How a Fly's Sense of Smell Reveals the Switch from Carnivore to Herbivore

Changing your lifestyle from eating meat to eating veggies doesn't seem like a big deal-and it isn't. For an entire species, though, it's a complete change in biology. Now, scientists have found out how this evolutionary process occurs in species.

In this case, the scientists turned to insects. Herbivorous insect species make up half of all known insect species. The switch from a non-plant diet to herbivory, though, only evolved in one-third of living insect orders. This particular discrepancy has long puzzled researchers.

"It implies that the transition to herbivory happened rarely, but when it happened, it turned out to be a major push for speciation spawning the evolution of a disproportionate number of species in that group," said Benjamin Goldman-Huertas, one of the researchers, in a news release.

In order to better understand this switch, the researchers focused on the fly species Scaptomyza flava, a close relative of the fruit fly, Drosophila melanogaster. These flies seek out healthy plants in the mustard family, pierce tiny holes in their leaves, drink the plant juice and then deposit their eggs within the leaves.

The researchers measured the electrical responses in the insects' antennae generated by olfactory receptors when presenting the animals with various scents. This revealed that the smell of yeast, which thrives on rotting fruit, actually wasn't attractive to the flies even though they're related to fruit flies, which are attracted. However, the flies were attracted to a compound that's responsible for the scent of freshly cut grass and is common in leafy plants.

"Scaptomyza is a very useful model to study genetic underpinnings because on the one hand, it is closely related to D. melanogaster, but on the other hand, it is evolutionary diverged," said Noah Whiteman, one of the researchers. "In these flies, herbivory evolved in the last 20 million years at most. The changes are recent enough that we are able to detect them and compared them to non-herbivorous sister species like D. melanogaster. But loss of behaviors doesn't make you go on to new feeding grounds, so some gain of function must have happened as well."

The researchers found a group of olfactory receptor genes that underwent a disproportionately large amount of sequence change. This suggests that evolution changed the function of the genes.

The findings reveal a bit more about evolutionary switches and show what genes needed to change to make this occur.

The findings are published in the journal Proceedings of the National Academy of Sciences.

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