The whitefly is an invasive pest that farmers worldwide rue for its resilience and ability to devastate crops, and scientists can now say where the insect has one major advantage: Genetically, it is part plant.
In the first study ever to describe gene transfer from a plant to an animal, Chinese researchers show that at least three species of whitefly possess a gene that allows them to circumvent a defense mechanism in plants. This same gene appears in virtually all plants, but no other insects. The groundbreaking findings appeared in Cell on March 25.
"There are so many extremely interesting plant-insect interactions that almost nothing surprises me anymore," said coauthor Ted Turlings of the University of Neuchâtel in Switzerland. "But this is spectacular. There is very solid evidence that this functional gene comes from plants."
The gene in question, BtPMAt1, allows whiteflies to tolerate the toxins plants produce to defend themselves from being devoured by scores of whiteflies or other insects. According to prior research, this gene appears to detoxify damaging compounds that can harm plants via air or other routes.
The researchers who discovered BtPMAt1 in whiteflies had been studying genes that help them handle these defensive compounds, also called phenolic glucosides. But when they looked for this gene in other publicly available genome data, they could only find it in plants, where it was very active — not in any other insect.
"They came into it by accident more than anything else, but they did recognize by these fellow genetic comparisons that it must have come from a plant," Turlings said in an interview with The Academic Times.
To confirm the function of this gene in whiteflies, which are actually more like aphids than flies, the researchers were able to "silence" it by changing the genetics of a tomato plant to make it produce RNA that blocks BtPMAt1. When the whiteflies in the experiment fed on the plant, they died, but mites and aphids lived, revealing a distinct link between whiteflies and BtPMAt1 in plants, and opening the possibility of a way to keep crops safe without pesticides.
Whiteflies are so harmful that they cost Georgia farmers $80 million to $100 million in lost cucumbers, squash and snap peas over the 2016 and 2017 fall seasons.
"We now have a very effective way of fighting this pest without affecting anything else," said Turlings. "I'd say it's extremely targeted."
According to Turlings, the whitefly may have acquired this gene about 35 million years ago, probably due to a virus or bacterium that infected a plant with its DNA, allowing the infector to multiply and integrate some of the plant genome into its own DNA. Since that time, the gene in both plants and whiteflies has evolved, so that today they are only about 45% similar, at most. But its detoxifying function has not changed.
The mystery, then, is how the altered plant gene moved from plant to insect. Turlings and his colleagues still do not know how this transfer occurred, but it probably began with this infection process.
Genes typically pass down from generation to generation within one species in vertical gene transfer. Genes passing from one species to another in horizontal gene transfer is extremely rare, but is known to occur in viruses and some bacteria to better resist antibiotic medication. A February study from Nature Communications established centipedes as the first animal known to have gotten genes for venom production from multiple gene families through this process.
Prior research had also suggested gene transfer from algae to slugs, but it has since been discredited, according to Turlings.
The researchers behind the latest discovery are now looking at a similar, second gene that whiteflies may have also taken from plants, along with the possibility that whiteflies could be detoxifying other toxic compounds. And while this new revelation may have implications for food production worldwide, Turlings believes that risk assessments are necessary before scientists create new varieties of corn, tomatoes or other plants that can kill whiteflies.
"You have to look at other effects or possible effects — which, I'm virtually 100 percent convinced that you won't find any, but you will have to test for that," he said. "Just like you have to keep testing the effects of new pesticides, or newly bred varieties."
Even if these plants do become ubiquitous someday, global whitefly populations should remain intact, as they will move on to eat more wild plants rather than those grown on farms, Turlings maintained. Any worry that removing a major food source for whiteflies will upset some ecological balance is unfounded, he argued.
"They have already upset the ecological balance," he said. "So, it should restore an ecological balance in most countries, but they will never completely disappear. There's no worry about that."
The study, "Whitefly hijacks a plant detoxification gene that neutralizes plant toxins," published March 25 in Cell, was authored by Jixing Xia, Zhaojiang Guo, Zezhong Yang, Haolin Han, Shaoli Wang, Haifeng Xu, Xin Yang, Qingjun Wu, Wen Xie and Youjun Zhang, Chinese Academy of Agricultural Sciences; Xuguo Zhou, University of Kentucky; Wannes Dermauw, Ghent University; and Ted C.J. Turlings, University of Neuchâtel.