Karyogamy

Karyogamy is the fusion of two haploid nuclei to form a single diploid nucleus, a central step in sexual reproduction. The term comes from Greek roots meaning “nucleus joining.” It is the nuclear counterpart to plasmogamy, the fusion of cytoplasm, and it typically follows plasmogamy in organisms with sexually dimorphic cells.

In fungi, karyogamy often occurs after a period during which two nuclei coexist in the same cell in a dikaryotic stage. Upon completion of karyogamy, a diploid zygotic nucleus is formed within the zygote. This diploid nucleus then undergoes meiosis (and often mitosis) to produce haploid spores, which can germinate to continue the life cycle. In other fungal lineages, such as basidiomycetes and ascomycetes, karyogamy occurs within specialized structures like basidia and asci, respectively.

In plants and animals, fertilization typically brings together genetic material from two gametes, leading to the formation of a zygote. The term karyogamy is sometimes used to describe the nuclear fusion that occurs within the fertilized egg, after which the zygotic nucleus may enter mitotic divisions and later, in organisms with alternating life stages, meiosis to restore haploidy.

Significance of karyogamy lies in restoring the diploid chromosome number and enabling genetic recombination through meiosis, which increases genetic diversity. Disruptions to karyogamy can prevent zygote formation or proper development of the offspring.