Selfish DNA

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

• the DNA sequence spreads by forming more copies of itself in the genome; and
• it does not help the reproductive success of its host organism. It may or may not have negative effects.

In his 1976 book, The Selfish Gene, Richard Dawkins suggested the idea of selfish DNA when the noncoding DNA in eukaryotic genomes was discovered. In 1980, two articles in the journal Nature expanded and discussed the concept. 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.

Normal genetically functional DNA might be seen as "replicating entities" that effect their replication by manipulating the cell that they control. In contrast, units of selfish DNA may exploit existing mechanisms in the cell, and multiply without affecting the fitness of the organism in other respects.

There is no sharp boundary between the concepts of selfish DNA and genetically functional DNA. Often it also is difficult to see if a unit of noncoding DNA is functionally important or not; or 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.

History of the idea

The idea that some genetic elements might not be useful to the organism is not a new one. In 1928, a Russian geneticist reported an X chromosome in Drosophila obscura. He claimed that the resulting female-biased sex ratio might drive a population extinct.

In 1941 it was first suggested that there might be a conflict between normal inherited nuclear genes from both parents and mitochondrial genes from one parent (the female). It could lead to cytoplasmic male sterility in plants.

Around the same time, several other examples of selfish genetic elements were reported. For example, a maize geneticist described how chromosomal knobs led to female meiotic drive in maize. Meiotic drive is when one copy of a gene is passed on to offspring more than the expected 50% of the time.

The Swedish botanist and cytogeneticist Gunnar Östergren in 1945 noted how chromosomes may spread in a population because of their own ”parasitic” nature. Discussing B chromosomes in plants he wrote: ”In many cases these chromosomes have no useful function at all to the species carrying them, but that they often lead an exclusively parasitic existence … [B chromosomes] need not be useful for the plants. They need only be useful to themselves.” - Gunnar Östergren.

Then, in the early 1950s, Barbara McClintock published a series of papers describing the existence of "transposable elements". These are among the most successful selfish genetic elements. The discovery of transposable elements led to her being awarded the Nobel Prize in Medicine or Physiology in 1983.