LSU AgCenter mycologist Vinson P. Doyle and LSU Department of Biological Sciences professor Brant C. Faircloth lent their combined expertise to a study led by Smithsonian Institution entomologist Ted Schultz, which demonstrates that ants began farming fungi after an asteroid struck Earth 66 million years ago, causing a global mass extinction.
In a paper published in the journal Science, scientists at the Smithsonian's National Museum of Natural History, LSU and other institutions analyzed genetic data from 475 species of fungi and 276 species of ants to craft detailed evolutionary trees. This allowed the researchers to pinpoint when ants began cultivating fungi millions of years ago, a behavior that some ant species still exhibit today.
The timing of the publication is particularly noteworthy because it falls on the 150th anniversary of the co-discovery of fungus farming by ants made by Thomas Belt and Fritz Mueller.
"Ants have been practicing agriculture and fungus farming for much longer than humans have existed," Schultz said. "We could probably learn something from the agricultural success of these ants over the past 66 million years."
The researchers believe decaying leaf litter likely became the food of many of the fungi that grew during this period, which brought them in close contact with ants. The ants in turn began to use the fungi for food and have continued to rely on and domesticate this food source since the extinction event.
"To really detect patterns and reconstruct how this association has evolved through time, you need lots of samples of ants and their fungal cultivars," Schultz said.
According to Faircloth, considerable amounts of DNA sequence data are needed to reconstruct the evolutionary history of both groups of organisms.
Collecting these types of data from fungal cultivars and ants is where Doyle and Faircloth, who began collaborating in 2015, came in. They developed the molecular methods used to capture genetic data from both the fungi and the ants analyzed in the manuscript.
Doyle said historical ideas about fungal farming by ants generally assumed there was a single origin of fungal cultivation, but what was hampering deeper insight into questions regarding how ants began farming was trying to capture sufficient DNA sequence data from the fungi that ants consumed. During the past 15 years, the cost of genome sequencing has plummeted, and the techniques for collecting many types of genomic data have significantly improved - enabling this and many other studies.
"If you have a mushroom, it's relatively straightforward to sequence its genome," Doyle said. "But when you have teeny-tiny fragments of a fungus that an ant is carrying inside of it, it's hard to get enough fungal material to generate sufficient genome sequence data to analyze. That's where the fungal bait sets we created come in. They allowed us to pull the DNA from miniscule bits of fungi, amplify it, sequence it and analyze it."
According to Doyle, these "capture-based" approaches to collecting sequence data from fungi allow researchers to study symbiotic relationships between organisms and fungi in a way that they couldn't before.
He gave the example of a doctoral student, Spenser Babb-Biernacki, whom he's co-advising with Jake Esselstyn, a mammalogist in the LSU Museum of Natural Science. Babb-Biernacki is studying a genus of fungus that occurs in the lungs of all mammals, Pneumocystis, commonly known as fungal pneumonia.
"It's hard to get DNA from the lungs of mammals when you have 100 million host cells and just a few fungal cells," he said. "We're using the same approach from this study to pull out genome-wide data, allowing us to start to understand the evolutionary history of the diverse group of organisms and address interactions between symbionts and their hosts.
Doyle's primary interest is in the influence of agriculture on fungi, saying that a pathogen doesn't typically originate in agricultural populations.
"It's out there in the wild, then because of agriculture, it jumps in and starts to change in the agricultural environments," he said. "This is similar, but it's studying the coevolution of the ants and fungus and looking at the impact of ant agriculture on fungal evolution and vice versa."
Doyle said there are parallels between what the researchers see happening in the fungi that the ants are cultivating and crops that humans have cultivated.
"The more examples you have of domestication across distantly related groups of organisms, the better you can start to develop a model for how domestication evolves and what happens to the genomes of organisms that become domesticated," he said. "This study provides an example from millions and millions of years before humans started domesticating plants, but it seems like the process is actually rather similar."
Research Report:The coevolution of fungus-ant agriculture
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