Question

Sexual reproduction in fungus occurs in the following sequential event. Arrange them properly. A. Fusion of two nuclei called karyogamy. B. Fusion of protoplasm between two motile or non-motile gametes. C. Meiosis in zygote resulting in haploid spores. (a) \(\mathrm{A} \rightarrow \mathrm{B} \rightarrow \mathrm{C}\) (b) \(\mathrm{B} \rightarrow \mathrm{A} \rightarrow \mathrm{C}\) (c) \(\mathrm{C} \rightarrow \mathrm{B} \rightarrow \mathrm{A}\) (d) \(\mathrm{C} \rightarrow \mathrm{A} \rightarrow \mathrm{B}\)

Short answer

The correct sequence of events for sexual reproduction in fungi is B → A → C, so the correct answer is (b) \(\mathrm{B} \rightarrow \mathrm{A} \rightarrow \mathrm{C}\).

Step-by-step solution

Identify the first step in the process

The first step in the sexual reproduction of fungi is the fusion of protoplasm between two motile or non-motile gametes.

Identify the second step

After the protoplasm has fused, the fusion of two nuclei takes place, known as karyogamy.

Identify the final step

The last step in the process is the meiosis in the zygote which results in the formation of haploid spores.

Key concepts

Karyogamy

Karyogamy is a critical phase in the sexual reproduction of fungi. It occurs following plasmogamy, where the nuclei of the fused gametes unite. This process involves the merging of two haploid nuclei to form a single diploid nucleus.
Karyogamy generates genetic diversity, a key benefit of sexual reproduction. Since the resulting nucleus contains genetic material from two different parent fungi, it can lead to offspring with new trait combinations.
In many fungi, especially those like basidiomycetes and ascomycetes, the phases of plasmogamy and karyogamy can be separated by a prolonged gap. During this time, the organism can enter a dikaryotic stage where cells contain two separate nuclei, one from each parent. This stage is unique to fungi, differentiating them from many other organisms.

Plasmogamy

Plasmogamy is the initial step in fungal sexual reproduction where the protoplasms (or cytoplasms) of two gametes fuse together.
This fusion can occur between motile gametes, those that can move, or non-motile gametes, those that can't. The result is a shared cytoplasm containing the distinct nuclei from the two parent fungi.
It's important to note that even after plasmogamy, the nuclei from the two gametes remain separate, resulting in a cell with two nuclei – a state often referred to as dikaryotic. This stage can persist for some time before the subsequent karyogamy step occurs.

• Plasmogamy initiates the combination of genetic material from two parent fungal cells.
• The fusion creates a new cell with two different nuclei.

Though it might sound straightforward, plasmogamy is an intriguing process, as it doesn't involve the immediate mixing of genetic material until the succeeding steps.

Meiosis in Fungi

Meiosis in fungi follows karyogamy and is an essential component for creating genetic variety within fungal populations.
Once a diploid nucleus is formed during karyogamy, meiosis ensures this nucleus splits and reduces its chromosome number back to the haploid state.
Meiosis results in haploid spores, crucial for the continuation of the fungal lifecycle. These spores, owing to their genetic mix from the prior processes, are essential for increasing adaptation and survival in varying environments.

• Meiosis helps in reducing the chromosome number back to haploid, maintaining genetic stability across generations.
• It leads to the formation of diverse haploid spores, ready to grow into new fungi.

The spores can then disperse, often over long distances, leading to colonization in new areas, helping fungi thrive in diverse ecosystems.

Haploid Spores

Haploid spores are the final product of the fungal sexual reproduction cycle initiated with plasmogamy. After meiosis in fungi, these spores are formed, each containing a single set of chromosomes.
These spores play a vital role as they facilitate the spread and reproduction of fungi across different environments.
The haploid spores are often produced in large numbers and can be adapted to survive in various conditions until they find a favorable environment to germinate.

• They contain one set of chromosomes, as opposed to the two found in diploid cells.
• They are essentially well-equipped vehicles for fungal propagation and biodiversity.

This process underscores fungi's unique reproductive capabilities, ensuring their survival and adaptation through genetic diversity.