We tend to think of agriculture as a human innovation. But insects beat us to it by millions of years. Various species of ants cooperate with fungi, creating a home for them, providing them with nutrients, and harvesting them as food. This reaches the peak of sophistication in the leafcutter ants, which cut foliage and return it to feed their fungi, which in turn form specialized growths that are harvested for food. But a variety of other ant species cooperate with fungi—in some cases strains of fungus that are also found growing in their environment.
Genetic studies have shown that these symbiotic relationships are highly specific—a given ant species will often cooperate with just a single strain of fungus. A number of genes that appear to have evolved rapidly in response to strains of fungi take part in this cooperative relationship. But it has been less clear how the cooperation originally came about, in part because we don't have a good picture of what the undomesticated relatives of these fungi look like.
Now, a large international team of researchers has done a study that traces the relationships among a large collection of both fungi and ants, providing a clearer picture of how this form of agriculture evolved. And the history this study reveals suggests that the cooperation between ants and their crops began in the wake of the mass extinction that killed the dinosaurs, when little beyond fungi could thrive.
Tracing the farmers
One of the key features of this work is its exhaustiveness; it obtained DNA from 475 species of fungus and 276 species of ants. These include both the agricultural species and their close relatives who don't engage in this practice. In addition, the researchers studied over 2,000 genes from each of these species in order to estimate which species were most closely related to each other, and when these species split off from a common ancestor.
The use of that many genes is critical, since some of these genes will likely have evolved rapidly in response to the altered conditions created by the adoption of agriculture. These genes likely have more mutations than would be expected based on the time between the present and when the species split off, making the split appear older than it actually is. By surveying a large number of genes, the effect of any outliers like this is much less likely to distort the analysis.
The researchers break the analysis down according to the kind of farming practiced by each ant species. Some of them farm yeast, others farm a group of species called coral fungi, and others engage in a more sophisticated form of agriculture involving fungi that are more adapted to this lifestyle. Leafcutter ants fall into this latter category. And, with a single exception (a group of leafcutters that aren't especially related to any of the rest), all of these groups cluster tightly together. All of these are embedded within a large group that opportunistically cooperates with fungi but don't specialize in growing a single species.
Both yeast and coral fungus farmers are all closely related to each other, and each derive from a single ancestral species. The most sophisticated farming species also cluster together. Leafcutter species are interspersed with these (aside from that one exception).
On the fungus side, similar things were true. The yeast species that are farmed all cluster together. Same with the coral fungi, although there are two wild-living strains within that cluster of species. The strains that are most adapted to farming form their own cluster, though they're all closely related to the yeast strains, with only a single wild-living strain separating them. Finally, all the species grown by leaf cutters are in a single cluster within this group.
Timing is everything
Tracing the lineages of agricultural ants to their most recent common ancestor revealed that the ancestor probably lived through the end-Cretaceous mass extinction—the one that killed off the dinosaurs. The researchers argue that the two were almost certainly related. Current models suggest that there was so much dust in the atmosphere after the impact that set off the mass extinction that photosynthesis shut down for nearly two years , meaning minimal plant life. By contrast, the huge amount of dead material would allow fungi to flourish. So, it's not surprising that ants started to adapt to use what was available to them.
That explains the huge cluster of species that cooperate with fungi. But most of the species that engage in organized farming don't appear until roughly 35 million years after the mass extinction, at the end of the Eocene (that's about 33 million years before present). The researchers suggest that the climate changes that accompanied the transition to the Oligocene included a drying out of the tropical Americas, where the fungus-farming ants had evolved. This would cut down on the availability of fungi in the wild, potentially selecting for the ability of species that could propagate fungal species on their own.
This also corresponds to the origins of the yeast strains used by farming ants, as well as the most specialized agricultural fungal species. But it doesn't account for the origin of coral fungus farmers, which seems to have occurred roughly 10 million years later.
The work gives us a much clearer picture of the origin of agriculture in ants and some reasonable hypotheses regarding the selective pressures that might have led to its evolution. In the long term, however, the biggest advance here may be the resources generated during this study. Ultimately, we'd like to understand the genetic basis for the changes in behavior of the ants, as well as how the fungi have adapted to better provide for their farmers. And to do that, we'll need to compare the genomes of agricultural species with their free-living relatives. The DNA gathered for this study will ultimately be needed to pursue those questions.
Science, 2024. DOI: 10.1126/science.adn7179 About DOIs
John Timmer
John is Ars Technica's science editor. He has a Bachelor of Arts in Biochemistry from Columbia University, and a Ph.D. in Molecular and Cell Biology from the University of California, Berkeley. When physically separated from his keyboard, he tends to seek out a bicycle, or a scenic location for communing with his hiking boots.