In biology, a heterokaryon is a multinucleate cell that contains genetically different nuclei. This is a special type of syncytium. This can occur naturally, such as in the

mycelium Mycelium (: mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Its normal form is that of branched, slender, entangled, anastomosing, hyaline threads. Fungal colonies composed of mycelium are fo ...

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

during sexual reproduction, or artificially as formed by the experimental fusion of two genetically different cells, as e.g., in hybridoma technology.

Etymology

The term ''heterokaryosis'' for the property of having genetically unlike nuclei is borrowed from the German ''Heterokaryosis'', which was coined by the German botanist Hans Burgeff in a 1912 paper about his work on the fungus '' Phycomyces nitens''. It is based on Greek ''hetero'', meaning "different," and ''karyon'', meaning "kernel" or in this case "nucleus.".

Occurrence

Heterokaryons are found in the life cycle of yeasts, for example ''Saccharomyces cerevisiae'', a genetic model organism. The heterokaryon stage is produced from the fusion of two haploid cells. This ''transient'' heterokaryon can produce further haploid buds, or cell nuclei can fuse and produce a diploid cell, which can then undergo mitosis.

Ciliate protozoans

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 Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, ...

properties.

True fungi

Many fungi (notably the arbuscular mycorrhizal fungi) exhibit heterokaryosis. The

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

nuclei within a mycelium may differ from one another not merely by accumulating

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...

s, but by the non-sexual fusion of genetically distinct fungal

hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

, although a self / non-self recognition system exists in Fungi and usually prevents fusions with non-self. Heterokaryosis is also common upon mating, as in Dikarya (

Ascomycota Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...

and

Basidiomycota Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basi ...

). Mating requires the encounter of two haploid nuclei of compatible mating types. These nuclei do not immediately fuse, and remain haploid in a n+n state until the very onset of meiosis: this phenomenon is called delayed karyogamy. Heterokaryosis can lead to individuals that have different nuclei in different parts of their mycelium, although in ascomycetes, particularly in " Neurospora", nuclei have been shown to flow and mix throughout the mycelium. In heterokaryons, the notion of ''individual'' itself becomes vague since the rule of “one genome = one individual” does not apply any more. Genetic heterogeneity within an individual is indeed usually considered to be detrimental, as selfish variants may be selected for and disrupt the integrity of the individual level.

Slime molds

Heterokaryosis is most common in fungi, but also occurs in slime molds. This happens because the nuclei in the 'plasmodium' form are the products of many pairwise fusions between amoeboid

individuals. When genetically divergent nuclei come together in the plasmodium form, cheaters have been shown to emerge. However, genetic homogeneity among fusing amoeboid serves to maintain the multicellular plasmodium.Kuzdzal-Fick, J. J., S. A. Fox, J. E. Strassmann, and D. C. Queller. 2011. High Relatedness is Necessary and Sufficient to Maintain Multicellularity in Dictyostelium. ''Science'' 334:1548-1551

Artificial heterokaryon

A medical example is a heterokaryon composed of nuclei from Hurler syndrome and Hunter syndrome. Both of these diseases result in problems in mucopolysaccharide metabolism. However, a heterokaryon of nuclei from both of these diseases exhibits normal mucopolysaccharide metabolism, proving that the two syndromes affect different proteins and so can correct each other in the heterokaryon.

References

External links

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