A heterokaryon is a combined cell with two separate nuclei. Pontecorvo (1) who worked with the fungus Aspergillus nidulans, which lacks a sexual cycle, observed that genetic recombination occurs during cell division in mitosis and employed it in the genetic analysis of fungi. If asexual spores from two genetically dissimilar strains of a filamentous fungus are planted very close together on the surface of a solid medium, the emerging hyphae fuse at points of contact. This fusion is followed by the transfer of nuclei from one hypha to the other, so that heterokaryons are formed whose hyphae contain nuclei from both strains. As the heterokaryotic hyphae grow, the nuclei multiply independently.
When asexual spores are formed, however, each initial sporogenous cell receives only one nucleus of one type or the other. Subsequent multiplication of these cells thus produces chains of spores. Each chain consists of genetically identical spores, homokaryons, although the spores of adjacent chains may differ in phenotype. Very occasionally, however, two of the nuclei in the heterokaryotic hyphae fuse to form a single diploid nucleus. This nucleus multiplies as such and is incorporated into spores in the same way as the haploid nuclei, so rare chains of diploid spores arise which, on subsequent germination, yield diploid individuals. If the haploid parental strains differ in complementary nutritional requirements, eg, one of genotype Ab (requiring B in the growth medium) and the other aB (requiring A), the diploid nucleus (Ab/aB) will possess genetic determinants to synthesize both and B substances needed for growth of the parents. Therefore the existence of diploid spores can be recognized and cultures obtained from them by plating on media lacking the growth factors A and B (2).
Cell fusion is usually carried out by treating a suspension of cells with some inactivated viruses or with polyethylene glycol, to alter the plasma membrane of cells so that they are induced to fuse with each other.
Heterokaryons provide a way of mixing the components of two separate cells to study their interactions (see Hybrid Cell).
