Selfish DNA

Selfish DNA is a term for sequences of DNA that sensu stricto have two distinct properties:

• the DNA sequence spreads by forming additional copies of itself within the genome; and
• it makes no specific contribution to the reproductive success of its host organism. (It might or might not have significant deleterious effects.)

In his 1976 book The Selfish Gene^[1] Richard Dawkins suggested the idea of selfish DNA in reaction to the then fairly new revelation of the large proportion of noncoding DNA in eukaryotic genomes. In 1980, two articles in the journal Nature expanded and discussed the concept.^[2][3] According to one of these articles:

The theory of natural selection, in its more general formulation, deals with the competition between replicating entities. It shows that, in such a competition, the more efficient replicators increase in number at the expense of their less efficient competitors. After a sufficient time, only the most efficient replicators survive.

— L.E. Orgel & F.H.C. Crick, Selfish DNA: the ultimate parasite.^[3]

In the purest forms of the concepts, units of genetically functional DNA might be viewed as "replicating entities" that affect their replication by manipulating the physiological activities of the cell that they control; in contrast, units of selfish DNA affect their replication by exploiting existing DNA and DNA-manipulating mechanisms in the cell, notionally without significantly affecting the fitness of the organism in other respects.

Irrespective of the strict definition of selfish DNA, there is no sharp, definitive boundary between the concepts of selfish DNA and genetically functional DNA. Often it also is difficult to establish whether a unit of noncoding DNA is functionally important or not, and if important, in what way. What is more, it is not always easy to distinguish between some instances of selfish DNA and some types of viruses.

Examples

• Transposons copy themselves to different loci inside the genome. These elements constitute a large fraction of eukaryotic genome sizes (C-values): about 45% of the human genome is composed of transposons and their defunct remnants.
• Homing endonuclease genes (HEGs) cleave DNA at its own site on the homologous chromosome, triggering the DNA double-stranded break repair system, which "repairs" the break by copying the HEG onto the homologous chromosome. HEGs have been characterized in yeast, and can only survive by passing between multiple isolated populations or species.
• Supernumerary B chromosomes are essential chromosomes that are transmitted in higher-than-expected frequencies, which leads to their accumulation in progenies.

See also

• Mobile genetic elements
• Transposable element
• Noncoding DNA
• Junk DNA
• Retrotransposon
• Endogenous retrovirus
• Introns: introns as mobile genetic elements
• Genome size
• C-value enigma
• Mutation
• Intragenomic conflict
• Gene-centered view of evolution

Notes and references

1. Dawkins, Richard R. (1976). The Selfish Gene. New York: Oxford University Press. ISBN 978-0-198-57519-1. OCLC 2681149.
2. Doolittle WF, Sapienza C (1980). "Selfish genes, the phenotype paradigm and genome evolution". Nature. 284 (5757): 601–603. doi:10.1038/284601a0. PMID 6245369.
3. Orgel LE, Crick FHC (1980). "Selfish DNA: the ultimate parasite". Nature. 284 (5757): 604–607. doi:10.1038/284604a0. PMID 7366731.

Further reading

• Burt, Austin & Trivers, Robert 2006. Genes in conflict: the biology of selfish genetic elements. Harvard University Press. ISBN 978-0-674-02722-0