Dr Terry Mabbett considers the potential introduction of gene-drive technology to control the UK population of grey squirrels, and the impact this could have on the wider environment.
GENE drive is a genetic engineering technique which modifies genes so they don’t follow the normal rules of heredity. Use of gene-drive technology allows scientists to modify the evolutionary trajectory and cause population collapse. And perhaps even extinction of the targeted species, according to Andrea Crisanti, a geneticist at Imperial College London who has been researching gene-drive technology for the control of malaria-carrying mosquitoes for a number of years.
Genes carrying the desired trait for induced female infertility are introduced into the population. In normal heredity there is a 50 per cent chance that any particular gene will be passed from parent to offspring, but gene-drive technology converts this 50 per cent chance into an almost 100 per cent guarantee. Gene-drive technology research has traditionally been targeted at insect vector species, but interest is now also focused on mammalian pest species. One of these is the eastern grey squirrel (Sciurus carolinensis). The rodent is not a tree and forest pest problem within its native North American distribution, the eastern seaboard of the United States and Canada, but very much so within its adopted United Kingdom home where it is responsible for huge economic losses in forestry, through tree bark-gnawing and stripping. Biodiversity losses also occur through successful competition with the native red squirrel (Sciurus vulgaris) and the transmission of squirrel pox virus disease, which is fatal to red squirrels but not greys.
Interest in gene drives for grey squirrel control could not have come at a more appropriate time for UK forestry. The UK Government has promised tree planting on a scale that can only be described as out of this world compared with what has happened (or not happened) for woodland cover over the last three decades.
The conservation charity ESL (European Squirrel Initiative) says 2019 research showed that grey squirrels were costing the UK economy £40 million a year and went as far as to claim that government tree-planting targets were pointless without grey squirrel control. Research carried out by the Royal Forestry Society has calculated accumulative losses of £1.1 billion over the next four decades, while forestry minister Lord Goldsmith freely acknowledges the threat of invasive, non-native species like grey squirrel.
READ MORE: Grey squirrels ‘remain top threat to broadleaf woodland’
The potential use of gene-drive technology to control grey squirrel is in the spotlight following publication of computer modelling findings published in Scientific Reports (4 March, 2021) by scientists at the University of Edinburgh (Faber et al., 2021). Gene drive would introduce squirrels with genomes altered to induce infertility in females and thus facilitate control of the population.
However, the technology faces technical challenges such as controlling spread of the altered genes as gene-drive individuals mate with wild individuals, and also development of genetic resistance which could make gene drive ineffective. Overcoming the risk of evolutionary resistance and ensuring self-limiting, localised effects are two important prerequisites for safe and effective deployment of gene-drive technology, to minimise risk to other populations in the wider environment.
To address these challenges the University of Edinburgh team used computer modelling to investigate the effectiveness of HD-ClvR, a combination of three CRISPR-based gene-drive technologies, using the grey squirrel as a case study. HD-ClvR combats resistance allele formation by combining a homing gene drive (H) with a cleave-and-rescue gene drive (ClvR), while the inclusion of a self-limiting daisyfield gene drive (D) allows for control and localisation of effects on the population.
The authors used both randomly mating and spatial models to simulate this strategy. Computer modelling showed the gene drive HD-ClvR could control a targeted grey squirrel population with little risk to other populations. HD-ClvR offers an efficient, self-limiting and controllable gene drive for managing invasive pests, said the authors.
In reality, genetically altered grey squirrels would be released into the resident population, but because gene-drive technology has not been tested in live grey squirrels, considerably more research is required beforehand, said the team. This would be to assess and consider, for example, what impact an abrupt and highly effective suppression of the grey squirrel population might have on the ecosystem as a whole, especially because gene-driven technology has the potential to completely wipe out a targeted species.
READ MORE: How you can help tackle tree destruction caused by grey squirrels
Such drastic and irreversible effects should be viewed in relation to the effect and consequences of rabbit haemorrhagic viral disease (RHVD) on another mammalian pest, the European rabbit, and how this could impact on the ecosystem and environment. Key predators of wild rabbits like red foxes and common buzzards, if deprived of wild rabbits, would most likely refocus their preying activities on native wild species, poultry and game birds.
Current plans for reintroduction of the native pine marten across the UK have grey squirrel control as one of the major benefits. However, complete removal of grey squirrel from the ecosystem and environment could mean pine martens prey on reds instead.
Other considerations may be social in nature and relate to diverging views on grey squirrels in town and country. Grey squirrels are causing massive amounts of economic tree damage in woodlands, with landowners and foresters largely unconcerned if grey squirrel was completely wiped out. However, in urban areas where the bulk of the UK population lives, grey squirrels are regarded as an important and generally much-loved component of urban and suburban wildlife. Complaints about tree damage in parks rarely arise, probably because grey squirrels are too well fed by the public at large.
A future forestry minister representing an urban parliamentary constituency could conceivably be faced with rural landowners and foresters on one side and their constituents on the other if it came down to the extinction of the grey squirrel in England. Failure to achieve complete control of grey squirrels in both town and country would mean urban squirrels migrating into the countryside to fill gaps left by control achieved through the deployment of gene-drive technology. All this could be avoided by appointing a member of the House of Lords who is not elected by anybody, and therefore does not have the constituency electorate to worry about.
Computer models have suggested that the release of 100 genetically altered grey squirrels into a resident population of 3,000 would wipe out the population within 15 years. Does all this really mean that grey squirrel could be gene-driven to the edge of extinction?
REFERENCE: Nicky R. Faber, Gus R. Mcfarlane, R. Chris Gaynor, Ivan Pocrnic, C. Bruce, A. Whitelaw and Gregor Gorjanc (2021) ‘Novel combination of CRISPR-based gene drives eliminates resistance and localises spread’, Scientific Reports volume 11.
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