Life cycle of Rhizopus
Rhizopus is a saprophytic fungus, commonly found on dead organic matter rich in carbohydrates. Mycelium is made up of white narrow thread like hyphae growing on the surface of sub substratum. Two types of vegetative hypha (Rhizoidal and Stoloniferous) arise from definite points called hold fast / apparent nodes. Third hypha is asexual. Called Sporangiophore and fourth type is sexual hypha called. Zygophore. The absorptive hyphae are called rhizoidal hypae which penetrate into the substratum. The hypae appeared over the surface of substratum are in the form of stolons. From the nodes of stolons arise branched rhizoidal hypae on the under surface and a group of aerial structures called sporangiophore (asexual hypha) from the upper surface of ‘apparent nodes’. The apical portion of each sporangiophore ends into a swollen structure called sporangium which is filled with sproes.
The hyphae are coenocytic. , aseptate and branched. It has many nuclei, oil drops, glycogen bodies and vacuoles in the cytoplasm.
The reproduction in Rhizopus is vegetative, asexual and sexaul.
1.Vegetative reproduction : This takes place by fragmentation of hypahe.
Asexual reproduction : Asexual reproduction takes place by the formation of non-motile spores inside a sporangium. The tip of aerial hypha swells in which cytoplasm migrates with nuclei. The tip swells considerably and the nuclei divide repeatedly. The content of the young swollen tip differentiate into a central zone called columellaplasm mainly filled with vacuolated cytoplasm surrounded by a peripheral zone called sporangioplasm containing dense cytoplasm and many nuclei. Vacuoles ultimately form a continuous vacuolated layer by fusing laterally one after the other and ultimately develop into a dome shaped septum known as columella (sterile part). In the meantime, cleavage of sporangioplasm takes place resulting into innumerable, small 2-10 nucleate portions which round up , become invested with spore membranes, and develop into nonflagellate spores, the Sporangiospores. These are formed under most favourable conditions. Thus, the sporangium is large, globose and contains many spores. Spores are dispersed by bursting o the thin wall of the sporangium due to pressure that is set up in the columella.
Other two asexual spores are oidia and chlamydospores ( formed under unfavorable conditions). The spores on germination produce a germ tube giving rise to new mycelium.
3.Sexual Reproduction : Sexual reproduction in Rhizopus takes place by the formation of two multinucleate gametangia. Both the gametangia are similar externally but are different physiologically i.e. they are of (+) and (-) type. It is called heterothallism (This was discovered by Blakeslee in Rhizopus stolonifer). Another species called Rhizopus sexualis is a homothalioc species.
When two mycelia of opposite strains, (+) behaving as male and (-) behaving as female respectively ], come near one another , under influence of a chemical called trisporic acid, this stimulates the formation of special sub-aerial hypha called zygophores. This hypha produces small outgrowths, called progametangia. Their apical ends are swollen and filled with multinucleate protoplasm. This apical portion of these progametagia, comes in contact with one another. A septum is laid down, separating the terminal portion which is now termed gametangium. The remaining part of progametangia is called suspensor. The multinucleate undifferentiated protoplast of each gametangium is termed as coenogamete. As the gametangia mature, the separating wall dissolves from the middle to outward and intermingling of the contents of two gametangia takes place (Gametangial copulation). Nuclear pairing and fusion of one (+) and another(-) nuclei gives rise to a large number and secretes several layers of thick wall (5 layered, 2 layered exosporium and 3 layered endosporium) around it. As the zygospore mature it breaks up the original gametangial wall into small pieces that fall apart exposing the outer thick , spiny and black exosporium.
Meiosis occurs at the time of germination of zygospore. The zygote germinates after rest period. On germination , the exosporium cracks and endosporium produces a germsporangiophora (promycelium) that terminally develops a germ sporangium (zygosporangium) which bears large number of spores.
The meiosis produces 4 haploid nuclei where only one remain functional. This divides repeatedly to produce coenocytic mycelium with many haploid nuclei.
Occasionally, failure of gametangial copulation results in parthenogenetic development of zygospore which are called azygospores (parthenospores)