What cellular steps lead up to karyogamy?

Karyogamy is the fusion of two haploid nuclei and represents the final stage of fertilization in organisms like fungi, algae, and some animals. It usually follows plasmogamy (fusion of cytoplasm), though the timing can vary among species.

The process generally involves the following steps:

1. Cell Signaling and Recognition
   Organisms release chemical signals, such as pheromones, to attract and identify compatible mating partners. This ensures that only gametes of the correct mating type fuse.
   • Example (yeast): Haploid cells of opposite mating types (a and α) release pheromones that direct growth toward each other.
2. Cell or Hyphal Fusion (Plasmogamy)
   The cell membranes and cytoplasm of the two haploid cells fuse, forming a single cell containing two separate haploid nuclei. This can create a dikaryon (n+n) or heterokaryon.
   • In fungi: Fusion occurs via specialized hyphae or gametangia.
   • In yeast: Cell walls and membranes of the two haploid cells fuse.
   • In flowering plants: Two sperm cells from a pollen tube are released into the embryo sac.
3. Nuclear Migration and Pairing
   The haploid nuclei move toward each other within the shared cytoplasm, often guided by the cytoskeleton.
   • In yeast: Microtubules from the spindle pole bodies pull the nuclei together.
   • In plants: Actin filaments guide the sperm nucleus toward the egg nucleus.
4. Nuclear Envelope Fusion
   As the nuclei approach, their nuclear membranes fuse in a stepwise manner: the outer membranes first, followed by the inner membranes.
   • In yeast: Proteins like Kar5p are essential for fusion.
   • In mammals: The sperm nucleus disassembles its envelope upon entering the egg and reassembles a single diploid nucleus after combining chromosomes.

Once fusion is complete, karyogamy forms a single diploid (2n) nucleus within the zygote, marking the completion of fertilization at the nuclear level.