We've known for a long time that we can limit malaria infections by controlling the mosquitos that transmit them. But that knowledge hasn't translated into control efforts that have always been completely successful. Many of the approaches we've used to control mosquitos have caused environmental problems, and mosquito populations are large enough that they have evolved resistance to many of our pesticides.

That made the development of what are called "gene drive" constructs exciting (if a bit scary). They have the potential to rapidly spread genes throughout a population—including a mosquito population. But the prospect of a modern genetic control of mosquito populations has run up against the very old problem of evolution, as the gene drives often stall due to genetic changes that allow mosquito populations to escape their impact.

Now, a team has figured out a way to possibly avoid this problem: use gene drive to target a gene that is fundamental to how mosquitos develop as male or female. In doing so, it makes the females sterile and, at least in the lab, causes mosquito populations to collapse.

The obvious question

What's gene drive? Essentially, it's a tool for converting an entire population to a single genotype over several generations.

Normally, the spread of genotypes within a population is slow, as each individual carrier will pass it on to only half its offspring, and the chance that two carriers will end up mating are small. The only way to speed things up is to have the genotype provide a powerful advantage in survival or mating success, but that's about the last thing you want to give to mosquitos. Gene drive changes that, accelerating the spread of any chosen genotype, even if it harms the organism carrying it.

The method works using the enzymes involved in gene editing. DNA carrying the genes for these enzymes are placed together with a gene that encodes a short RNA that directs the editing to the location of your choice within an organism's genome. All of this then gets inserted into that same spot where the gene editing system will target the DNA. Typically, this spot resides within one of the organism's genes.

In the next generation, this gene-editing DNA will reside on one chromosome, and the organism's normal DNA will be present on that chromosome's partner. The gene editing system will recognize the normal DNA and make a cut in it. The cell will then attempt to repair this cut using the DNA from this region on the other copy of the chromosome. But that other copy will have all the genes of the gene editing system inserted in it, so the repaired chromosome will end up with these as well. As a result, both copies of the chromosome will end up carrying the DNA-editing system.

Also as a result, all of their offspring will end up inheriting the engineered gene from this parent. And, in each of these offspring, the process will repeat, converting all the chromosomes to the engineered version. If you start a population where only 12 percent carry the gene-drive system, it will be present in 100 percent of the population in about a dozen generations.

Targeting sex

This process has a lot of potential for things like mosquito control. You can take a population and potentially eliminate its resistance to existing pesticides. Or you can make it sensitive to a chemical that doesn't normally work as a pesticide. Or, in some of the more extreme versions, you can simply kill off all the members of one sex.

There has just been one small problem: so far, initial attempts to use gene drive in this manner haven't worked out so well. The problem has been that trying to kill off a population places a strong evolutionary pressure on said population, selecting for animals where the editing doesn't work. This typically involves changes to the DNA at the site the editing system targets, changes that mean the system no longer recognizes it. And, at least with the targets chosen so far, these sorts of changes appear to either already be present in the population at low levels or arise frequently enough that resistance to the gene drive spreads quickly through the population.

These sorts of changes may even occur more often when gene-drive systems are present, since the gene-editing system doesn't always neatly edit and may create deletions of the DNA it targets.

The goal behind the new work is to find a gene where the changes that would make it immune to editing would also damage the gene. To do so, the researchers took advantage of our knowledge of how insects determine sex, largely generated in the fruit fly Drosophila. That work has identified a gene called doublesex that is essential for both males and females to develop properly.

The male and female activities are somewhat separate. If you simply damage the entire gene, then both sexes are affected and will develop as a confused intermediate of male and female traits. But there's a specific part of the gene that is needed for female development. If it is damaged, then females develop as a mix of traits, while males develop perfectly normally. Because this part is so essential for the gene's function, changes to the DNA there aren't well tolerated.

Driven

This is precisely the area that the researchers, from Imperial College in the UK, targeted with their gene drive system. Males that carry one or two copies of the edited version of the doublesex gene develop perfectly normally and are fertile. Females with only one copy also develop normally. In all these cases, these animals will experience gene editing, and all their offspring will end up receiving a copy of the edited version of the gene from them. And females where both copies have been edited develop with a mix of male and female traits and can't reproduce.

Thus, once this gene-drive construct starts to spread, every mosquito is likely to either spread it further or be sterile.

The authors tested this on two groups of mosquitos by mixing in males carrying the gene-drive construct until these unlucky fellows were 12.5 percent of the total population. In one cage, only seven generations were needed for every mosquito to inherit the gene-drive-carrying chromosome. All the females were sterile, and the population collapsed—there was no generation eight. In the second group, this took until generation 11, but that population collapsed as well.

The researchers checked, and they did find a few altered versions of the doublesex gene that could no longer be edited. But all of these deleted part of the female-specific portion of the gene and, therefore, caused female sterility as well. The researchers also sequenced African mosquito populations and found only a single naturally occurring variant at the site where gene editing takes place. Tests show that it wouldn't interfere with the editing. So, as far as they can tell, the evolution of resistance isn't an issue here.

That doesn't mean it can never be. The researchers plan to try their method out in much larger populations to determine if very rare events allow resistance. And they cite a similar approach that targets a different gene involved in the development of the two sexes yet still suffered from frequent resistance. Figuring out exactly how these approaches differed will be important for this to move forward.

Even aside from that, the gene drive isn't ready for use in the field. Doublesex is so central to insect sex determination that every species we have looked at has a version, and the ones in closely related species are similar enough that the gene-drive construct could potentially hop species. While targeting other mosquitos might not be a terrible thing, we probably want to have a clear idea of potential issues before releasing anything like this into the wild.

Back to basics

The work also highlights the potential value of the foundational research behind these developments. We didn't actually know what the mosquito version of doublesex did before this work. Instead, all of our knowledge had been generated in the fruit fly Drosophila; the authors note that this work helps clarify the poorly understood mosquito version of the sex determination pathway. And Drosophila isn't a major agricultural pest or a disease vector. People were just studying it in order to have a better understanding of how biology operates.

That's precisely the sort of open-ended, impractical research that finds itself at risk whenever budgets get tight and funding has to be cut. (In fact, fruit-fly research was specifically singled out by Sarah Palin as having "little or nothing to do with the public good.") But science has a funny way of finding uses for knowledge that was developed without any purpose in mind—just like the gene-editing technology itself, which grew out of trying to understand how bacteria protect themselves from viruses.

Nature Biotechnology, 2018. DOI: 10.1038/nbt.4245 (About DOIs).