The grey squirrel is a familiar creature in our parks and gardens. This fluffy-tailed mammal has been making mischief in the UK for a while now, overwhelming our native reds since being introduced here from North America in the late 19th century. Before the greys arrived, the UK was home to about 3.5 million reds; today, only 120,000–160,000 remain.
Reds have suffered at the paws of greys. Their larger, non-native cousins outcompete them for food and also carry the squirrel pox virus, which is often fatal to reds but not to greys. Scientists have worked to develop a vaccine to protect red squirrels from the disease, but inoculation isn’t much use when it comes to a ransacked larder.
Currently, attempts to control the grey squirrel population involve poisoning or trapping and shooting. But conservationists are exploring more humane ways of keeping their numbers in check. One idea is to use pine martens, which prey on squirrels. Research has shown that woodlands with higher numbers of pine martens host fewer greys and more reds; scientists think that the smaller reds can escape predation by perching on thinner branches that can’t support the weight of greys or martens. However, some conservationists are concerned that increased numbers of pine martens would have an impact on bird populations, too.
A proposal to suppress the grey squirrel population using a contraceptive has also received Government backing, but there are concerns that this approach wouldn’t be species-specific – that food laced with drugs could be consumed by other animals, too.
Clearly, current strategies are largely failing to control numbers of grey squirrels – aliens that cost the UK £40 million a year from damage to broadleaved forest. So eyes are turning to other methods.
“ Current strategies are largely failing to control numbers of grey squirrels.”
Enter CRISPR-Cas9, a gene-editing technique that enables scientists to rewrite the DNA of a species. (A CRISPR is a specialised stretch of DNA; Cas9 is an enzyme that acts rather like a pair of molecular scissors, cutting strands of DNA.)
This technique allows scientists to find a specific part of a genome (the complete set of genes of a cell or organism) and edit it – to remove an unwanted gene or insert a new one. The ability to simply change the DNA within a species’ genome, without adding any DNA from a different species, could provide a faster route to a desired outcome that would otherwise require decades of traditional selective breeding.
“The beauty of CRISPR-Cas9 is its ability to target a specific location in the genome in a more precise manner than previous genetic engineering tools enabled us to do,” says Todd Kuiken, senior research scholar in the Genetic Engineering and Society Center at North Carolina State University.
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