Population suppression

Two main strategies for population suppression are

(i) the release of sterile mosquitoes and

(ii) methods to impose a substantial genetic load by introducing deleterious, e.g. recessive, mutations into the target population.

Sterile release methods

These are essentially variations or improvements on the classical Sterile Insect Technique (SIT).  In SIT, pest insects are reared in large numbers, sterilised (e.g. by irradiation) and released to interbreed with the wild population.  The progeny of any such interbreeding die, due to inheritance of dominant radiation-induced mutations.  If enough sterile insects can be release for sufficient time, the target population will decline and collapse.  SIT has been proven in very large-scale control programmes against several agricultural pests including the New World Screwworm and Mediterranean fruit fly.

Several trials of classical SIT were conducted in the 1970s and 1980s, with variable success.  Potential improvements involving GM mosquitoes include genetic sterilisation, to eliminate the need for radiation and genetic sexing, to allow females to be eliminated from the sterile insect population, so that only males are released.

Progress

Strains with the necessary genetic properties already exist and some contained field trials have been performed, as well as extensive laboratory analyses.  Similar strains of agricultural pest species are also in trials; one strain began open field trials in 2006.  It is anticipated that the first field use of GM mosquitoes will be in the context of a sterile-release population suppression strategy.

Genetic load

One early suggestion was to spread a trait through the target population that would be lethal at some predictable point in the future, e.g. winter.  More recently, Austin Burt suggested  a method to drive a recessive lethal (or sterile) mutation into a population – this would be relatively harmless at low allele frequency but impose a significant genetic load if it accumulated to a higher frequency.  Simulations indicated that this approach, applied to several target genes simultaneously, could render the target population – or species – non-viable.  The key requirement for this approach is a way to impose a genetic load that will accumulate in a target population.  Since natural selection would normally tend to eliminate deleterious mutations, this implies the use of gene drive system  to spread the deleterious trait; in Burt’s proposal these are engineered homing endonucleases (HEGs).  HEGs have several other potential applications, some of which resemble sterile-release methods (above); others a gene drive system.

Progress

Several advances are required for this method including: the ability to engineer HEGs to recognise selected sequences; identification of suitable target genes and sequences; efficient homing of engineered HEGs in insects. Progress has been made towards each of these, but not yet a working prototype.

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