Growing Dependency

Entomologist Ted Schultz holds a nest of an ant that grows but does not domesticate fungus, while standing next to a lab nest of a fungusgrowing, leaf-cutting ant that domesticates its crops, demonstrating the difference in agricultural scale between these two systems.

James DiLoreto, Smithsonian

Before the human transition from nomadic hunter-gatherers to societies predominantly based on agriculture roughly 10,000 years ago, ants had already been farming fungi for millions of years. Some fungal species farmed by ants are capable of living without the insects, while others have become dependent upon their ant hosts—in other words, domesticated. The evolution of ant agriculture, and in particular, when and how ants began to domesticate fungi, has been presumed to have occurred in South American rainforests, but this hypothesis has remained untested. 

To recreate the history of ant agricultural systems, entomologists Michael G. Branstetter at the University of Utah, Ted R. Schultz at the Smithsonian Institution, and five US colleagues used a next-generation genetic sequencing approach. Traditionally, morphological (body shape and size) data have been used to draw inferences about farmer ant evolutionary history, “but there are a limited number of morphological traits that can be used,” says Branstetter. On the other hand, sequencing whole genomes is prohibitively expensive. Instead, the group applied a new method to sub-sample ant genomes in 119 taxa representing a range of ant species that do or do not farm fungi.

The team focused on about 1500 ultra-conserved elements—regions of DNA that are identical across multiple species—enabling them to generate an order of magnitude more data than had been used before to create a farmer ant phylogeny, or evolutionary family tree. They discovered that “certain key events in fungus farming ant evolution correspond in an almost uncanny way with certain key events in global climate and habitat change,” says Schultz.

First, the transition to fungal farming coincided with the asteroid strike that spurred the Cretaceous-Paleogene mass extinction. This global cooling event may have been advantageous for fungi, and may have promoted ant lineage diversification.

Second, their phylogenetic tree concurred with the historical “out of the rainforest” hypothesis for the origins of ant agriculture, but—in contrast to this hypothesis—the transition to domesticating fungi occurred in dry environments. In dry habitats, fungi would have been completely dependent upon farmer ants to maintain the dark, moist, underground environment they needed to survive. That isolation presumably spurred an evolutionary divergence during which these fungi became genetically distinct from free-living fungi, and dependent upon their hosts. Schultz plans to explore this idea further by creating a phylogeny for the fungi. (Proceedings of the Royal Society B)