Mucor: Vegetative Body and Reproduction | Phycomycetes | Fungi | Botany

In this article we will discuss about:- 1. Vegetative Body of Mucor 2. Reproduction in Mucor 3. Torula State or Yeast Condition 4. Physiological Control of Sexual Reactions.

Vegetative Body of Mucor:

The vegetative body of the fungus, which is a filamentous and much-branched thallus, is known as the mycelium, and the fila­ments of which the mycelium is made up are called the hyphae. Some of these hyphae penetrate the substratum, serve as anchors for the superficial mycelium and draw nourishment therefrom.

The mycelium is normally non-septate and contains abundant granular cytoplasm, numerous minute nuclei and small vacuoles; it is, there­fore, a coenocyte. While non-septate condition is characteristic of the young vigorously growing mycelium, cross walls are frequently found in the older hyphae, and the reproductive parts are generally separated from the vegetative ones by septa.

Reproduction in Mucor:

Mucor reproduces both asexually and sexually.

When conditions are favourable, the plant reproduces asexual­ly by the formation of spores (or gonidia). During the process some aerial hyphae bear at their tips some spherical sporangia (or gonidangia), and each simple aerial branch bearing a sporangium is called a sporangiophore (or gonidangiophore).

The formation of a sporangium takes place in the following way:

An aerial hypha gradually elongates, and after it has elongated to a certain height, its tip begins to enlarge into a sporangium. In­to this enlargement a certain amount of cytoplasm with many nuclei and a considerable amount of reserve food flow from the adjoining region. Then, a dome-shaped area of vacuoles appears within the cytoplasm towards the base of the sporangial wall.

This region of cytoplasm is known as the columellaplasm, which is surrounded by a peripheral layer of nucleated cytoplasm, known as the sporeplasm. The vacuoles then flatten, fuse end to end into one conti­nuous cavity, around which gradually a wall is laid down forming the so-called columella.

Then, there follows a progressive cleavage of the sporangial cytoplasm from the periphery to the centre and vice versa, with the result that the cytoplasm becomes gradually cut off into smaller and smaller polyhedral blocks, eventually resulting into small multinucleate or rarely uninucleate units.

These ultimate units take on a definite wall and become the spores without any further nuclear division. When mature, the spores are thin-walled and ovate in shape, hyaline or dully coloured in mass. The wall breaks up and ultimately disappears on coming in contact with water. In some species, where the sporangial wall is thin, a liquid droplet appears at the junction of the sporangiophore and sporan­gium on one side.

This droplet then gradually increases in size and envelops the entire sporangium. After some time, the sporangial wall disintegrates, and the droplet forms a conspicuous sporangial drop holding the spores in suspension. The sporangiophore on losing turgidity then collapses, usually brings the drop against a solid sur­face, and it spreads out rapidly owing to surface tension.

Ultimately on drying, the spore masses remain firmly adherent together with mucilage, which, however, is readily dissolved and the spores are later on separated in water. Each spore, under favourable condi­tions, puts forth one or more germ tubes, which ultimately ramify­ing develop into a new mycelium.

Under certain conditions, especially when the food supply of the mycelium is exhausted, the plant reproduces sexually and the organs of sexual reproduction are formed. The reproduction is isogamous, which consists in the conjugation of two like gametes.

During the process, two club-shaped hyphae (progametangia) of different sexes (plus and minus strains) approach, come in con­tact end on end with each other, and a conjugate cell is cut off at the apex of each progametangium by the formation of a transverse wall.

The two conjugating cells are the two gametangia, and their undifferentiated protoplasmic contents form the gametes. The re­mainder of the special branch bearing a gametangium is known as a suspensor. The wails between the gametangia generally break down, the two multinucleate gametes thereupon coalesce, cytoplasm with cytoplasm but the nuclei fuse in pairs and a zygospore is formed.

Those nuclei which do not fuse ultimately degenerate. The resulting zygospore increases in size and forms a heavy, dark and often spiny or otherwise roughened wall. After a considerable period of rest, the zygospore increases in size and then germinates, its outer wall (exospore) bursts and the inner wall (endospore) puts forth a long germ tube, known as promycelium, which ultimately be­comes terminated by a germ sporangium.

Each of the spores contained therein, on being set free germinates under suitable conditions, puts forth a germ tube, which ultimately ramifying produces a new mycelium. It is to be noted that reduction division of the diploid nuclei only occurs in the germ sporangium, and since Mucor is heterothallic, the separation of the sexes (+ and —) occurs probably in the formation of sporangia, that is, the spores of one sporangium are either all + or all —.

The gametes may develop without conjugation into a kind of spore known as azygospore or parthenospore.

Torula State or Yeast Condition of Mucor:

If a portion of the mycelium of Mucor be placed in a sugar solution, it becomes divided by transverse walls into thin-walled cells, known as oidia. These cells go on budding like the yeast and can excite alcoholic fermentation in the sugar solution producing alcohol and carbon dioxide.

Physiological Control of Sexual Reactions in Mucor Mucedo:

Plempel and his associates (1957, 1960, 1963) made extensive studies on the regulation of sexual reactions between + and — strains of Mucor mucedo. He discovered a system which could ex­plain the physiological control of sexual reactions in M. mucedo.

This system is shown in the following diagram:

According to Plempel, each strain + or — if grown separately in liquid culture produced a substance known as progamone, i.e. a + strain produced a + progamone and a — strain a — progamone When fungus free culture filtrates from either of the two were applied to mycelia of the opposite strain, synthesis of another sub­stance called gamone was induced.

In other words, culture filtrate containing a + progamone applied to—mycelia induced the for­mation of — gamone and vice versa. The gamones in turn induced zygophores (i.e. swollen hyphae which would develop into game­tangia in normal circumstances) in the opposite strains and this was highly specific, i.e. — gamone was ineffective in a — strain but could form zygophores only in a + strain and not in a — one.

The zygophores of the two opposite strains grew towards each other and showed zygotropic curvature due to the production of two zy­gotropic hormones. Plempel also demonstrated that when two agar blocks containing zygophores of two opposite strains + and — were placed at a distance of 2.5 mm from each other and separated by a mica membrane having a hole (3.5 mm diameter) the — zygophores began to grow towards the hole in the mica membrane after 6-8 hr. After 14 hr., the – zygophores were in contact with the + ones.

If the two agar blocks were separated by a greater distance, the zygophores just opposite the hold elongated more than those away from the hole. When the + strain was replaced by — strain, the two opposing zygophores were of the similar strains so instead of growing towards each other original — zy­gophores produced sporangiophores. But when the – strain was replaced by the + strain, now hyphae began to grow from the lips of — zygophores towards the + zygophores within 12 hr. of replacement.