Why Is a BU Researcher So Fascinated with the Diets of Dung Beetles?
Biologist Sofía Casasa studies tiny insects to learn whether their genes or diet are most responsible for their size, shape, and structure
Why Is a BU Researcher So Fascinated with the Diets of Dung Beetles?
Why Is a BU Researcher So Fascinated with the Diets of Dung Beetles?
Scientists know it as Onthophagus—a type of horned scarab beetle found in just about every part of the world. But you probably know it for what it eats: poop. Whatever else dung beetles are good for—they are super strong, unparalleled navigators, and important plant seed spreaders—they just can’t shake the rep they get for their dietary choices.
In her Boston University lab, biologist Sofía Casasa has trays full of Onthophagus and fridges stocked with their favorite food—cow dung collected from a farm in Lincoln, Mass. An expert in evolutionary developmental biology, she’s studying the insects for clues to how organisms develop and grow. Casasa’s goal is to understand how much of an organism’s appearance—its size, shape, and structure, or morphology—is dictated by its genes, and how much by its environment, particularly what it eats.
With the dung beetles, she’s starting with one question: why do some beetles grow longer horns than others?
“Knowing how body parts are made, how they grow, why some grow more than others is something we’re all interested in,” says Casasa, a BU College of Arts & Sciences assistant professor of biology. “And this happens in all organisms, whether that’s humans or a dog or a fly. Just understanding how the environment is affecting our development, I think that’s really important.”
In one project, she mixed up the diet for three closely related species of dung beetle to see how it impacted their horn growth. One batch of larvae from each species was fed with nutritious dung—collected during summer when the cows have been chewing on lush green grass. The other batch got the winter dung—hay-filled and relatively nutrient-poor.
“All species respond in body size,” says Casasa. More nutrients gave the beetles a noticeable growth boost. But in one species, large body size also meant extravagant horn growth. Biologists call this adaptation to different environments and conditions phenotypic plasticity. Casasa says dung beetles are a great model for study because they can exhibit extreme plasticity—as shown by the horns of the bull-headed dung beetle, Onthophagus taurus. In the study, the grass-fed bull-headed beetle grew long, lavish horns; its hay-fed peer grew little stumpy ones. Another species had a more moderate difference in horn size based on diet; the third didn’t grow any horns at all, no matter what it ate.
To figure out why the species reacted differently to the two diets, Casasa is exploring the beetles’ genomes.
“We know the environment affects organisms, so we’re studying this at the molecular level. We’re trying to understand what genes could be involved in regulating this,” she says. “We use a technique called RNA sequencing across these three species to try to uncover the genetic mechanisms that regulate whether a species has an exaggerated degree of plasticity or has completely lost the ability to do this.”
Using RNA interference, Casasa can effectively turn genes off, or downregulate them. She and her team can then repeat their nutrition experiments to see what happens. “If we see, for example, a beetle that normally would have horns suddenly has no horns, then we know this gene was probably very important in building horns.”
One future area she’d like to investigate is how longer horns seem to shape the beetles’ behavior.
“Beetles with horns fight to gain access to females; beetles with no horns use sneakier tactics and try to bypass large males,” says Casasa. “But we don’t really understand the molecular mechanisms that regulate this behavior or the connection with having large horns.”
She also plans to expand her research to a roundworm, or nematode, known as Pristionchus pacificus. Like the dung beetle, this one-millimeter-long creature has a dubious diet: it likes to dine on microbes inside rotting scarab beetles. But biologists like Casasa love it, in part because—like the beetles—its genome has been fully mapped, which allows them to scrutinize its development in fine detail.
Although she admits her work—and frequent trips to local dairy farms—might be an unusual conversation starter (or should that be stopper?), Casasa remains fascinated by the beetles she first started studying as a PhD student interested in evolution and ecology. “There’s so much you can do with them,” she says. “They have a really interesting behavioral ecology, but we can also do a lot at the molecular and developmental level. They’re the perfect system.”
Watch the video above to see Casasa at work on the farm and in the lab—and to see just how big those beetle horns can get.
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