An Aedes aegypti mosquito in Tanzania, 2009. Credit: Muhammad Mahdi Karim/Wikimedia Commons, GNU 1.2.
A new year means new beginnings. But for the residents of Florida Keys, a small archipelago off Florida’s coast, the dawn of 2021 seems to portend ill winds.
In August 2020, the local government approved a plan to release 750 million genetically modified (GM) mosquitoes. A British biotech company named Oxitec has planned to execute this release over two years. However, over 236,000 people have signed a petition against this decision because they fear the unknown long-term effects of releasing GM mosquitoes in the environment.
Oxitec’s foray in the US followed a decade of trial runs in the Cayman Islands and in Brazil. On its website, the company showcases publications spanning two decades.
The Florida government has given Oxitec an ‘experimental use permit’. Pursuant to this, the country’s Environment Protection Agency and Centres for Disease Control and Prevention assessed 25 scientific studies.
But the presence of GM material in these mosquitoes has proved sufficient to stoke the apprehensions of Florida Keys’ residents.
Farther to the north, researchers at the University of Minnesota have developed a novel way to resolve this problem. They used genetic engineering to create organisms for release that are not genetically modified.
Maciej Maselko was a postdoctoral associate at the university when he was part of the study. “Slow and expensive regulatory approvals for GM insect release” inspired the team’s work, he told The Wire Science. “We looked for a way to get the benefits achieved with GM insect release but without needing to release GM insects.”
He conceptualised the experiment with PhD scholar Siba Das and molecular biology professor Michael Smanski. The results were published in November 2020.
In a typical control intervention, researchers release sterile male mosquitoes into the environment. These compete with wild males to mate with wild females. Mosquitoes mate only once in their lifetime. Since mating with sterile mosquitoes produces no offspring, the local mosquito population begins to fall.
The methods to select these male mosquitoes to subsequently release are either labour intensive or need specialised equipment. The colony that scientists rear is also often three times larger than the number of males released. Third, a mosquito lives typically for 8-10 days. So scientists must select the males to release close to the site of intervention.
Genetic modification eases these issues. In Oxitec’s method, males carry a self-limiting gene. When these males mate with an unmodified female in the wild, the resultant offspring inherits the gene. This gene pushes the insect’s protein production machinery to over-produce a particular protein, which then interferes with normal development of the offspring. Eventually, the young mosquito dies before maturing into an adult. In genetic engineering parlance, this genetic pathway is called a lethal circuit.
Maselko’s method took the lethal circuit a few notches up the difficulty graph. By way of demonstration, the team used two strains of the model organism of genetic modification experiments: the fruit fly (Drosophila melanogaster). Like humans, fruit flies have two sex chromosomes, X and Y. Females have two X chromosomes and males have one X and one Y.
In Maselko’s method, one strain has a lethal circuit on the X chromosome (XL) while another has a lethal circuit on the Y chromosome (YL). These lethal circuits can be switched on and off as required by modifying the composition of the substance in which the flies are nurtured.
In the first step, the team grew the YL strain in conditions where the lethal circuit is activated. These mate with wild type females (XX). All males (XYL) die, leading to female offspring, XX, that don’t have the lethal circuit.
In a separate second step, the team grew the XL strain in conditions where the lethal circuit is activated. Males carrying only one copy of the lethal variant (XLY) survive, while all females (XLXL) die.
In the third and final step, the surviving females from the first step (XX) are mated with surviving males (XLY) from the second step. The team again grew the offspring in conditions where the lethal circuit is activated. This produced only males (XY) that don’t have the lethal circuit.
“Our approach enables a more centralised production approach, where only eggs can be shipped,” Maselko said. “This needs modest facilities to rear, sterilise and release the males.”
Max Scott, a professor of entomology at Genetic Pest Management, North Carolina State University, called the approach “a clever scheme for producing only non-GM males”.
He also said “the Y chromosome has few genes. Finding a location where the lethal circuit works is a real challenge.”
Maselko’s team seems to have addressed this challenge. In the three batches that the researchers raised using this method, 2,932 males were generated, and one female.
Omar Akbari, an associate professor of cell and developmental biology at the University of California, San Diego, echoed Scott’s assessment. He said he would like to see this model work in actual mosquitoes.
In response to Akbari’s point that the resulting males are not sterile, Maselko said the team would “either sterilise them with X-rays or use the Wolbachia infection approach.”
Wolbachia are bacterial species that occur naturally in 60% of insect species – but they don’t occur in Aedes aegypti mosquitoes. A 2009 study showed that infecting Aedes mosquitoes with Wolbachia reduces the transmission of dengue, Zika and other viruses. Scientists are trialling the technique in many countries, under the World Mosquito Program. At least one scientific paper discussing trial results is expected later this year.
The Minnesota team screened over a thousand fruit flies looking for lethal circuits. They found none. The mathematical possibility of the lethal circuit surviving their experimental process, according to the team’s estimate, is 0.1%.
The Non-GMO Project is a non-profit organisation that verifies the absence of GM material in seeds, retail goods, drugs, livestock feed and supplements. According to its latest standards, its threshold for certification ranges from 0.25%-1.5%.
However, Scott is unsure if this will help people. “Whether a non-GM male derived from two GM strains would be acceptable [to people] is hard to know.”
Phil Taylor, director of the ARC Centre for Fruit Fly Innovation, Australia, said that “from a regulatory perspective, it does test the boundaries a bit – as most novel technologies do.”
Maselko is now a CSIRO Synthetic Biology Future Fellow at Macquarie University in Australia. He wants to conduct larger trials in mosquitoes before scaling up their solution.