A journal article showing that DNA from genetically modified (GM) mosquitoes had spread to the local mosquito population in Brazil prompted alarming news reports and backlash from some scientists after its publication last month. Now, one co-author of the paper contends it overstates the potential risks of the GM insects, and several co-authors have reportedly requested that it be retracted.

The study tracked the effects of a field trial of GM Aedes aegypti mosquitoes created by the company Oxitec and released in Jacobina, Brazil, between 2013 and 2015. The Abingdon, U.K.–based company released nonbiting male insects endowed with a gene that would kill most of their offspring. The trial was designed to see whether such engineered insects could reduce populations of wild mosquitoes that carry viruses such as Zika and dengue.

Before that release, lab tests had shown that when GM males mated with wild females, a small percentage of their offspring survived. To test whether these survivors had managed to reproduce and pass on their genes, a Yale University team led by population geneticist Jeffrey Powell collaborated with Brazilian researchers to collect and analyze DNA from mosquitoes before, during, and up to 3 months after the release. These collaborators included biochemist Margareth Capurro and colleagues at the University of São Paulo in São Paulo, Brazil, who had an agreement with Oxitec to monitor the effects of the Jacobina release.

By the researchers’ estimates, somewhere from 5% to 60% of the mosquito population carried DNA from the Oxitec strain in its genome at different points in the study and in different Jacobina neighborhoods. The team didn’t find mosquitoes carrying the transgenes, foreign to A. aegypti, that Oxitec introduced to kill offspring and to fluorescently label the mosquitoes as GM.

The study also didn’t test whether the hybrid mosquitoes were resistant to insecticides or how readily they transmitted viruses. But the paper, published 10 September in Scientific Reports, suggests DNA from the Oxitec strain could have made wild mosquitoes somehow fitter or stronger. The creation of hybrids “very likely result[ed] in a more robust population than the pre-release population,” it says.

Capurro publicly objected to that suggestion, telling Revista Questão de Ciência, a publication of the Brazilian nonprofit Science Question Institute, that there were no data supporting the idea of “hybrid vigor.” She also said she had not seen the final version of the text before its publication. The article reported that Capurro and five co-authors had asked the journal editors to retract the paper.

Capurro’s lawyer, Lucas Sampaio Santos, based in São Paulo, told ScienceInsider yesterday that Capurro was “trying to solve this issue directly and amicably with the other authors. … Dr. Capurro does not support the inflammatory, dramatic and speculative statements contained in the manuscript,” he wrote in an email. Santos did not comment on the reported request for a retraction.

Oxitec has reacted to the paper’s publication with a list of objections and has filed a complaint with the journal. (On 17 September, Scientific Reports added an editor’s note to the study, promising “a further editorial response” to criticisms raised about the conclusions.) The company, now a subsidiary of U.S. biotech company Intrexon, has run several large trials in Brazil since the Jacobina release, including a test last year of a second-generation GM strain designed so that male offspring survive and pass on their genes.

Powell, listed in the Scientific Reports paper as the author who “conceived and directed the project and wrote the manuscript,” says he stands by the validity of the paper’s data and analysis. The paper clearly states that the effects of DNA spread from the Oxitec strain aren’t known, he says, and it raises some possible consequences without claiming that they’ve been proved.