Ants learned to farm fungi during a mass extinction

We tend to think of agriculture as a human innovation. But insects beat us to it by millions of years. Various ant species 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 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, partly 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 after 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 closely related to each other, and each derives 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 species cluster. 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.