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Leafcutter Ants' Success Due to More Than Crop Selection

Leafcutter Ants' Success Due to More Than Crop Selection
A rare peak inside the garden of a leafcutter ant colony reveals the queen (center-left), brood, and an extensive matrix of fungal hyphae that form both the nest structure and the insects' exclusive diet. Public domain image by Alex Wild, for the University of Texas at Austin's "Insects Unlocked" project.

A complex genetic analysis has biologists re-evaluating some long-held beliefs about the way societies evolved following the invention of agriculture—by six-legged farmers.

Like humans, leafcutter ants grow crops, and like humans, farming allows the ants to produce enough food to support millions of individuals who work at specialized jobs. But while people invented agriculture at the dawn of civilization about 10,000 years ago, leafcutters began cultivating massive subterranean fungus gardens more than 10 million years ago.

Scientists have long thought that leafcutter ants were able to become "ecologically dominant"—in other words, widely destructive to ecosystems and agriculture—throughout much of South, Central and North America precisely because the fungus they domesticated is a kind of superfood. But a new study published this week in the journal Molecular Ecology shows that the ants can also be ecologically successful with other, more ordinary fungi once thought to be farmed only by other ants.

Ulrich Mueller, professor of integrative biology at the University of Texas at Austin, led the research, which also included scientists from Rice University and Brazil's São Paulo State University.

"This shows that it's not just the type of cultivar, but the combination of farming strategy and type of cultivar that are the key to their success," said Mueller. "The same is also true for human agriculture, but leafcutter ants figured this out 15 to 20 million years ago."

"The ability to grow domesticated crops was a major turning point in human history and evolution, and we thought, until recently, that a similar thing was true for leafcutters," said study co-author Scott Solomon, an evolutionary biologist at Rice University who collected many of the study's samples as a graduate student and postdoctoral researcher at UT Austin and the Smithsonian Institution in Washington, D.C. "Our findings suggest that several of the things we thought we 'knew' about leafcutters are not true."

The research, led by Mueller, Solomon's longtime UT collaborator and mentor, is available in both the newly published paper and a 2017 companion study, also published in Molecular Ecology.

Leafcutter ants. Photo by By Geoff Gallice from Gainesville, FL.

"This study started 20 years ago as a collaboration between Brazilian and Texan labs and developed into a huge collaboration involving 22 labs surveying leafcutter ants in 17 countries," said Mueller. "Because of this international effort, we now have a comprehensive understanding of leafcutter ecology and evolution."

Leafcutter ants are found only in the Americas. More than 40 species range from Argentina to the southern United States, and they are a dominant ecological player in any forest or grassland they inhabit.

"They aren't the only ants that grow fungi, but if you compare leafcutter ants with other ants that grow fungi, there are many differences," Mueller said. "For starters, no other ants use freshly cut leaves to grow their fungi."

Ants that grow fungus on dead and decaying leaves have been around even longer than leafcutters, probably about 50 million years, Solomon said. But leafcutters' ability to use living leaves was a quantum leap in evolutionary terms because it opened up the entire ecosystem.

The new studies, which are the first to analyze the genes of fungi from hundreds of leafcutter colonies across the Americas, found instances where other ants grew the specialized "leafcutter-only" fungus, as well as instances where leafcutters grew more generic fungal crops.

"It's not the crop that makes them special," Mueller said. "We found that leafcutter ants and their fungi have co-evolved, and while that's not a surprise, the evidence suggests that this co-evolution occurred in a more complex way than previously believed."

Additional co-authors on the new Molecular Ecology study include Melissa Kardish, Heather Ishak, Sofia Bruschi and Alexis Carlson, all of UT, and April Wright of Southeastern Louisiana University. Solomon is an associate teaching professor in the Department of BioSciences at Rice.

Agencies that supported the research include the National Science Foundation, the Brazilian Ministry of Education's CAPES Foundation and the São Paulo Research Foundation.

Read an extended version of this release from Rice University, from which this was adapted.

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Wednesday, 26 September 2018

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