Fruit flies steal bacterial defense to survive parasitic wasps

In the continual arms race between parasites and their hosts, innovation was thought to be the key to a successful attack or defense that one-ups the competition.

But sometimes, as in the corporate world, outright theft can be a quicker way to achieve dominance.

University of California, Berkeley biologists have shown that several species of fruit fly have stolen a successful defense from bacteria to survive predation by parasitic wasps, which in some flies can turn half of all fly larvae into surrogate wombs for baby wasps -; a gruesome fate that inspired the creature in the 1979 movie "Alien."

Bacteria and other microbes are famous for stealing genes from other microbes or viruses; this so-called horizontal gene transfer is the source of troublesome antibiotic resistance among disease-causing microbes. But it's thought to be less common in multicellular organisms, such as insects and humans. Understanding how common it is in animals and how these genes are co-opted and shared can help scientists understand the evolution of animal immune defenses and could point the way to human therapies to fight parasitic or infectious diseases or cancer, itself a kind of parasite.

It's a model for understanding how immune systems evolve, including our immune system, which also contains horizontally transferred genes."

Noah Whiteman, UC Berkeley professor of molecular and cell biology and of integrative biology and director of the campus's Essig Museum of Entomology

Last year, the researchers and their colleagues in Hungary used CRISPR genome editing to knock out the gene responsible for the defense in one widespread fly species, Drosophila ananassae, and found that nearly all the genetically modified flies died from predation by parasitic wasps.

In a new study published Dec. 20 in the journal Current Biology, the biologists demonstrated that this defense -; a gene that encodes a toxin -; can be edited into the genome of the common laboratory fruit fly, Drosophila melanogaster, to make them resistant to parasitoid wasps as well. The gene essentially becomes part of the fly's immune system, one weapon in its armamentarium to fend off parasites.

The results demonstrate how crucial the stolen defense is to fly survival and highlights a strategy that may be more common in animals that scientists suspect.

"This shows that horizontal gene transfer is an underappreciated way that rapid evolution happens in animals," said UC Berkeley doctoral student Rebecca Tarnopol, first author of the Current Biology paper. "People appreciate horizontal gene transfer as one of the major drivers of rapid adaptation in microbes, but these events were thought to be super uncommon in animals. But at least in insects, it seems like they're fairly frequent."

According to Whiteman, senior author of the paper, "The study shows that in order to keep up with the barrage of parasites that are continually evolving new ways to overcome host defenses, a good strategy for animals is to borrow genes from even more rapidly evolving viruses and bacteria, and that's just what these flies have done."

Gene flow from virus to bacteria to fly

Whiteman studies how insects evolve to resist the toxins that plants produce to prevent being eaten. In 2023, he published a book, "Most Delicious Poison," about the plant toxins that humans have come to enjoy, such as caffeine and nicotine.

One plant-herbivore interaction he focuses on is that between the common fruit fly Scaptomyza flava and sour-tasting mustard plants, like the cresses that grow in streams throughout the world.

"The larvae, the immature stages of the fly, live in the leaves of the plant. They're leaf miners, they leave little trails in the leaves," Whiteman said. "They're true parasites of the plant and the plant's trying to kill them with its specialized chemicals. We study that arms race."

What he's learned, however, likely applies to many other insects, among the most successful herbivores on Earth.

"These are obscure little flies, but if you think about the fact that half of all living insect species are herbivores, it's a very popular life history. Understanding the evolution of that is really important for understanding evolution in general in terms of how successful herbivores are," he said.

Several years ago, after sequencing the fly's genome in search of genes that allow it to resist mustard toxins, he discovered an unusual gene that he learned was widespread in bacteria. A search through earlier published genome sequences turned up the same gene in a related fly, Drosophila ananassae, as well as in a bacteria that lives inside an aphid. Researchers studying the aphid uncovered a complicated story: The gene actually comes from a bacterial virus, or bacteriophage, that infects the bacteria that live inside the aphid. The bacteriophage gene, expressed by the bacteria, makes the aphid resistant to a parasitic wasp that plagues it.

These wasps lay their eggs inside the larvae, or maggots, and remain there until the larvae turn into immobile pupae, at which point the wasp eggs mature into wasp larvae that consume the fly pupa, eventually emerging as adults.

When Tarnopol first used gene editing to express the toxin gene in all cells of D. melanogaster, all the flies died. But when Tarnopol expressed the gene only in certain immune cells, the fly became as resistant to parasites as its cousin, D. ananassae.

Whiteman, Tarnopol, and their colleagues subsequently discovered that the gene found in the genome of D. ananassae -; a fusion between two toxin genes, cytolethal distending toxin B (cdtB) and apoptosis inducing protein of 56kDa (aip56), that the researchers called fusionB -; codes for an enzyme that cuts up DNA.

To discover how this nuclease is able to kill a wasp egg, the UC Berkeley researchers reached out to István Andó at the Institute of Genetics of the HUN-REN Biological Research Centre in Szeged, Hungary, which had previously shown that these same flies have a cellular defense against wasp eggs that essentially walls off the eggs from the fly's body and kills them. Andó and his lab colleagues created antibodies to the toxin that allowed them to track it through the fly's body and found that the nuclease essentially floods the fly's body to surround and kill the egg.

"We've been finding this huge untapped world of humoral immune factors that might be at play in the immune system of invertebrates," Tarnopol said. "Our paper is one of the first ones to show, at least in Drosophila, that this type of immune response might be a common mechanism by which natural enemies like wasps and nematodes are dealt with. They are way more lethal in nature than some of the microbial infections that most people work with."

Whiteman and his colleagues are still exploring the complexities of these interactions between fly and wasp, and the cellular and genetic changes that allowed the flies to synthesize a toxin without killing itself.

"If the gene is expressed in the wrong tissue, the fly is going to die. That gene is never going to sweep through populations through natural selection," Whiteman said. "But if it lands in a place in the genome that's near some enhancer or some regulatory component that expresses it a little bit in fat body tissue, then you can see how it can get this leg up really quickly, you get this amazing advantage."

Horizontal gene transfer in any organism would pose similar problems, he said, but in the arms race between predator and prey, it may be worth it.

"When you're a poor little fruit fly, how do you deal with these pathogens and parasites that are rapidly evolving to take advantage of you?" he said. "One way is to borrow genes from bacteria and viruses because they're rapidly evolving. It's an ingenious strategy -; instead of waiting around for your own genes to help you, take them from other organisms that are more rapidly evolving than themselves. And that seems to have happened many times independently in insects, given that so many different ones have taken up this gene. It gives us a picture of a new kind of dynamism that is occurring even in animals that have just innate immune systems and don't have adaptive immunity."

Whiteman's work was funded by the National Institute of General Medical Sciences of the National Institutes of Health (R35GM119816). Other co-authors of the paper are Josephine Tamsil, Ji Heon Ha, Kirsten Verster and Susan Bernstein of UC Berkeley, Gyöngyi Cinege, Edit Ábrahám, Lilla B. Magyar and Zoltán Lipinszki of Hungary and Bernard Kim of Stanford University.