When 8-year-old Hugo Deans found a handful of BB-sized objects near an ant nest under a log in his backyard, he thought they were seeds. However, his father, Penn State entomology professor Andrew Deans, knew immediately what they were — oak galls, or insect-triggered plant growth. He didn’t immediately realize that the galls were part of a complex relationship between ants, wasps and oaks, the discovery of which would upend a century of knowledge about plant-insect interactions.
Looking back, Hugo, now 10, said he “thought they were seeds, and I was excited because I didn’t know ants would collect seeds. I always thought ants would eat food scraps and stuff around the house. Then I became more excited when [my dad] tell me they’re cowards because [my dad] So excited. I’m surprised ants collect galls because why do they do it? “
Many plant-insect interactions are well documented, according to Andrew Deans, curator at Penn State’s Frost Insect Museum. For example, it has long been known that most “cynipid” wasp species induce oak trees to develop protective galls or growths around their larvae to ensure the safety of their developing offspring. In addition, certain plants—including Sanguinaria canadensis, a wildflower native to North America—produce edible appendages on the seeds, called elaiosomes, to attract ants, which then carry the seeds back to the nest, thereby spreading the Seeds are scattered. The latter example is known as a “myrmecochory” – or seeds spread by ants.
“In myrmecochory, the ants get a little nutrition when they eat the elaiosomes, and the plants disperse the seeds into spaces where there are no enemies,” Deans explained. “This phenomenon was first documented more than 100 years ago and is often taught to biology students as an example of plant-insect interactions.”
The team’s new research — which begins with Hugo’s discovery of galls located near ant nests — reveals a more complex myrmecochory that combines wasp-oak-gall interactions with edible appendage-ant interactions effect.
“First, we observed that while these galls typically contained fleshy pale pink ‘caps’, galls close to the nest did not have these caps, suggesting that they may have been eaten by ants,” Deans said. “Ultimately, this led us to discover that gall wasps were manipulating oak trees to produce galls before taking it a step further and manipulating the ants to retrieve the galls back to their nests, where the wasp larvae could be protected from gall predators or have other benefits. This multi-layered interaction is exciting; it’s almost hard to wrap your mind around it.”
The team’s findings were published in the journal American naturalist.
To better understand the interaction, the researchers conducted a series of field and laboratory experiments. First, to determine whether oak gall caps—which the researchers named kapéllos (Greek for “caps”)—are indeed edible and attractive to ants like eliaosomes, the team looked directly at New West wild ant colonies Oak Galls in York and Central Pennsylvania. Additionally, they set up cameras to capture additional animal/bile interactions. In both locations, they saw ants transporting galls to their nests. Inside the nest, all edible lids have been removed, while the galls themselves are left intact.
In a second set of experiments to determine whether kapéllos function similarly to elaiosomes, the researchers investigated the ants’ preference for oak galls and bloodroot seeds. They established seed/gall bait stations and observed that the ants removed the same number of seeds and galls, suggesting no differences in the ants’ preferences.
Next, the scientists conducted a laboratory experiment to document whether the ants collected galls for their nutrient-rich kapéllos. They set up three petri dish treatments—containing whole galls, galls with kapéllos removed, or kapéllos with galls removed—and a control dish containing different types of galls without edible appendages. They introduced ants into petri dishes. They found no difference in ant interest between control galls and no kapéllo-treated galls, both of which lack edible components. In contrast, ants were more interested in galls with intact kapéllos and kapéllos alone than in control galls.
“We found that galls with covers were more attractive to ants than galls without covers, and the covers themselves were attractive to ants,” said John Tooker, a professor of entomology. “This suggests that the hat must be Evolved to attract ants.”
Finally, the team asked, “What is it about kapéllos that makes them so attractive to ants?” According to Tooker, the chemistry of elaiosomes is well-studied and known to contain nutrient-rich fatty acids. So the team compared the chemical composition of kapéllos with elaiosomes and found that kapéllos also contain healthy fatty acids.
“Abundant fatty acids in the gallbladder and liposomes seem to mimic dead insects,” Tucker said. “Ants are scavengers trying to find and grab whatever is suitable to bring back to their colony, so it’s no accident that both their gall caps and lecithin contain fatty acids typical of dead insects.”
Which came first?
The last and, according to the researchers, the most interesting question the team pursued was, “Which came first in evolutionary time? The lecithin interaction or the bile interaction?”
“Given that myrmecochory was described more than a century ago and has been well studied and taught in schools, one might assume that enzymatic interactions emerged first, but this assumption may be wrong for several reasons, said Robert J. Warren II, professor of biology at SUNY Buffalo State University.
One reason, he explained, is that hairless plants like blood roots make up only a small percentage of all plant species, and so may not provide enough food resources to drive ants’ natural selection. However, oak galls are widespread. In fact, Warren said, they were once so abundant that they were often used to fatten livestock.
“If these galls are so abundant, and this strategy of growing this hat evolved thousands of years ago, that could be a strong driver of natural selection in ants,” Warren said. “It could be that ants have gotten used to it long ago. Pick up galls with a hat, and then when spring wildflowers start to produce seeds that happen to have edible appendages, ants already tend to pick up with fatty acid appendages.”
Deans noted that the team recently received a grant to conduct phylogenetic work to further investigate which of these interactions first emerged during evolutionary times.
“Understanding how these interactions evolved and how they work can help unravel the complexity of life on Earth,” he said.
On what it’s like to contribute to such an important discovery, Hugo said: “I bet other kids made similar discoveries, but never knew how important they might be. Knowing that I was involved in an important science I’m very happy and proud to find out. It’s strange to think that only a few ants collect what I think are seeds that are actually an important scientific breakthrough.”
When asked if he wanted to be an entomologist like his father when he grew up, now that he has made his first scientific discovery, Hugo said: “Not really. I want to be different… Unique…grown up.”
Crypt breeder wasp found parasitizing multiple gall wasps
Robert J. Warren et al., Oak Galls demonstrated ant dispersal with Myrmecochorous seeds convergent, American naturalist (2022). DOI: 10.1086/720283
Courtesy of Penn State University
Citation: Boy’s Discovery Reveals Highly Complex Plant-Insect Interactions (September 2, 2022) Retrieved September 2, 2022 https://phys.org/news/2022-09-boy-discovery-reveals -highly-complex.html
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