Crops farmed by leafcutter ants show signs of domestication

Earth 11 September 2015

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Farming fungus in underground chambers (Image: Alex Wild)

Leafcutter ants in the rainforests of South America beat us to the invention of farming by some 50 million years. Now it seems that their fungus crop has undergone the same genetic changes as human crops.

As people selectively bred new crop plants, they often inadvertently made changes to their genomes. Wheat, bananas, tobacco and strawberries are all polyploid – meaning they have three or more copies of each chromosome rather than the usual two.


Now, a team at Copenhagen University have discovered that leafcutter ant crops are the same. Pepijn Kooij and colleagues compared the fungus farmed by leafcutter ants with fungus kept by their less-specialised relatives. The latter ants’ crop consistently had two copies of each chromosome, whereas leafcutter fungus was polyploid, with between five and seven different copies.

Fast-track farming

“Polyploidisation is the fastest way to make a domesticated crop,” says Rachel Meyer from New York University. It makes it larger and more robust because it increases the number of copies of each gene, producing more gene products like growth hormones and immune proteins.

Early humans would likely have favoured polyploid crops for their increased productivity, and the same may be true for ants.

“About 50 million years ago, fungus-growing ants gave up their lives as hunter-gatherers to become fungal farmers,” says Kooij. He thinks the leafcutters took it further by selecting the more productive, polyploid fungi and encouraging their growth.

This final step may have helped leafcutters to increase their colony size by several orders of magnitude, just as crop domestication helped human populations expand and grow. Primitive fungus-farmers have colonies of up to 2000 workers, while the leafcutters can number into the millions. These remarkably successful insects dominate the rainforest, with a single colony taking as much as 500 kilograms of plant material each year to feed their fungal crops.

“The results of our study provide yet another piece of the puzzle to explain how these ants have been so extremely successful,” says Kooij.

Tastier bananas

Polyploid species are often unable to reproduce sexually. This speeds domestication for both humans and ant farmers as it prevents a crop species outbreeding with wild relatives. A loss of sexual reproduction also means no seeds, which allowed us to make fruits such as bananas more palatable.

“Humans have made edible bananas, bigger sugarcane and strawberries,” says Meyer. “And we’re currently making new polyploids for bigger kiwi fruit and seedless watermelon.”

Polyploidy also has other benefits – the extra genes provide raw material for natural selection. The team suggest this has driven leafcutter fungus to evolve new genes to digest fresh plant material, and to produce specialised edible growths called staphylae that made it easier for the ants to harvest their crop.

Journal reference: Journal of Evolutionary Biology, DOI: 10.1111/jeb.12718

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