokinesis.