Genetically Modified Mosquitoes
Given great concern in the Florida Keys about the region’s fragile ecosystem, mosquito control officials in the Keys have been searching for alternatives to pesticide use, which can and have negatively impacted public health and wildlife. Local mosquito control officials in the Florida Keys are awaiting approval from the federal government to begin releasing hundreds of thousands of genetically modified mosquitoes developed by British company Oxitec to stop the spread of the disease.
Oxitec is in discussions with the FDA regarding potential release its RIDL® (Release of Insects carrying a Dominant Lethal) male mosquitoes in the Keys. As Oxitec explains, by “sterile” it means that although the males do produce sperm and can fertilize the female’s eggs. But their offspring are “inviable”, meaning that they die at a very early stage of development due to the lethal gene construct passed on by the altered males.
The sterile OX513A mosquitoes were produced by introducing, via a piggyBac helper plasmid injected in mosquito eggs, the OX513 transposon. The OX513 construct encodes the tetracycline-repressible transcription activator (tTA). Once the mosquitoes were developed with the stably-integrated construct, the company was able to breed them, and says it has been breeding them now for over 100 generations.
When expressed at high levels the tTA protein proves lethal, probably due to transcriptional “squelching” and/or interference with ubiquitin-dependent proteolysis.
In a deadly genetic positive feedback cycle, when expressed in LA513 the tTA protein can keep its own expression turned on by binding to an upstream operator sequence, thus driving expression of tTA from a nearby minimal promoter, which in turn binds to the activator tetO, creating a positive feedback system.
But tetracycline, which binds tTA, prevents the activator from interacting with tetO, allowing batches of transgenic mosquitoes to be grown in the presence of the antibiotic. The resultant transgenic Aedes eggs are collected for hatching at a trial site, and the smaller male pupae are sorted from females and on maturity released into the field, where breeding with wild-type female mosquitoes results in sterile mating.
Oxitec scientists have conducted two series of field tests, one in the Cayman Islands and one in Brazil, to determine whether the altered mosquitoes could survive in the wild, their ability to compete for female attention with wild-type males, and whether they could successfully pass on the repressible lethal genetic system that would ensure their offspring would not survive.
Oxitec founder and chief scientific officer Luke Alphey, Ph.D., and the developer of the RIDL system, told GEN, “The trials turned out as exactly as we hoped with all endpoints met. The first trial asked the question whether these mosquitoes could successfully survive in the wild and mate with females in competition with wild-type males. That is fundamental to any kind of sterile insect method. The sterile mosquitoes survived and mated and we could then calculate how many males we would need to release over a specific time period in order to suppress the wild population.”
The key end point of second trial, he said, was “to determine whether we could suppress the population of wild type mosquitoes, and we saw an 80% suppression in the population.”