Depict with a neat diagram and explain the mode of reproduction in rhizopus, the bread mould.
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Answer:
Rhizopus is a genus of common saprophytic fungi on plants and specialized parasites on animals. They are found in a wide variety of organic substances , including "mature fruits and vegetables", jellies, syrups, leather, bread, peanuts, and tobacco. They are multicellular. Some Rhizopus species are opportunistic agents of human zygomycosis (fungal infection) and can be fatal. Rhizopus infections may also be a complication of diabetic ketoacidosis.This widespread genus includes at least eight species.
Rhizopus Structure
They are fast-growing fungi and have a cottony appearance
The body of rhizopus consists of branched mycelium. The mycelium is coenocytic and composed of three types of hyphae; stolon, rhizoids and sporangiophores
Stolon is the internodal region, it is aerial, forms an arch and touches the substratum forming nodal region
Rhizoids are formed where the stolon touches the substratum at nodes. They are branched, anchor the mycelium to the substratum and absorb food
Life Cycle of Rhizopus
Rhizopus reproduce by all the three processes, i.e. vegetative, asexual and sexual
Vegetative reproduction is by fragmentation and each of the fragments of a stolon develops separately making a complete mycelium
Asexual reproduction is by the formation of sporangiospores and chlamydospores
Formation of Sporangiospores: Sporangiospores are formed terminally in sporangia of aerial mycelium called sporangiophores. They are formed under favourable conditions.
Sporangiophores develop from the upper side of the rhizoidal node
The apical part swells up forming sporangium, as nuclei and cytoplasm move apically
The cytoplasm of sporangium differentiates into the denser peripheral region with more nuclei and the central columella region with fewer nuclei and more vacuoles
Sporangiospores develop inside sporangium. They are multinucleated and non-motile
The wall of sporangium ruptures after maturation and sporangiospores come out as a powdery mass
After getting suitable condition and substratum, each spore germinates into a new mycelium
Formation of Chlamydospores: Chlamydospores are formed during unfavourable conditions. An intercalary segment of mycelium develops due to the formation of septae and accumulation of protoplasm. It is thick-walled and detaches from the mycelium once it dries. They remain dormant until the favourable conditions return and then germinate to form a new mycelium.
Sexual reproduction is by fusion of two compatible hyphae. Most of the Rhizopus species (R. stolonifer) are heterothallic, i.e. having different mycelium for + and – mating strains. R. sexualis is homothallic.
The compatible hyphae come together. In both the mycelia, a small outgrowth develops. It is known as progametangia
Nuclei and cytoplasm move towards the apical region and progametangia make contact
The apical region is separated from the rest of the hyphae by septae formation. This is known as gametangia
Gametangia conjugate to form a multinucleated structure
Plasmogamy is followed by karyogamy and diploid (2n) zygote is formed. It is known as zygospore. Rest of the unpaired nuclei degenerate
Zygospores enlarge and become thick-walled and resistant to adverse environmental conditions. On getting favourable conditions, zygospores germinate
The inner wall of zygospore develops into promycelium forming germ sporangiophore, with germ sporangium formed apically
Meiosis occurs and haploid meiospores are formed. They come out after rupture of the germ sporangium wall and develop into new mycelia
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