Just when you thought there wasn’t anything worse to fear than the hydrogen bomb and the Ebola virus, The Economist brings you an article (“Exterminate, exterminate”, March 20th, 2003) on something new and chilling. Apparently, a new technique exists with the potential to “lead to the extinction of almost any sexually reproducing species.”
The technique exploits site-specific selfish genes which have a tendency to be particularly selfish, in keeping with Richard Dawkins’ theory of evolutionary selection. Basically, SSSG’s use a cell’s own machinery to introduce themselves into new chromosomes: a means of dispersal not unlike that of a virus. If an SSSG falls in the middle of a gene, that gene will no longer function – denying the organism the ability to produce that protein, unless, of course, the gene on the other chromosome is fine. In fact, that backup system is why these genes don’t cause death or horrible cancer in everyone all the time.
If, however, the particular SSSG in question is a homing endonuclease gene (HEG), this safety system doesn’t work. That is because when a HEG gets into a chromosome, it produces an enzyme called endonuclease. This enzyme cuts the DNA strand at any place where a particular string of nucleotides is found.
The next part is sufficiently tricky that I defer to the superior writing ability of The Economist’s writers:
“Cells have two copies of most chromosomes (one deriving from the mother, and one from the father). If only one of these carries an HEG, the other will be cut by the enzyme which that gene produces. The site of the cut corresponds to the site of the HEG on the “infected” chromosome. The infected chromosome itself is not cut, because the HEG is in the middle of the enzyme-recognised sequence, and thus disguises it. But, since cells repair chromosomal damage by replacing the corrupted DNA with a copy from the same place on the partner chromosome, the HEG is copied over as part of the repair process. Now, instead of one copy of the broken gene, the cell has two.”
Once this process has taken place, every gamete (sperm or egg) produced by the organism will contain the HEG disrupted gene, even though the parent only has one copy. This process leads to the gene spreading very rapidly through the population. Finally, if you choose a gene where the loss of one copy does little harm, but the loss of both is fatal, you end up with a situation where 80-95% of embryos produced by parents who each have one HEG gene will die before they come to term. The number of offspring falls below the natural replacement rate and the species goes extinct.
Now, there are some potentially useful applications for this technology. The Economist suggests the worthy idea of wiping out malaria carrying mosquitoes. At the same time, the potential danger of this technique seems to be considerable, especially if it proves relatively easy to do.