Which organisms commonly undergo karyogamy?

Karyogamy is the fusion of two haploid nuclei during sexual reproduction. It is a critical process in the life cycles of eukaryotic organisms that reproduce sexually, including fungi, protists, plants, and animals.

Fungi

Karyogamy is a key event in the sexual reproduction of fungi. Its timing varies across different fungal groups:

• Zygomycota: In species like Rhizopus and Mucor, karyogamy occurs immediately after plasmogamy (fusion of cytoplasm), forming a diploid zygospore.
• Ascomycota (Sac Fungi): Karyogamy takes place inside a specialized sac called the ascus, producing a diploid nucleus that undergoes meiosis to form haploid ascospores. Examples: yeasts (Saccharomyces), molds (Aspergillus).
• Basidiomycota (Club Fungi): In mushrooms like Agaricus, karyogamy happens in a club-shaped structure called the basidium, forming a diploid nucleus that gives rise to haploid basidiospores through meiosis.

Protists

Many protists undergo karyogamy as part of their sexual life cycle:

• Ciliates: In Paramecium, karyogamy follows conjugation, where two individuals exchange haploid micronuclei, which then fuse to form a new diploid micronucleus.
• Slime molds: Cellular slime molds and other amoebozoans have the machinery for both karyogamy and meiosis.
• Algae: In algae like Chlamydomonas, gametes fuse, and the resulting diploid zygote undergoes karyogamy before forming a protective wall.

Other Eukaryotes

Karyogamy also plays a vital role in fertilization in plants and animals:

• Plants: In angiosperms (flowering plants), karyogamy occurs during double fertilization. One sperm nucleus fuses with the egg to form a diploid zygote, while another fuses with the central cell nucleus to form the triploid endosperm.
• Animals: In mammals, the sperm and egg nuclei fuse in a process similar to karyogamy. The sperm pronucleus migrates toward the egg pronucleus, forming the diploid nucleus of the zygote.