Between 2013 and 2015, genetically modified mosquitoes of the species Aedes aegypti were released in Brazil. The aim was to dramatically reduce the number of these insects that are carriers of diseases such as dengue, chikungunya, Zika, and yellow fever.
These GM insects were designed to cull the population by producing offspring that cannot become adults. But it appears that some managed to survive into adulthood and their genes spread to the local population they were designed to cull. The findings are published in Scientific Reports.
“The claim was that genes from the release strain would not get into the general population because offspring would die,’’ said senior author Jeffrey Powell, professor of ecology and evolutionary biology, in a statement. “That obviously was not what happened.”
The mosquitoes were released in the city of Jacobina in the Bahia region by British company Oxitec. The city experienced 1,800 cases of dengue fever in 2012. Roughly 450,000 male mosquitoes were released every week with official permission from the government over a period of 27 months.
This approach was successfully conducted previously in other areas of Central and South America, leading to a reduction of 85 percent of the mosquito population. The Jacobina trial was the biggest trial for the company and it did show a reduction in 92 percent of the wild Aedes aegypti population. However, this reduction didn’t last as the effectiveness of the program began to break down at about the 18-month mark.
These mosquitoes were designed (and thus released) with the assumption that most of the offspring between the GM and wild population would die before maturity; the few that could survive would likely be weak and infertile. At least, this was the assumption based on laboratory tests. The new study reveals this was not the case.
The team sampled the wild population of mosquitoes six, 12, and between 27 and 30 months after the release commenced and estimated that between 10 and 60 percent (depending on the criterion used) now have genetic material belonging to the GM mosquito.
“It is unclear how this may affect disease transmission or affect other efforts to control these dangerous vectors. These results highlight the importance of having in place a genetic monitoring program during such releases to detect unanticipated outcomes,” the authors write in the paper.
One concern the authors present is not the presence of the modified gene in the wild population, but the fact that the mosquitoes used were a Cuban/Mexican hybrid. The new tri-hybrid population might result in enhanced resistance against insecticides due to hybrid vigor.