Mucorales: Occurrence and Reproduction | Zygomycetes

In this article we will discuss about:- 1. Occurrence of Mucorales 2. Somatic Structures of Mucorales 3. Reproduction.

Occurrence of Mucorales:

This order is by far the largest and includes about 250 species which are placed under 45 genera. They are chiefly terrestrial saprophytes. A few species are parasitic on plants and animals. The saprophytic species are commonly found on moist, decaying, organic matter such as dung or decaying plant or animal matter rich in starch and sugars. Mucorales are familiarly known as the ‘black molds’.

Somatic Structures of Mucorales:

The thallus is a well-developed, branched mycelium which is freely exposed to air. In some species it produces rhizoidal hyphae at the points of contact with a hard surface. Ultrastructurally the hyphal protoplast, besides the nuclei, contains mitochondria, ribosomes, endoplasmic reticulum, and lipid droplets.

Dictyosomes consisting of stacks of cisternae and centrioles are absent. Vesicle-like structures are common in the growing hyphal tips.

The hyphae are coenocytic and aseptate. Septa, however, are formed at the bases of reproductive structures (sporangia and gametangia) or occasionally in old hyphae. Such septa are solid plates forming complete partitions.

The septa, as usual, arise as annular ingrowths beginning at the hyphal wall. The ingrowth advances towards the centre forming a complete portion. In some of the advanced Mucorales, the hyphae are regularly septate.

The septa are perforate, each has a central pore. In some forms there are tubular extensions projecting forward in the direction of the protoplasmic flow and others have special type plugs. Pracker et al (1970) reported the occurrence of chitosomes in Mucor cousil which they speculated to function in the formation of chitin-chitosan in the walls of mucorraceous fungi.

Some mucorales are dimorphic. Under typical conditions they produce a well-developed mycelium but when placed in liquid media under normal environmental conditions they exist as yeast-like forms.

Asexual Reproduction in Mucorales:

It takes place by means of non-motile spores (sporangiospores) which are produced in large numbers within globose usually columellate sporangia borne singly at the tips of simple or branched sporangiophores.

Some species produce small, few spored or even monosporous sporangia which are called sporangioles or sporangiola (sing, sporangiolum). The sporangiola may be with or without columella. Some species produce both sporangia and sporangia on the same sporangiophore which is branched. The deciduous monosporous sporangia or sporangiola are regarded as conidia.

In certain species the sporangial and spore walls are completely fused. The term conidia applies appropriately to such one-spored sporangiola.

It is obvious from this account that within the Mucorales, we can trace the evolution of conidium—a typical asexual spore of the Ascomycetes from a multispored sporangium by reduction in the number of spores from several to one and subsequent fusion of the sporangial and spore walls.

The development of true conidia in the genus Cunninghamella of Mucorales has led some mycologists to advance the view that the Ascomycetes and Zygomycetes have evolved from a common ancestor. Khan and Talbot (1975), however, showed by TEM that the spore and sporangial walls in the aforesaid organs are closely applied but not fused with one another. The use of term conidium is thus inappropriate.

Spore Structure of Mucorales:

The mature spores of Mucorales are typically multinucleate. In form they vary from globose to ovoid but cylindrical in some species. Ultrastructurally the spore wall is bilayered. The outer layer is electron-dense and the inner electron-transparent.

The cytoplasm contains one or more nuclei, ribosomes, mitochondria, vacuoles and reserve food in the form of lipid droplets. Endoplasmic reticulum is sparse. The production of asexual spores is determined by environmental factors such as temperature, humidity and light conditions.

(a) Effect of temperature:

Investigations of Barnett and Lilly (1950, 1955) on Choanephora cucurbitarum, which produces both multispored sporangia and monosporous sporangiola have shown that sporangial production is favoured by high temperature (30-31°C) and 100 per cent relative humidity. Conidial production is inhibited under such conditions. It is favoured by low temperature (25°C) and low relative humidity.

(b) Effect of light:

Under conditions of continuous dark, continuous bright light (60 foot candles) or darkness followed by bright light no conidia are formed. Abundant conidia are produced when a period of darkness is preceded by bright light. The conclusion is that substance which induces production of conidia is synthesized in two steps. The first step requires light and the second step is inhibited by light.

Sexual Reproduction in Mucorales:

Some species of mucoraceous fungi are homothallic but the majority are heterothallic. Sexual reproduction is isogamous or nearly so and takes place by gametangial copulation. Two short, side branches the progametangia arise from the adjacent hyphae. The hyphae bearing the progametangia are called zygophores. The progametangia grow towards each other and come into contact end to end.

A small, terminal multinucleate cell, the gametangium is cut off by a septum at the tip of each progametangium, the remainder of which is called a suspensor. In many species the gametangia are similar but in a few others one is larger than the other.

The walls between the two gametangia dissolve and their contents mingle in the fusion cell. Karyogamy eventually takes place. A single zygospore is formed within the fusion cell termed the prozygosporangium.

It enlarges and secretes a thick multilayered pigmented sculptured wall around it to become the Zygosporangium. Single zygospore develops within the zygosporangium. After the resting period, the zygospore germinates. The thick zygosporangium wall ruptures and a hypha emerges.

It grows to a certain height and forms the terminal germ sporangium. The protoplast of the germ sporangium undergoes cleavage to produce non-motile spores called the germ spores which are genetically haploid. It means meiosis takes place during zygospore germination.

In meiosis segregation of strains takes place.

Thus, the germ sporangia in the heterothallic species may contain:

(i) All the spores of the same mating type (+ or –), or

(ii) Of three mating types: +, – and ± or

(iii) Of two types : + and -. How it happens we shall see shortly.

The zygosporangium in Phycomyces blakesleanus is protected by sterile, forked bristles arising from the suspensors. In the genus Mortierella it is surrounded and protected by a cottony sheath of sterile hyphae. Endogone goes a step further. A number of neighbouring zygosporangia are enclosed in a common hyphal sheath or peridium to form a tiny fruiting body resembling the ascocarp of the Ascomycetes.

This tendency to increased protection of fruiting body is a distinct approach to the Ascomycetes and lends support to the view that such species of Mucorales represent the ancestral line from which the Ascomycetes may have developed.

Differentiation of Sex in Mucorales:

In many species of Mucorales, the fusing gametangia are borne on the zygophores arising from the some mycelium. Such species are called homothallic. Several species, however, have two distinct types of mycelia. These are similar outwardly but function differently in reproduction. They belong to two different mating types.

One of these is labelled as plus strain and the other minus strain. The zygospores are produced only when a plus strain zygophore and a minus strain zygophore come into contact. The species which require two distinct types of mycelia (plus and minus strain) to form zygospores are called heterothallic. It was Blakeslee who first discovered the phenomenon of heterothallism in Mucorales in 1904.