Classification
Kingdom-Fungi
Phylum-Zygomycota
Class-Zygomycetes
Order-Mucorales
Family-Mucoraceae
Genera-Rhizopus
Species- R. stolonifer
Occurrence: Rhizopus is a common and widely distributed saprophytic fungus. It lives as a saprophyte and is frequently found producing white mycelium over the surface of moist bread and other foodstuffs. It is commonly known as 'bread mould'.
Vegetative structure: The vegetative body of Rhizopus is coenocytic which form white cottony growth on the substratum. The mycelium is differentiated into three types of hyphae they are branched root like structure called rhizoids which penetrate the substratum, the hyphae which present on the surface of the substratum which connects the rhizoids are called stolons and the hyphae which grows upward in clusters from stolons, these unbranched hyphae are called sporangiophores which contains sporangium.
The mycelium is multinucleated, aseptate, coenocytic. The cells of young hyphae contain a granular protoplast which contains nuclei, glycogen, and oil globules. The wall of the hyphae is made up of fungal chitin.
Reproduction
It reproduce both sexually and asexually.
Asexual reproduction
Asexual reproduction takes place by means of non motile spores which are produced within sporangia, borne at the apex of unbranched erect hyphae, called sporangiophores.
Under suitable conditions, certain vertical hyphae grow up in clusters from the mycelium. These unbranched upright hyphae are called sporangiophores. The tip of each sporangiophore swells to produce a globose sporangium. A considerable amount of cytoplasm with nuclei ad food material migrate into this enlarged tip. The tip of the sporangiophore enlarges and the nuclei undergo repeated divisions. The protoplasmic contents accumulate densely in the peripheral portion of the developing sporangium, leaving central much less dense portion with few nuclei and vacuolated protoplasm. A layer of vacuoles now appears between the peripheral and central portions. These vacuoles unite laterally with one another to form a dome shaped septum. This separates the sporangium into a peripheral fertile portion and an inner dome shaped sterile portion, cleavage furrows begin to start. These cleavage furrows divide the protoplasm into numerous small pieces, each with several nuclei. Each piece secretes a wall and becomes a spore.
At maturity, the wall of the sporangium becomes fragile and is disintegrated by the slightest disturbance of air current. The spores, which form a dry powdery mass, are scattered by air currents as the sporangium ruptures. The columella usually persists at the end of the sporangiophore even after the bursting of the sporangium.
The spores are small, dark coloured and globose to ellipsoidal in shape. Upon reaching a suitable substratum, each spore germinates by a germ tube, which gives rise to a new mycelium.
Sexual reproduction
Sexual reproduction occurs by the conjugation of two gametangia, some species of Rhizopus are homothallic and some others are heterothallic.
In heterothallic species, the sexual reproduction occurs when two hyphae of opposite strains come in close proximity to each other. From these hyphae, develop short lateral branches, the progametangia, the tips of which grow and ultimately come in contact. A considerable amount of cytoplasm with numerous nuclei, mitochondria and food materials migrate into these progametangia. At the end of each progametangium, a multinucleate segment is cut off by a cross wall from the remainder of the branch. Each such segment which has densly multinucleate protoplast is called gametangium, and the remainder of the branch which has more vacuole protoplast is called suspensor. The wall between the two gamatangia dissolves, their protoplasmic contents mingle and the opposite nuclei fuse in pairs to form a multinucleate zygospore. The nuclei which which fail to fuse become disorganised. The resulting zygospore enlarges and develops a dark, thick, warted wall about itself to become a resting structure. It undergoes a period of dormancy for many months.
During germination, the outer wall splits and the inner one grows out to form a hypha like germ tube or promycelium. It grows erect to form a sporogiophore, bearing a terminal sporangium. Such a sporangium is also known as germ sporangium or zygosporangium. Meosis occurs during the germination of zygospores so that the germ spores, which are produced in the germ sporangium, are all haploid. The germ spores that are produced in the germ sporangium are either all(+), or all (-), or mixtures of both types(+) and (-).
The germ spores on reaching a suitable substratum germinate to produce new hyphae. Sometimes, zygospores are produced parthenogenetically. Such parthenogentically produced zygospores are called azygospores.
Occassionally, the gametangia fail to fuse. The gametangia become surrounded by a thick wall resulting in the formation of azygospore.