Whereas Different Bugs Performed, This Species Developed the Blade

They’re fruit flies, but they eat leaves. And to feed, the females scrape and scoop vegetation using what researchers actually sometimes call them vaginal teeth. Then they turn their mouths to the plant juices they have released and lick them up.

In a study published last week in the journal Proceedings of the Royal Society B, researchers identified evolutionary clues to these flies, including genes that may have helped transform their bristly genitalia into blades that help them eat.

Fruit flies of the Drosophilidae family, including these leaf-eaters, first evolved to feed on decaying vegetation and the yeasts it harbors. Her mouth is “almost like having a pool noodle with a sponge on the end,” said Andy Gloss, an evolutionary biologist at New York University. But near their hindquarters, female flies have devices that mimic a mouth: egg-laying organs called ovipositors, with valves that open and close like jaws. Hair on the edges of the valve can harden into tooth-like spikes.

dr Gloss and colleagues hypothesized that the evolution of these specialized, sharpened ovipositors helped some flies become herbivorous leaf eaters.

To test this hypothesis, the researchers looked at how changes in the laying apparatus corresponded to the flies’ feeding strategies. They counted the number of bristles that different species had on their genitals and took flies under a microscope when there were no answers in the scientific literature. They then assigned the number of bristles to a tree of evolutionary relationships. In a group called the Scaptomyza, bristle count increased with the onset of herbivory around 10 million years ago, suggesting that greater density of these tooth-like hairs helped herbivory.

“Since you’re trying to saw into very tough, fibrous plant material, you need something that’s a little sharper,” said Julianne Peláez, an evolutionary biologist who was involved in this research at the University of California, Berkeley.

The team then combed through the genome of one of these lettuce eaters: Scaptomyza flava, a small yellow fly that one of the study’s authors, Noah Whiteman, also from Berkeley, has been studying for a long time. This herbivorous species evolved relatively recently, making it a promising target for unraveling how insects first started eating plants, he said.

By sequencing DNA from groups of laboratory-bred S. flava flies, Dr. Whiteman’s team genes that may be responsible for the development of vaginal teeth. One gene they found accounted for about 10 percent of the variation, the team reported.

“This is really a start in understanding the genetic basis of this trait,” said Joel Atallah, a developmental biologist at the University of New Orleans who was not involved in the study. He added that additional research is needed to examine whether the same candidate genes contributed to the development of the bristles in other species of fruit flies.

These multipurpose ovipositors are “some of the most medically and economically important body parts on the planet,” said Catherine Linnen, an evolutionary biologist at the University of Kentucky who was not involved in the study. Plant-piercing ovipositors have turned some insects into crop pests, while some bees and wasps have co-opted egg-layers for piercing weapons. They are central to how many insects reproduce and survive. “But we know so little about how these are made,” said Dr. linen.

Further study of the ovipositor could lead to a better understanding of herbivorous insects, which make up about a quarter of all animals on Earth. “It’s so important, but so difficult to achieve because the sands of time have obscured the changes in most major lines,” like butterflies, moths and weevils, said Dr. Whiteman. But it is not the case that all insects have switched to snacking on plants: herbivores are found in only nine out of about 30 insect orders.

But herbivores were likely central to these flies and their relatives spreading around the world, said Dr. Glossy, and toothy genitalia may have helped them find plants in new habitats, whether suburban ditches or mountaintops.

“The ability to slice and scoop with their rear ends, frivolous as it sounds, may have been one of the innovations that unlocked this potential,” he added.