Coenocytic fungi which lack motile cells and produce zygospore as the sexual spore are placed under phylum Zygomycota. The zygospore typically results from the complete fusion of the two gametangia during gametangial copulation. These fungi reproduce asexually by means of aplanospores (also called sporangiospores, formed inside sporangia) or conidia. The phylum has two classes – Zygomycetes and Trichomycetes. The class Zogomycetes has 10 orders. We shall study the orders Mucorales and Entomophthorales.
Order Mucorales (Pin Molds or Sugar Fungi):
Mucorales are mostly saprobes living on dead plant or animal materials or dung. Some are weak parasites on plants and animals, while still others are obligate parasites on other fungi, especially on other mucorales or mushrooms. Mucorales are called ‘pin molds’ because of the sporangia that appear as black dots on the hyphae.
These are also called ‘sugar fungi’ because they utilize only the simplest carbohydrates (sugars), and not the complex ones, like polysaccharides. Enormous number of spores is produced in the sporangia by some genera, while only few-to-one spore is produced by others.
Sporangia having a few spores are called sporangiole. The single spore in the sporangium, by fusion with the wall of the sporangium, might have given rise to a conidium. This suggests the possible origin of conidia from sporangia. Many members of Mucorales are important in industry, for their metabolic products.
There are 13 families that are differentiated on the basis of sporangiospores and sporangia, and to a lesser extent on habitat and sexual reproduction. We shall study Mucor and Rhizopus belonging to family Mucoraceae, and Pilobolus of the family Pilobolaceae.
Family Mucoraceae:
In Mucoraceae – (1) the sporangia have a distinct columella projecting into the sporangia, and (2) the spores are passively discharged. There are 20 genera that grow on a variety of substrates.
Habitat:
Mucor is a large genus with 50 known species which are essentially saprobes. Some species, e.g., M. ramannianus and M. hiemalis, occur in soil. Other species such as M. mucedo and M. hiemalis appear regularly on the dung of herbivore animals.
The harmful effects of Mucor in the spoilage of food, textile and leather are well-known. Lesser known are their beneficial activities. They are important in the biodegradation of complex organic materials. Species of Mucor are used in the fermentation process in industry to convert starch into glucose, which is then acted upon by yeast. Yeast does not produce the enzyme amylase and, therefore, it cannot degrade starch.
Sporangiospores:
These are produced within spherical sporangia borne on unbranched sporangiophores. The sporangiophore grows as an erect hypha. Its apical growth ceases and the tip swells into a spherical structure full of protoplasm. The spherical structure is separated from the hypha by a septum which later bulges and projects into the former as a dome-shaped structure, called columella.
The spherical structure, including the columella, is called the sporangium. The protoplasm cleaves into numerous multinucleate segments, each of which secretes a wall and becomes a spore, called the sporangiospore. Meanwhile, the sporangial wall on the external surface develops minute needle-like crystals of calcium oxalate.
When the sporangial wall dissolves on maturity, the spores remain firmly adhered to the columella in a drop of fluid and are not blown off even by strong winds. The spores when dispersed, usually by insects, germinate immediately or later by forming a germ tube.
Chlamydospores:
Chains of round and thick walled chlamydospores are formed on the hyphae. When rest of the hyphae die and disintegrate, the chlamydospores survive which later germinate to give rise to the fungus.
Torula Condition:
The hyphae of M. racemosus form yeast like ‘sprout cells’ or oidia in sugar solutions. This is called yeast or torula condition.
It occurs through fusion of similar gametangia (gametangial copulation) and subsequent production of a thick-walled zygospore. Both homothallic and heterothallic species are known. The phenomenon of ‘heterothallism’ was discovered in Mucor in 1904 by A. F. Blakeslee.
The opposite mating types are designated as ‘plus’ (+) and ‘minus’ (-) strains which are morphologically alike and hence, the terms male and female could not be used.
Differentiation of sex organs starts when hyphae of similar (in homothallic strains) or of opposite strains (in heterothallic strains) come in contact. Gametangial branches, called progametangia, arise from each of these hyphae. Each progametangium enlarges at the tip which is delimited by a cross wall.
The terminal multinucleate swollen structure is called the gametangium. The remainder of the progametangium is called the suspensor. The two gametangia are of equal size. The wall between them dissolves and the protoplasm of the two intermingles. The nuclei pair and later fuse to form diploid nuclei.
The cell formed by the fusion of the two gametangia, enlarges considerably; its wall thickens and becomes black and warty. This is zygospore, though it may more appropriately be called a zygosporangium, containing a closely-fitting zygospore. Meiosis occurs before the zygospore becomes dormant.
The zygospore, on the return of suitable conditions, germinates to form a hypha which bears a germ sporangium. Half of the spores formed in the germ sporangium are of (+) plus strain while the other half are of (-) minus strain. However, in Mucor mucedo, occasionally, all the spores are of one strain only, all ‘plus’ or all ‘minus’.
This is explained as being due to the disintegration of all but one nucleus, resulting from meiosis. The surviving nucleus multiplies and the nuclei formed, which are of one kind only, enter the germ sporangium and get incorporated in the spores. Whether (+) or (-) mating type will survive is simply a matter of chance.
Genus Rhizopus:
Rhizopus, which is often considered similar to Mucor is, however, much different. Cultures of Mucor cause no contamination in the laboratory while Rhizopus is a thorough nuisance. This is because of the rapid dispersal of the spores of Rhizopus that contaminate the cultures.
Many saprobic species of Rhizopus are used in the production of industrially important substances, e.g., R. stolonifier (= R. nigricans) produces fumaric acid, R. oryzae produces alcohol, while R. sinensis, R. stolonifer and R. nodosus produce lactic acid.
Species of Rhizopus are used in the fermentation process to convert starch to sugar, to be later acted upon by yeast. Strains of Rhizopus are used in Indonesia in the preparation of a delicious food called ‘tempeh’ from soybeans which is regularly eaten by millions of people. As a parasite, Rhizopus often attacks apples and other fruits, especially during storage, causing soft rot. Sporangiophores develop on the rotted surface.
Rhizopus, as compared to Mucor, has short, stout, and stiff sporangiophores which bear black sporangia. The mycelium consists of two kinds of hyphae; the aerial hyphae called stolons, which arch over the root-like rhizoids that enter the substratum. Sporangiophores arise from the junction of stolon and rhizoids. There is no such distinction of hyphae in Mucor.
There is also a difference in the dispersal of spores. The wall of the sporangium of Rhizopus cracks into a number of small fragments; the columella collapses and the sporangiospores are quickly blown away by the wind.
The sexual reproduction, though similar to Mucor, differs in cytological details. Karyogamy takes place before the onset of dormancy of zygospore, the meiosis is delayed until its germination. In Mucor, meiosis precedes dormancy.
Family Pilobolaceae:
The family Pilobolaceae is characterized by the explosive shooting of the entire sporangium.
Genus Pilobolus (‘Shot Gun’ Fungus or the ‘Hat-Thrower’):
Pilobolus is a ‘coprophilous’ fungus, i.e., it is found on the dung of herbivore animals (horse, cow, sheep, rabbit, etc.)
For isolation, fresh dung is incubated in a damp chamber under a bell jar kept in light. After 1-3 days, a forest of sporangiophores of Pilobolus appears which can be seen with a hand lens. These may persist up to about 15 days, but usually decline after 7 days. Dung samples should not be placed in airtight containers as in that condition nematodes and insects break up the dung and produce anaerobic conditions.
Thallus:
The hyphae are coenocytic, and much branched; characteristic sporangiophores arise from the hyphae.
Asexual Reproduction:
Sporangiophores may be terminal or intercalary in position on the mycelium. If it arises terminally, the sporangiophore initially is cut off by a single septum, and by two septa if intercalary.
The sporangiophores, 0.5 to 1 cm high, are strongly phototropic. It consists of a bulbous basal cell (trophocyst) embedded in the dung; a straight stalk, which enlarges apically to form a crystal-clear sub- sporangial vesicle, and a comparatively small, black sporangium. The sporangium is separated from the sporangiophore by a columella. The sporangium contains 15,000 to 30,000 spores.
The entire sporangium is thrown off violently with a speed of 14 metres per second towards light up to a distance of about 2 metres. This forceful discharge gives the fungus names such as the ‘shot-gun fungus’ and ‘hat-thrower’. The sporangia fall on grasses, and along with the grasses reach the alimentary canal of herbivore animals. The spores pass undamaged through the alimentary canal, where they receive the pre- treatment stimulation for their germination later in the deposited dung.
The periodicity of sporangial production in Pilobolus is a rhythmic phenomenon. Successive crops of sporangia appear each morning at a definite interval of 24 hours and are discharged in the afternoon. Such rhythmic phenomena (which are several and occur in many organisms other than fungi) are called circadian rhythms or 24 hour ‘biological clock’. Sporangical production in Pilobolus occurs only in alternating periods of darkness and light. The ‘biological clock’ can be regulated by shifting the 12 hour dark and light periods so as to get sporangial production at any time during the day or at night.
Sexual Reproduction:
The genus has both heterothallic and homothallic species. Fusion of two multinucleate gametangia of opposite strains leads to the formation of a thick, smooth-walled zygospore. The suspensors are twisted around each other and then separated like the jaws of a pair of tongs. The zygospore is held by their tips.
After a period of dormancy, the zygospore germinates and forms a sporangiophore and sporangium. Meiosis occurs and haploid spores are produced in the sporangium. The germination of the zygospore, however, has been infrequently seen. It can germinate directly producing a mycelium.
Order Entomophthorales:
This is a small order comprising chiefly parasites of insects (rarely of plants), or saprobes growing in soil or on dung. A characteristic feature of the order is the breaking of the mycelium into hyphal bodies, which form conidiophores. Conidia are violently discharged. Sexual reproduction occurs by copulation between two enlarged hyphal bodies that act as gametangia (i.e., gametangial copulation). There are six families in the order. We shall study genus Entomophthora belonging to the family Entomophthoraceae.
Genus Entomophthora (The ‘Fly Fungus’):
The genus Entomophthora has eleven species, which attack several insects. E. muscae attacks houseflies and causes ‘fly cholera’ which is quite common, especially during summers. Dead flies surrounded by a halo of conidia are frequently seen on window panes. Healthy flies contact the sticky conidia and get infected. The disease quite often appears in an epidemic form.
When the sticky conidia come in contact with a fly, they adhere to its body with the help of sticky mucilage that surrounds the conidia. In absence of the host, the conidia produce secondary conidia by budding. The conidia germinate and the germ tube enters the body through thinner portions of the body, such as the inter-segmental areas, or through the joints of the appendages.
Inside the body, the free end of the germ tube buds-off to produce cells which give rise to hyphae. The hyphae soon become septate and break to form hyphal bodies. The hyphal bodies multiply by budding and soon the entire body of the fly (head, thorax, abdomen, and even the tarsi) gets choked with the fungus.
Finally, all the internal structures are destroyed. Such fungus-stuffed flies become restless and cling to any substratum (glass panes, branches, or leaves) with the help of their mouth parts. The insect dies in a firmly attached position. Shortly after death, the hyphal bodies form conidiophores which emerge through the inter-segmental portions.
The conidiophores are club-shaped and bear a single conidium at the tip. Conidiophores, protruding out from the inter-segmental regions form white bands on the body and the fly looks like a different species. Flies die characteristically in the afternoon after 5-8 days of infection. Conidia are discharged violently as in the case of Pilobolus.
The conidiophore bursts, shooting the conidium to a distance of 2-3 cm. A biconvex water- filled cavity appears in the conidium above the septum, which generates a hydrostatic pressure that brings about the violent discharge. The enormous number of ejected conidia forms a white ring (halo) around the dead fly.
Conidium formation is followed by sexual reproduction leading to the formation of zygospores. Hyphal bodies act as gametangia.
The zygospore is formed:
1. By enlargement of one of the two gametangia,
2. On an outgrowth arising between the two hyphal bodies, or
3. From one of them after fusion.
The role of the zygospore is not known; it probably germinates by a germ tube after a resting period. Parthenogenetic development of zygospore is known; thus the zygospore is called an azygospore.