Reproduction in Fungi

Fungi can reproduce in three main ways: Vegetative reproduction, Asexual reproduction, and Sexual reproduction. Each method has specific characteristics and mechanisms.

A. Vegetative Reproduction:

Vegetative reproduction allows the fungus to grow and spread without sexual reproduction. This process occurs through various mechanisms:

1. Fragmentation of Hyphae:
   • When a portion of the hyphal thread (the body of the fungus) breaks off, it can grow into a new colony.
   • This form of vegetative reproduction is common in nature and can be used in the laboratory to propagate fungi by transferring small pieces of hyphae into new growth mediums.
2. Production of Spores (Oidia or Arthrospores):
   • Oidia or arthrospores are thin-walled, non-motile spores formed from the fragmentation of hyphae. These spores are capable of germinating and growing into new hyphal colonies.
3. Chlamydospores:
   • Chlamydospores are thick-walled, swollen spores that are capable of storing food, such as starches or lipids, to survive in adverse conditions (e.g., during droughts or when nutrients are scarce).
   • These spores can remain dormant and germinate when favorable environmental conditions return.
4. Budding:
   • In budding, a small protrusion or “bud” forms from a cell. The daughter nucleus migrates into the bud, and the bud is pinched off from the parent cell.
   • This process may sometimes result in a chain of buds that remain attached, forming a structure called pseudomycelium.
5. Fission:
   • Fission is the division of a single cell into two daughter cells. This process occurs in certain yeasts, where the cell divides along the transverse plane, and each resulting cell can grow into a new individual.

B. Asexual Reproduction:

Asexual reproduction in fungi is more common than sexual reproduction and involves the production of spores that can germinate to form new fungal individuals. Asexual reproduction plays an essential role in the spread and survival of fungi.

1. Sporulation:
   • Fungi produce spores through two methods: internal (endogenous) or external (exogenous). These spores are essential for dissemination, survival, and infection.
2. Sporangium Formation (Straminopila and Zygomycota):
   • In some fungi like the Oomycota (Straminopila) and Zygomycota, asexual spores are produced inside a sporangium, which is a sac-like structure.
   • The spores can be motile (zoospores) or non-motile (aplanospores).
     • Zoospores are motile, with one or two flagella, and can move to find new environments.
     • Aplanospores are non-motile and are released from the sporangium.
3. Conidia Formation (Ascomycota and Deuteromycota):
   • Conidia are another type of asexual spore produced on specialized hyphal branches called conidiophores.
   • Conidiophores can be unbranched or branched and produce conidia either individually or in chains.
   • The conidia can vary in shape and size and are adapted to spread and disperse quickly.
4. Structures Supporting Sporulation:
   • Conidiophores may aggregate to form complex structures such as coremia, synnema, sporodochia, acervuli, and pycnidia. These specialized structures are designed to produce and house large numbers of spores in a compact space.
5. Flagella Types in Zoospores:
   • Zoospores can have two types of flagella:
     • Whiplash flagella, which are thinner at the tip.
     • Tinsel flagella, which are found in members of the Oomycota and have many small hair-like projections called mastigonemes along their length.
6. Flagella Structure:
   • The flagellum in fungi has a 9+2 structure, with nine peripheral pairs of fibrils surrounding two central fibrils. The flagella are connected to the blepharoplast inside the motile cell, and the membrane of the flagella is continuous with the plasma membrane of the cell.

C. Sexual Reproduction:

Sexual reproduction in fungi involves the fusion of specialized sexual cells and the formation of new individuals with a combination of genetic material. The main stages are plasmogamy and karyogamy.

1. Plasmogamy: refers to the fusion of two sexual cells (gametes), resulting in the formation of a dikaryotic or heterokaryotic cell, which contains two genetically distinct nuclei.
2. Karyogamy:
   • Following plasmogamy, the nuclei fuse in a process known as karyogamy, forming a diploid nucleus.
   • The diploid nucleus then undergoes meiosis, producing four haploid nuclei, which can develop into new haploid individuals.
3. Mechanisms of Plasmogamy:
   • Fungi exhibit different types of gametogamy and fusion mechanisms:
     • Gametogamy: The fusion of two gametes (e.g., isogametes or anisogametes). Isogametes are equal in size, whereas anisogametes differ in size, with one being male and the other female. The fusion between motile and non-motile gametes is known as heterogamy.
     • Gametangiogamy: The fusion of gametangia (sexual organs). The gametangia may be of similar or different shapes and sizes.
     • Spermatization: The transfer of male gametes (spermatia) to the female gametangium. This occurs in Ascomycota and Basidiomycota.
     • Somatogamy: The fusion of somatic cells without the formation of gametes. This can occur between cells of the same or different fungal thalli.

D. Parasexual Reproduction:

In some fungi, especially those lacking sexual reproduction, parasexual reproduction can occur. This process does not involve typical meiosis and karyogamy but still allows for genetic recombination.

• Haploidization occurs through aberrant mitosis, where chromosomes are randomly distributed during cell division.
• It was first discovered by Pontecorvo and Roper in Aspergillus nidulans in 1952.