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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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When genes look out for themselves

The antics of selfish DNA in worms and plants

My latest Mind and Matter column in the Wall Street Journal is on selfish DNA:

 

The theory of selfish DNA was born as a throwaway remark in the book "The Selfish Gene" by Richard Dawkins, when he pondered why there is so much surplus DNA in the genomes of some animals and plants.

Perhaps, he suggested, our genomes are riddled with digital parasites, sequences of code that proliferate at our expense and aren't there to help "us"-that is, the organism as a whole-at all.

This theory has since explained a lot of puzzling phenomena. It might be the reason that there are so many insect species on the planet. Consider Wolbachia, a bacterium that lives inside the cells of more than two-thirds of insects. This parasite is passed through the female line, because insect sperm are stripped of their cell contents when they enter the egg.

That means that Wolbachia in a male insect is in an evolutionary dead-end. A gene trapped in a reproductive dead-end that mutates so as to find a way to get into another generation will prosper-even if it damages the interests of the creature it is part of. Indeed, Wolbachia's genes are so manipulative that they can convert male insects to females, kill males, induce virgin birth in females and even cause the offspring of males to die unless they carry the very same strain of Wolbachia as the father.

The genes of insects respond by mutating to suppress Wolbachia's gerrymandering of sexual reproduction. A sort of arms race results. But in the process, lots of different insect species emerge because of the incompatibility of the Wolbachia strains. The Creator's "inordinate fondness for beetles" (a phrase attributed to J.B.S. Haldane) may be caused partly by this parasite.

Wolbachia isn't the only object inside the cell with its own genes. Mitochondria, a cell's electrochemical "batteries," also have genes, left over from when they were bacteria that set up shop inside our distant ancestors' cells. Mitochondria pass through the female line too, and sure enough, their genes have a habit of castrating the male functions of more than 150 species of hermaphroditic plants. (This "cytoplasmic male sterility" is exploited by plant breeders seeking to generate hybrid varieties, because it prevents self-fertilization.)

Now it emerges that mitochondrial genes also can run amok in animals. This month a new scientific paper discussed the finding that small nematode worms of a certain species often carry a particular mitochondrial mutation-the absence of 786 letters of code from a crucial gene.

Experiments by scientists at Oregon and Portland State Universities leave no doubt that this mutation puts its carriers at a disadvantage. But they also show that, far from dying out, in the lab the mutation increases in abundance by 1% per generation.

How can this be? Genes bad for organisms are supposed to die out, not spread. The answer is that the mutation probably makes the worms breed as self-fertilizing hermaphrodites, rather than as males, so it thrives, selfishly, even as its owner suffers.

In recent years, some scientists have argued that such selfish genetic elements serve the greater good in the long run by making species more "evolvable," like insects, and that this explains their existence. Persuasive evidence against this, and in favor of the idea that they are harmful parasites that generate greater evolvability only as a side effect, comes from asexual species.

In asexual animals or plants, evolutionary diversification is much slower because of the lack of genetic remixing that happens during sex. So selfish elements should be welcomed in such lineages if evolvability is a net benefit. Instead-as shown by some tiny animals, called rotifers, that have not had sex for 80 million years-selfish DNA is generally purged from the genomes of asexual species as a nuisance.

Truly, the more we understand what happens within and between genes, the more innocent the human world seems.