Life cycle of Rhizopus

Rhizopus belongs to the phylum Zygomycota.

It is often found on decaying organic matter like bread and fruits and is commonly known as a bread mold. 

The life cycle of Rhizopus involves both asexual and sexual phases.

Asexual Reproduction

Asexual reproduction is the primary and most rapid method of reproduction for Rhizopus. It allows the fungus to quickly colonize a food source under favorable conditions, which include a moist environment and an abundance of nutrients.

1. The main body of the fungus is the mycelium, a network of branched, thread-like filaments called hyphae. In Rhizopus, these hyphae are coenocytic, meaning they lack septa (cross-walls) and contain multiple nuclei within a single cytoplasm.

2. Specialized hyphae called rhizoids anchor the mycelium to the substrate, while others, known as stolons, spread across the surface. 

3. From the stolons, aerial hyphae called sporangiophores grow upwards. These sporangiophores are unbranched and support a reproductive structure at their tip.

4. At the apex of each sporangiophore, a spherical, sac-like structure called a sporangium forms. Inside the sporangium, the cytoplasm and nuclei undergo multiple mitotic divisions to produce a large number of haploid, asexual spores. The sporangium is differentiated into two parts. The peripheral sporoplasm with dense cytoplasm where spores develop and a central columella with vacuolated cytoplasm. The sporoplasm is separated from the columella by a dome-shaped partition.

5. Tiny, spherical, smooth walled spores are formed inside the sporoplasm. These spores are sporangiospores.

6. When the spores are mature, the sporangium wall ruptures, releasing the spores into the air. If a spore lands on a suitable substrate, it absorbs water, swells, and germinates. It then grows into a new mycelium, starting the asexual cycle over again.

Sexual Reproduction

Sexual reproduction in Rhizopus is a survival mechanism triggered by unfavorable environmental conditions, such as a lack of nutrients or the presence of a dry environment. It involves the fusion of two different mating types, typically designated as + and - strains.

1. Hyphae Contact and Gametangia Formation: When hyphae of opposite mating types (+ and -) grow close to each other, they are chemically attracted. They develop short, swollen branches called progametangia. A wall then forms to separate the tip of each progametangium, creating a multinucleate structure called a gametangium.

2. Plasmogamy: The walls between the two opposing gametangia dissolve, and their cytoplasm merges. This process is called plasmogamy, resulting in a single cell with fused cytoplasm but separate nuclei from both parent strains.

3. Karyogamy and Zygospore Formation: The nuclei of the two parent strains then fuse in pairs in a process called karyogamy. This forms a diploid nucleus. The resulting structure, a zygospore, develops a thick, warty, and protective wall that makes it highly resistant to harsh environmental conditions like desiccation and temperature extremes.

4. Germination and Meiosis: The zygospore enters a period of dormancy. When conditions become favorable again, it germinates. The diploid nucleus within the zygospore undergoes meiosis to produce new haploid nuclei.

5. Spore Production and Dispersal: The germinating zygospore produces a single, short filamentous structure called promycelium with a germ sporangium at its tip. This germ sporangium contains haploid spores, which are a mix of both parental genotypes due to genetic recombination during meiosis. When the sporangium bursts, these genetically diverse spores are released, disperse, and can grow into new mycelia, completing the sexual life cycle and increasing the fungus's ability to adapt.

Feature	Asexual Reproduction	Sexual Reproduction
Triggering Conditions	Favorable conditions (abundant food and moisture)	Unfavorable conditions (lack of nutrients, stress)
Parental Involvement	A single parent is involved.	Two parents of opposite mating types (+ and -) are involved.
Reproductive Structures	Sporangia form on sporangiophores.	Gametangia fuse to form a zygospore.
Spore Type	Asexual, haploid spores (sporangiospores) are produced through mitosis.	A sexual, thick-walled zygospore is formed. It undergoes meiosis during germination to produce haploid spores.
Genetic Variation	No genetic variation, offspring are genetically identical to the parent.	High genetic variation due to the fusion of genetic material from two different parents and meiotic recombination.
Function	Rapid multiplication and colonization of a substrate.	Survival during harsh environmental conditions and adaptation to new environments.
Outcome	New mycelium is a clone of the parent.	New mycelium is genetically different from both parents.