In this article we will discuss about:- 1. Occurrence and Distribution of Physarum 2. Economic Importance of Physarum 3. Origin and Phylogenetic Relationship.
Occurrence and Distribution of Physarum:
The slime molds feed on bacteria, protozoa, other small organisms and dead organic matter. The Physarales usually live as saprophytes.
They can easily be encountered on dead and decaying undersides of fallen rotting leaves, twigs and logs of wood often under bark, decaying vegetable matter, and fleshy fungi, on damp soil and frequently on damp rocks in high humidity, cool temperature and away from the direct rays of sun.
In fact they love shady, cool, damp and dingy places in the forests where there is plenty of decaying vegetable matter. They have been reported from all parts of world but are quite common and active in the humid tropical forests.
They also appear in abundance following warm, rainy periods in temperate zone forests, especially in late spring when there is plenty of decaying vegetable matter. A few species have been reported to occur on the alpine region. A little after and during rainy season the slime molds are found on the leaves of grasses or other plants on the lawns.
Economic Importance of Physarum:
The slime molds are not of much economic value. During the rainy season and just after, they are most abundant on lawns and in gardens. They creep over ornamentals and render them unsightly. Sometimes they even smother the seedlings in beds. Like many bacteria and true fungi, the slime molds bring about decay and decomposition of organic materials on and in the soil.
In recent years, the slime molds have become the subject of intensive laboratory studies on biological problems which challenge the biologists. Particular attention has been given to protoplasmic streaming, reproduction and morphogenesis.
The Plasmodia of true slime molds are acellular, hence form an excellent material for the study of structure and physiology of protoplasm. They are considered ideal tools for experimental studies on physiology and development in lower organisms. Their remarkable beauty, attractive colours and delicately constructed fruiting bodies of intricate designs are of immense artistic value.
Origin and Phylogenetic Relationship of Physarum:
The evolutionary origin of slime molds and their inter-relationships are obscure. Both are a matter of speculation. Some biologists hold that the slime molds form a connecting link between the plants and animals as they possess characteristics common to both.
The somatic phase is animal-like but the fruiting bodies are more plant-like. The supporters of this view consider the slime molds to have evolved from the flagellate ancestors and consider them closely related to animals like Amoeba of the class Sarcodina, phylum Protozoa.
The formation of a fruiting body (sporangium) in the slime molds and the cellulose nature of the spore wall are plant characteristics. The absence of chlorophyll indicates their fungal or bacterial nature. The slime molds resemble one group of bacteria called Myxobacteriales in several ways.
Thus the possibility of Myxobacteriales as a potential ancestral stock of slime molds is not excluded. Some mycologists are, however, of the opinion that the resemblance between the slime molds and Myxobacteriales is simply an instance of parallel evolution.
Furthermore the slime molds differ from the bacteria in their cellular organisation. In slime molds, the nuclei are bound by membranes and chromosomes are involved in mitosis. Besides, they are unlike the bacteria and true fungi in their plasmodial stage and phagotrophic nutrition.
Some mycologists, however, include the slime molds in the fungi and connect them to the Phycomycetes by such forms as Plasmodiophoromycetes. They consider that the slime molds (Myxomycetes) are related to the Plasmodiophoromycetes which include the endoparasitic slime molds. The two resemble in having Plasmodium as the vegetative phase and in the type of motile cells.
The recent discovery that the slime molds and Plasmodiophoromycetes are similar in having two unequal whiplash type flagella has tempted them to group the two together under the division Myxomycophyta. However, the differences between the Myxomycetes and Plasmodiophoromycetes are more fundamental.
In their life cycle, general organisation, mode of life (being obligate endoparasites), haploid somatic phase and absence of fructifications, the Plasmodiophoromycetes differ from the slime molds.
The presence of naked biflagellate swarm spores in the life history of slime mold is suggestive of the fact that they have originated from biflagellated ancestors among the Phycomycetes (Oomycetes).
Since both the flagella of Myxomycetes are of whiplash type though unequal in size indicates that the slime molds originated independently of the Phycomycetes in which one of the two flagella is of whiplash type and the other of tinsel type.
Moreover, occurrence of a prolonged diploid phase is unusual among the true fungi. Martin, therefore, considers the Myxomycetes to have evolved independently of the true fungi and thus places the slime molds in a subdivision of their own which he calls Myxomycotina. They have given rise to no higher forms of life. Alexopoulos subscribes to this view.