When Mating Kills: Rewiring Nature to Kill Mosquitoes
Image: “Mosquito bite” by James Jordan / CC 2.0
Before coming to Canada from Bangladesh, going to sleep most nights required a lot of preparation. I had to spray my room with insecticide, put up a bed net, or both. The precautions were mainly for a good night’s sleep – being bitten all over by mosquitoes isn’t exactly conducive to sleep, and makes for a very itchy morning. But dengue fever, a mosquito-borne viral illness, was a big concern too – I got it once, and my sister twice. We were privileged enough to have access to great healthcare, but many people don’t, and subsequently die. In 2012, malaria – also carried by mosquitoes – killed approximately 630,000 people worldwide 1 , while dengue infected over 100 million 2 . Traditionally, insecticides, bed nets, and environmental clean-up have been the standard for combatting mosquito-borne illness. These do work, and have saved countless lives, but resistance in mosquitoes, as well as cost and adherence, are extreme challenges. Over the last decade, genetically engineering mosquitoes and the bacteria that infect them has emerged as a promising new alternative.
Last month, scientists in the Brazilian community of Tubiacanga, Rio de Janeiro made news when they started field trials looking at intentionally infecting Aedes aegypti (the mosquito species that transmits dengue virus) with Wolbachia, a bacterial parasite of the mosquito itself 3 . The trials in Tubiacanga are not the first – they are a follow-up of successful testing done halfway across the world, in Australia. Based at Monash University in Melbourne, the Eliminate Dengue team is taking a global approach, with projects in Colombia, China, Indonesia, and Vietnam, as well as collaborators from other countries 4 . What’s more, this isn’t the only team bringing the latest in molecular and microbial science to the fight against such historically neglected tropical diseases. The Oxford University-spawned company Oxitec is also working in Brazil, after initial trials in the Grand Cayman Islands 5, but rather than studying infection with an existing parasite, they have produced a genetic vector designed to decimate wild mosquito populations. And it doesn’t end there – labs all over the world are designing promising new strategies against disease-bearing mosquitoes 6 . An attractive aspect of Wolbachia infection, which is one of the most promising of these strategies, is that the bacterium can spread through populations by co-opting natural selection on mosquitoes for their own purposes. Certain strains cause infected males to be unable to produce viable offspring with uninfected females, a process known as cytoplasmic incompatibility. Because of this, uninfected females are less likely to mate successfully. Infected females mate and produce eggs that – lo and behold – are also infected. In this way the bacteria are passed on from generation to generation, spreading through the population. Of course, if these bacteria are just symbionts – that infect and spread, but don’t kill – what’s the use? It has been found that certain Wolbachia strains can block mosquitoes from carrying pathogens like dengue virus, and that others can kill off mosquitoes before they begin to bite for blood. By mixing and matching characteristics from different bacterial strains and infecting mosquitoes with the resulting genetic hybrids, scientists have created mosquitoes that could be introduced into the wild and propagate these traits through a local population 7 . Australian field trials of Aedes aegypti infected with such dengue-suppressing Wolbachia strains had hopeful results – even two years after release, most mosquitoes were stably infected in the population 8 .
Unfortunately, Anopheles mosquitoes, which transmit the malaria-causing Plasmodium parasite, are not naturally infected by Wolbachia. Generating stable infections in this species has proved difficult, and strategies based on genetic modification seem more promising 6 . The previously mentioned company Oxitec created a population of male Aedes aegypti mosquitoes that carry a repressive lethal gene. These males can be grown to maturity in lab conditions that repressed the gene – then they could be continually released into the wild to mate with females, producing offspring that died when the gene was expressed 9 . Another of Oxitec’s products – albeit one that hasn’t yet been field-tested – features a female-specific gene. Released males have a silenced copy of the gene, and go around mating with wild-type females. The female offspring of this mating express the gene, which gives them faulty flight muscles, and are eaten up by predators, while the male offspring, still carrying the deadly gene, keep on mating, eventually causing the whole population to crash 10 . While both of these studies targeted Aedes aegypti, they will hopefully be translated to work in Anopheles mosquitoes as well.
One study does target Anopheles gambiae, the mosquito species mainly responsible for malarial transmission. A team headed by Roberto Galizi at Imperial College London has developed a fascinating and elegant new technique that has high potential. It is based on something called a homing endonuclease – an enzyme that recognizes an extremely specific sequence in DNA and chops it up. In this particular method, the scientists made a transgenic mosquito (containing genes from a different organism – in this case a mold) with the gene for a homing endonuclease called I-PpoI embedded in its DNA. The enzyme recognizes sequences in the mosquito’s X-chromosome and attacks it. But where the science gets really neat is in the way the team tweaked the enzyme. They engineered the homing endonuclease gene to be expressed only when the males produced sperm, and to make an unstable enzyme that would only be active between the time when the sperm was made and the time when the male mated. Like humans, male mosquitoes have one X and one Y chromosome, while females have two X chromosomes. In these genetically engineered mosquitoes, X chromosomes in sperm are chopped up, and only the sperm with Y chromosomes survive intact. After mating, then, the only viable offspring that these mosquitoes produce will have inherited a Y chromosome – that is, they will always be male. Moreover, these male offspring will have the gene coding for I-PpoI, so they can, ironically, keep on destroying their own kind by mating. Within a few generations, the team found that populations became almost 100% male and subsequently died off 11 .
Field trials are hopefully coming soon for this study. Unfortunately, field trials are often where promising initiatives go horribly wrong. In the 1970s, when the WHO tried to introduce sterile male mosquitoes to bring down the mosquito population in parts of India, misinformation and lack of community consultation led to the belief among many that the project was a US plot to develop biological weapons. Needlessly, the program shut down 12 . Fast forward to 2011, when Oxitec published the result of trials in the Grand Cayman islands. The trials worked and there was no local public backlash, but the scientific community did not know about the existence of the trial until after its results were published, and there had not been any great effort in community consultation. Having seen the negative effects that a lack of communication is having in the ongoing GM crop fiasco, the scientific community was much more aware this time around, and spoke out against Oxitec’s neglect 13,14 . Subsequent Oxitec trials in Florida are placing much greater emphasis on ensuring that locals know the science and data behind the intervention, but there still seems to be significant opposition to address 15 . Eliminate Dengue is involving local scientists, and working with local groups for months in advance to ensure that the implementation of their projects is a community-based effort, and not just the result of scientists flying in from Australia and releasing GM mosquitoes with no explanation 4.
The results from field trials and lab experiments have been extremely promising. If things go smoothly, lab-grown mosquitoes might be released into the wild in the near future. That is definitely still some way off, but it promises to be as revolutionary in tackling the likes of malaria and dengue as insecticide and bed nets were when first used.
References
World Health Organization. World Malaria Report 2013. (2013).
Ratnam, I., Leder, K., Black, J. & Torresi, J. Dengue fever and international travel. Journal of travel medicine 20, 384–393 (2013).
BBC. Brazil releases ‘good’ mosquitoes to fight dengue fever. (2014).
Eliminate Dengue. Eliminate Dengue.
Oxitec. Oxitec Products – Aedes aegypti OX513A.
McGraw, EA & O’Neill, SL. Beyond insecticides: new thinking on an ancient problem. (2013). doi:10.1038/nrmicro2968
LePage, D & Bordenstein, SR. Wolbachia: Can we save lives with a great pandemic? (2013). at http://www.sciencedirect.com/science/article/pii/S1471492213001013
Hoffmann, A. A. et al. Stability of the wMel Wolbachia Infection following invasion into Aedes aegypti populations. PLoS neglected tropical diseases 8, e3115 (2014).
Harris, A. F. et al. Field performance of engineered male mosquitoes. Nature biotechnology 29, 1034–7 (2011).
Fu, G. et al. Female-specific flightless phenotype for mosquito control. Proceedings of the National Academy of Sciences of the United States of America 107, 4550–4 (2010).
Galizi, R. et al. A synthetic sex ratio distortion system for the control of the human malaria mosquito. Nature communications 5, (2014).
Curtis, C. & Borstel, R. Allegations against Indian research unit refuted. Nature 273, (1978).
Subbaraman, N. Science snipes at Oxitec transgenic-mosquito trial. (2011). at <http://www.nature.com/nbt/journal/v29/n1/full/nbt0111-9a.html>
Enserink, M. GM Mosquito Trial Strains Ties in Gates-Funded Project. (2010).
Sagastume, P. Genetically modified mosquitoes set off uproar in Florida Keys. (2013).