Question

Describe the life cycle of Rhizopus stolonifer. Include the Class it belongs to and the structures found in their sexual and asexual stages. Name the four (include Fungi imperfecti) classes of fungi we dealt with in the laboratory and describe how these fungi are classified. Include the Latin definition for each of the classes named.

Answer 1

ans:) Fungi are usually classified in four divisions: the Chytridiomycota (chytrids), Zygomycota (bread molds), Ascomycota (yeasts and sac fungi), and the Basidiomycota (club fungi)

• Rhizopus stolonifer is commonly known as black bread mold.[1] It is a member of Zygomycota and considered the most important species in the genus Rhizopus

the life cycle of Rhizopus stolonifer  is:

asexual stucture found in the life cycle of this fungai are sporangiospore and chlamydospore:

(a)Sporangiospore Formation:

During favourable condition, the non-motile spores such as sporangiospores or aplanspores are formed inside the spo­rangium. The sporangium develops singly at the apex of sporangiophore.

The sporangiophore develops in tuft from the upper side of node opposite to the rhizoidal hyphae . Initially, a number of elon­gated hyphae develop aerially from the upper- side of the node which elongate upto a certain height.

The nuclei and cytoplasm push more and more towards the apical side, consequently the apex of the aerial hyphae swells up . The swollen part enlarges and develops into a large round sporangium

With matu­rity, the protoplast inside the sporangium is differentiated into a thick dense layer of multi­nucleate cytoplasm towards the peripheral region just inside the sporangia! wall, called the sporoplasm and a vacuolated portion with a few nuclei towards the centre, called columellaplasm. A series of small vacuoles then appears between the sporoplasm and columellaplasm .

These vacuoles become flattened and coalesce to form a continuous cleavage cavity. This is followed by the formation of a septum towards innerside of the cavity. With further development, the septum becomes dome-shaped and pushes its way into the sporangium .

Protoplast of the sporoplasm then undergoes cleavage to produce many small multinucleate (2-10 nuclei) segments. These segments are transformed into globose non-motile sporangiospores

After the maturation of spores, the wall of sporangium dries and columella collapses like an inverted cup with irregular surface. The sporangial wall breaks in different fragments leaving a portion as collar on the sporangiophore. The powdery mass of spores are exposed to the atmosphere .

(b) Chlamydospore:

During unfavourable condition, thick-walled, nutrition-rich, intercalary mycelium segments arise by septation of mycelium, termed as chlamydospores. They get separated from each other when the connecting mycelium dries up. With the onset of favourable condition, the chlamy­dospore germinates and gives rise to a new mycelium.

Sexual Reproduction:

Sexual reproduction takes place during unfavourable condition by means of gametangial copulation. The gametangia look alike, but equal or unequal in size and, by conjugation, they give rise to zygospore. Most of the species of Rhizopus are heterothallic (Rhizopus. stolonifer), but few species (R. sexualis) are homothallic.

In heterothallic species, zygospores are pro­duced by the union of two gametangia deve­loped from mycelia of compatible strains ; whereas, in homothallic species, the uniting gametangia develop from mycelia that derived from a single spore.When heterothallic species are cultured, two mycelia of compatible strain come near to each other, the mycelia produce small out­growth, called progametangia .The apical region of the two progametangia come in close contact Nuclei and cytoplasm of each progametangium push more and more towards the apical region which swell up with dense protoplasm.

The rear region becomes vacuolated. A septum is laid down separating the apical region, which is called gametangium; and the basal region, is called suspensor. The undifferentiated multi-nucleate protoplast of the gametangium is called aplanogamete or coenogamete

There is much variation in size of the gametangium pairs. In some pairs, the uniting gametangia are equal in size, but in other pairs they are unequal.

After maturation of gametangia, the common wall at the point of their contact dissolves and the protoplast of both the gametangia unite to form zygospore The nuclei of opposite gametangia fuse together to form diploid (2n) nuclei and unpaired nuclei gradually degenerate.

The young zygospore enlarges and probably secretes five layered (two in exospore and three in endospore) thick wall, which undergoes a period of rest

After resting period, the zygospore germi­nates. On germination, the innermost layer comes out after cracking the outer walls and produces a promycelium. With further develop­ment, the promycelium is differentiated into a lower stalk like germsporangiophore and an upper spherical germsporangium

• four class of fungi are classfied on the basis of presence and absance of mycellium septa:

mycellium septa present in:

• .ascomycetes(sac fungi)
• basidomycetes (club fungi)

mycellium septa absent in :

• oomycetes( oogamus fungi)
• zygomycetes( conjugation fungi)

and one more known as fungi imperfecti in which sexual reproduction is absent. ​​​​​​​deuteromycetes.

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