Literally, "fusion of karyon", or nucleus. The fusion of the nuclei, as separated from the fusion of the cytoplasm, or plasmogamy. The terms plasmogamy and karyogamy are usually only used when the events are separated by a period of time, as in the life cycle of a mushroom, in which karyogamy only occurs in cells which are involved in the sexual cycle. The majority of cells instead spend their life as a dikaryon (n+n).