Heterokaryosis (from the Greek heteros, meaning other and karyon, meaning kernel) 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.
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 (notably the arbuscular mycorrhizal 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.
Heterokaryosis is most common in fungi, and lichen, but also occurs in slime molds. This happens because the nuclei in the 'plasmodium' form are the products of many pairwise fusions between amoeboid haploid individuals.
- King R.C., Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford. p204