Tusk-like mandibles protrude from a screwworm larva’s mouth
Scott Camazine/Alamy
While controversial, the idea of eradicating certain species could be beneficial. For instance, eliminating malaria-carrying mosquitoes could greatly improve the world.
Genetic technology now offers a way to achieve this through gene drives, which can propagate detrimental traits within a population. However, the application of this technology to malaria-transmitting mosquitoes is not imminent. Instead, Kevin Esvelt from the Massachusetts Institute of Technology, known for developing the first CRISPR-based gene drive, anticipates the screwworm (Cochliomyia hominivorax) being its initial target.
Esvelt suggests, “I would bet on the New World screwworm, a notorious bot fly recently appearing in Texas, being more despised than malaria mosquitoes.”
Screwworm flies deposit eggs in animal and sometimes bird wounds. Upon hatching, the larvae burrow into the host’s flesh, consuming it alive. This can exacerbate wounds as more eggs are laid. If not removed, larvae can inflict severe injuries and may kill the host. This is a significant issue for livestock farmers and those afflicted by screwworms.
Once widespread across the Americas, screwworms were eradicated from North and Central America in the 1960s, yet they persist as a major problem in South America.
The eradication in North America was achieved using the sterile-insect technique, which involves sterilizing male screwworms with radiation and releasing them in numbers to outmatch the wild population. Since female screwworms mate only once, mating with a sterile male results in no offspring.
This method is costly, as are newer genetically modified versions, which is why it hasn’t been attempted in South America. Gene drives, however, present a potential alternative.
How do gene drives work?
Gene drives are mechanisms that bias inheritance patterns. Typically, a DNA segment from one parent is inherited by only half of their offspring. Harmful DNA is eventually weeded out as fewer offspring survive to pass it on.
Gene drives include DNA segments that ensure more than half of offspring inherit them. Some do this by impairing rival sperm. The CRISPR gene drive developed by Esvelt copies itself from one chromosome to another.
This allows a gene drive, along with any associated traits, to proliferate within a population, even if it faces natural selection pressures. This can lead to the extinction of entire populations.
For example, gene drives can disrupt genes crucial for fertility. If only one parent carries the drive, offspring remain fertile. However, offspring are infertile if both parents carry it. As the drive spreads and more parents carry it, populations decline.
Gene drives offer an advantage over the sterile-insect technique by self-propagating, eliminating the need for mass insect releases. They work with species that mate multiple times and are preferable to pesticide use, which harms many species, including humans.
No controversy needed
Although I support gene drives to eliminate malaria-carrying mosquitoes or stop malaria transmission, this seems unlikely soon. Campaigns against genetically modified crops in Europe have spread to Africa, casting genetic engineering as dangerous and immoral. A promising gene drive initiative in Burkina Faso was shut down following a police raid.
I believe genetic modification, like hammers, is a tool—most food is genetically modified to some extent. Its application matters.
This applies to gene drives too. Though they might seem uncontrollable, they are natural phenomena. Many gene drives exist in the wild, likely in most species, including humans.
Disadvantageous gene drives rarely spread due to evolving resistance. This would likely occur if a single gene drive attempted to eradicate an insect pest.
Esvelt notes, “Resistance always emerges,” but can be countered with multiple gene drive versions.
Extinction of a widespread insect requires releasing gene drive carriers in multiple countries. While unlikely in Africa due to opposition, Esvelt believes it could happen in the Americas, where genetically modified crops are common and screwworms are detested.
Two projects are underway to develop screwworm gene drives: one at Uruguay’s National Institute of Agricultural Research and the other in the US under the GUARDIAN program by DARPA. The progress of these projects is unclear, with no response from INIA project leader Alejo Menchaca and DARPA providing no substantial information. Given the development of working gene drives in mosquitoes, screwworm gene drives seem feasible with enough effort.
This month, Colossal Biosciences, a de-extinction company, also proposed creating a screwworm gene drive, but Esvelt notes the company lacks experience with gene drives or insect work.
One argument against using gene drives to eradicate species like mosquitoes is potential ecological impact. I find this absurd, considering human-driven extinctions, land transformations, and climate change. Is saving lives by eradicating invasive mosquito species truly riskier?
In the screwworm’s case, North America’s experience showed no obvious ecological harm. Esvelt notes screwworms can be frozen and revived if necessary.
Keep an eye on developments. In a few years, we might witness gene drives eradicating screwworms across the Americas. Should it succeed, the technology could address many pests, including those spreading malaria and dengue.
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