In biology, a heterokaryon is a

multinucleate Multinucleate cells (also known as multinucleated cells or polynuclear cells) are eukaryotic cells that have more than one nucleus, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordinate ...

cell that contains genetically different nuclei. This is a special type of

syncytium A syncytium (; : syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell that can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus), i ...

. 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 Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

''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 The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...

protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

ns such as '' Tetrahymena''. This has two types of cell nuclei, a large, somatic

macronucleus A macronucleus (formerly also meganucleus) is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, such as metabolism Metabolism (, ...

and a small,

germline In biology and genetics, the germline is the population of a multicellular organism's cells that develop into germ cells. In other words, they are the cells that form gametes ( eggs and sperm), which can come together to form a zygote. They dif ...

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 An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'') is a type of mycorrhiza in which the symbiosis, symbiont fungus (''Arbuscular mycorrhizal fungi'', or AMF) penetrates the Cortex (botany), cortical cells of the roots of a vascular plant f ...

) 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 ''Neurospora'' is a genus of Ascomycete fungi. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores that resemble axons. The best known species in this genus is '' Neurospora crassa'', a common model organ ...

", 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 mold Slime mold or slime mould is an informal name given to a polyphyletic assemblage of unrelated eukaryotic organisms in the Stramenopiles, Rhizaria, Discoba, Amoebozoa and Holomycota clades. Most are near-microscopic; those in the Myxogastria ...

s. This happens because the nuclei in the 'plasmodium' form are the products of many pairwise fusions between

amoeboid An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and r ...

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 Hunter syndrome, or mucopolysaccharidosis type II (MPS II), is a rare genetic disorder, inherited lysosomal storage disease in which large sugar molecules called glycosaminoglycans (or GAGs or mucopolysaccharides) build up in body tissues. Hunte ...

. 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

{{wiktionary
MedicineNet.com
Eukaryotic cells Mycology Cell biology