Show the stages of karyogamy in fungi

Karyogamy is a key stage in the sexual cycle of fungi, where the nuclei of two parent cells fuse to form a single diploid nucleus. It comes after plasmogamy (fusion of cytoplasm) and before meiosis.

In some fungi, such as ascomycetes and basidiomycetes, karyogamy occurs later, with a dikaryotic (n+n) stage existing between plasmogamy and nuclear fusion.

Stages of Karyogamy

Although karyogamy is essentially a single nuclear fusion event, the process can be broken down into steps:

1. Pronuclear Migration: The two haploid nuclei (pronuclei) from the fused cells move toward each other.
2. Nuclear Congression: The pronuclei align properly in preparation for fusion.
3. Nuclear Envelope Fusion: The membranes of the two nuclei merge, allowing their genetic material to combine.
4. Formation of a Diploid Zygote: The outcome is a single diploid nucleus (zygote) containing chromosomes from both parent cells.

Karyogamy within the Fungal Sexual Cycle

1. Plasmogamy: The cytoplasm of two compatible haploid fungal cells fuses, but the nuclei remain separate, forming a dikaryotic cell.
2. Karyogamy: The two haploid nuclei in the dikaryotic cell fuse to form a single diploid nucleus (zygote).
3. Meiosis: The diploid zygote undergoes meiosis, reducing the chromosome number to produce haploid spores.
4. Spore Germination: These haploid spores germinate and develop into new haploid fungal individuals, completing the sexual cycle.