Heterokaryosis
Heterokaryosis is a term used in biology meaning to have two or more
genetically different nuclei within the same mycelium of a fungus or other life form. This is a special type of syncytium. The term
heterokaryon was coined in 1965 by B. Ephrussi and M. Weiss; H. Harris and J. F. Watkins; Y. Okada and F. Murayama. A
heterokaryon is a cell with more than one nucleus of differing genetic origin. The term was first used for ciliate protozoans such as
Tetrahymena. This has two types of cell nuclei, a large, somatic macronucleus and a small, germline micronucleus. Both exist in a single cell at the same time and carry out different functions with distinct cytological and biochemical properties. Many fungi exhibit heterokaryosis. The haploid nuclei within a mycelium may differ from one another not merely by accumulating mutations, but by the non-sexual fusion of genetically distinct fungal hyphae. This can lead to individuals that have different nuclei in different parts of their mycelium. In this case, the notion of
individual itself becomes vague since the rule of one genome = one individual does not apply any more.