A Pesky Insect Took an Evolutionary Shortcut
Millions of years ago, whiteflies pilfered a plant defense, and they have been benefiting ever since.
Among the vegetable world’s most incorrigible villains, the whitefly ranks high. Pale, squishy, and smaller than a sesame seed, these sap-sucking bugs terrorize more than 600 plant species, infecting them with deadly viruses and smearing their leaves with sweet, sticky liquids that encourage the growth of molds. Whiteflies resist pesticides. They dupe plants into mounting the wrong defenses. They can overtake greenhouses by the millions and sicken entire fields of crops; their dastardly deeds grace the pages of many an agricultural guide, and perhaps a horticulturist’s burn book or two.
Against such an enemy, plants deploy poisons powerful enough to stunt insect growth or bring about the bugs’ untimely demise. But whiteflies (which aren’t flies, but cousins of aphids) have found a work-around for this obstacle, too, through thievery. To protect themselves against toxins called phenolic glycosides, a common botanical defense, whiteflies imported a stolen good: a gene that encodes a kind of antidote, likely pilfered from an ancient poisonous plant, according to a study published today in Cell. The plant had probably created the chemical cure to protect its own tissues. In doing so, it revealed a vulnerability for the sticky-fingered whitefly to exploit.
“It’s so clever,” says Naomi Pierce, an entomologist and evolutionary biologist at Harvard who wasn’t involved in the study. “It’s taking advantage of a defense that has been carefully honed by plants … I’m just contemplating how incredible it is.”
Genetic theft is very difficult to prove, and by all accounts, it appears to be somewhat rare. The evidence is generally circumstantial, and by the time scientists clue in, millions of years may have passed. “There’s really no way to know—none of us were there,” says Seemay Chou, a biochemist at UC San Francisco, who led the discovery of a similar phenomenon in ticks but wasn’t involved in the new study.
When grand genetic heists do occur, they can create evolutionary shortcuts, and blur the boundaries between organisms. The tree of life is often presented as full of discrete branches, spaced farther and farther apart. Every so often, however, two organisms come close enough to intersect, and leave relics of their intimacy in their genetic codes. In abducting its enemy’s antidote, the whitefly may have rapidly armed itself with a new weapon, skipping over the chore of creating the gene itself, and becoming a touch plantier in the process. The study’s authors think the insect may have even carried out its felony with an accomplice: a microbial smuggler ferrying bits of genetic material between multicellular hosts.
Typically, genes can be traced back in time, because ancestors tend to pass them down like heirlooms to the generations that follow. But the new study’s researchers, led by Youjun Zhang, a plant biologist at the Chinese Academy of Agricultural Sciences, noticed that this one didn’t follow the usual pattern. They found the gene that protects these bugs from poison in the genomes of only one close-knit group of species on the whitefly family tree, as if it had appeared out of nowhere. Curious about the gene’s origins, the researchers searched its sequence in a massive database. They found that no other insect genomes encode the gene, or anything that even vaguely resembles it.
But a version of the gene appears to be a fixture of many plant species—a hint that it might have hopped from one organism into an entirely different one.
These genome-jumping events are known as horizontal gene transfers. They’re common among bacteria, which swap genetic material willy-nilly, often as a sort of primitive pantomime of sex. Among eukaryotes such as plants and animals, though, they’re relatively rare. For another organism’s genes to integrate, foreign DNA must make its way through a creature’s complex body, and into a cell. It must breach the barriers of the nucleus, where genetic material is cloistered. It must assimilate itself into a new molecular context in a usable form, without diminishing the integrity of the genome that’s already there. It must accomplish all of this without being nuked or jettisoned by the host’s many defenses against unfamiliar matter.
After all that, the stolen genes must then find their way into an egg or a sperm cell, perhaps carrying some sort of benefit that will make offspring more fit.
The chances of all those pieces falling into place are low, to say the least. But in recent years, as genetic-sequencing technologies have become more advanced, scientists have discovered more and more examples of gene hopping. Microbes, they found, can punt genes not just to one another, but to more complex creatures as well. Bacterial genomes are gold mines: They’re where ticks got the genes to manufacture defensive antimicrobial compounds, and where beetles acquired the ability to destroy coffee plants. Parasitic wasps have weaponized the genes of viruses. Tiny freshwater creatures called rotifers seem to be career criminals, habitually lifting genes from bacteria, fungi, and plants. In perhaps the closest parallels to whiteflies, aphids snatched DNA from fungi to paint themselves in a rosy red hue, and caterpillars burglarized bacteria to protect themselves from the cyanides in certain plants. Humans, too, have likely plundered a few microbial genomes.
There can also be red herrings—strange sequences of DNA that appear to point to horizontal gene transfer, but are actually the result of something else, such as laboratory contamination. That seems to have been the case with a study examining the genomes of tardigrades, and the original draft of the human genome.
But several outside experts told me that the new study by Zhang’s team seems solid. “It’s convincing to me,” Nancy Moran, an entomologist at the University of Texas who pioneered work showing horizontal gene transfer in aphids, said. “My skeptical radar is usually raised with horizontal gene transfer,” said Harmit Malik, a geneticist at the Fred Hutchinson Cancer Research Center who collaborated with Chou on the tick study. But a whitefly pinching the gene from a plant is “the most parsimonious hypothesis.”
Still unclear, though, is exactly how, when, or from whom the whiteflies filched the gene. Ted Turlings, a chemical ecologist at the University of Neuchâtel in Switzerland and an author on the new study, told me that the event must have occurred at least 35 million years ago. Back then, plants looked a bit different.
Most cases of horizontal gene transfer seem to have involved prolonged close contact between donor and recipient. It’s certainly possible that plant DNA, adrift in the whitefly’s digestive tract, somehow meandered its way over to a reproductive cell. But Turlings said he and his colleagues think the whitefly had help from a virus with privileged access to parts of the insect’s body. Viruses, after all, are used all the time in labs to deliver genetic intel to cells; this superpower has even been leveraged in some COVID-19 vaccines. Perhaps what the researchers found is not a direct plant-to-insect dispatch, but a protracted relay that could have taken place over many years—a molecular drug trade, with microbes acting as the mules.
Julie Dunning Hotopp, a microbiologist and horizontal-gene-transfer expert at the University of Maryland who wasn’t involved in the study, told me she favors the idea of a bacterial intermediary. A bacterium could even have been the original source of the detoxifying gene, she noted, doling it out first to a very early plant, then again to whiteflies further down the road.
Details aside, it’s clear that whiteflies somehow gained a serious wing up on the plants that try to poison them. That might sound like bad news for plant lovers. “It’s an endless problem for us,” Pierce, of Harvard, told me. “We’re constantly fighting against whiteflies.”
But the world’s vegetation isn’t doomed. Evolution trudges on; perhaps some plants have wised up to the whiteflies’ tactics, and are already plotting their next toxic move. Humans, too, could help by engineering crops that turn the whitefly’s gene off, making the insect vulnerable once again. Pierce delights in this possibility. Watching whiteflies over the years, “you wonder, How do they do it?” she told me. “Now I look at them and I think, Aha—we’ll get you in the end.”