Using gene drive for vermin control

Controlling 'vermin' with gene drive technology is both interesting .. and a possible fount of unintended consequences.

From The Telegraph:

"There are thought to be more than 10 million rats living in Britain and pest control is estimated to cost the UK around £1.2 billion each year.

The technique suggested for rodents is known as ‘x-shredding.’ Male mammals have both an ‘x’ and ‘y’ sex chromosome, while females need two ‘x’ chromosomes.

The scientists want to insert ‘x shredder’ code into the DNA of male rats which would destroy the ‘x’ chromosomes in their sperm, meaning they could only pass on a ‘y’ chromosome, so their offspring would never be female. With fewer and fewer females over time, the population would have to decline."

Here's an example, starting with sixteen rats, 8 male and 8 female, in population equilibrium. One of the males is a mutant with the gene drive. We assume, for clarity and simplicity, that the mutated males outcompete normal males and always 'get their gal' preferentially.

The population goes extinct in five generations. Initially the population exhibits an exponential increase in mutant males from a low base; in the terminal phase the process is dominated by the ever-decreasing number of females, and - no doubt - tremendously elevated inter-male aggression.

According to Wikipedia,

"Since it can never more than double in frequency with each generation, a gene drive introduced in a single individual typically requires dozens of generations to affect a substantial fraction of a population.

Alternatively, releasing drive-containing organisms in sufficient numbers can affect the rest within a few generations; for instance, by introducing it in every thousandth individual, it takes only 12–15 generations to be present in all individuals.

Whether a gene drive will ultimately become fixed in a population and at which speed depends on its effect on individuals fitness, on the rate of allele conversion, and on the population structure.

In a well mixed population and with realistic allele conversion frequencies (≈90%), population genetics predicts that gene drives get fixed for selection coefficient smaller than 0.3; in other words, gene drives can be used not only to spread beneficial genetic modifications, but also detrimental ones as long the reproductive success is not reduced by more than 30%. This is a great contrast with normal genes, which can only spread in large populations if they are beneficial."

There are of course issues:

• Mutations: It is possible that a mutation could happen mid-drive, which has the potential to allow unwanted traits to "ride along" on the spreading drive.


• Escape: Cross-breeding or gene flow potentially allow a drive to move beyond its target population.


• Ecological impacts: Even when new traits' direct impact on a target is understood, the drive may have side effects on the surroundings.

In particular, natural genetic variation may prevent the drive 'taking' in some males; as a consequence a resistant population may soon emerge. It's been stated that the biggest danger with a gene drive is that it just won't work at all. There is talk of targeting several loci in parallel.

Something phenomenologically similar to a gene drive is at work in human populations (such as India, China) where female foetuses have been selectively aborted. But the numbers involved are well below population-lethal.