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the fusion of the nuclei of gametes into the nucleus of a zygote.
Karyogamy is the essence of fertilization, in which the paired nature of the homologous chromosomes carrying genetic information from the maternal and paternal gametes is restored. Karyogamy may occur immediately after the gametes fuse (for example, in the sea urchin) or, more often, somewhat later, in the metaphase of the first division. In some animals (for example, the water flea), algae, and fungi, the nuclei of the male and female draw together to form a dikaryon.