Jonathan and his team from the University of Copenhagen recently returned from tropical rainforests near the Panama Canal, where they worked in a research station at the Smithsonian Tropical Research Institute.  These tropical forests are important habitats of leafcutter ants, which range from South and Central America to the southern United States. 

The ecologically advanced ‘farming system’ of these leaf-cutting and fungus-growing ants is fascinating to Shik.  “In the wild, these ants navigate rainforest environments with hundreds of plant species to forage a specific blend of nutrients in fresh leaves, flowers, and fruits. Yet, leafcutter ants do not directly eat this vegetation. Instead, they convert these plant fragments into a nutritional mulch used to provision their fungal cultivar, which in turn provides the ant farmers with edible nutritional rewards, swollen hyphal tips called gongylidia. But how do they know what nutritional blends are best for their crops? In our research, we often take this super organism apart to understand which nutrients are best for the fungal crop and how ant farmers detect their cultivar’s nutritional needs.”

Complex farming societies

Leafcutter ants’ colonies are the ultimate superorganisms. They begin when a virgin queen ant departs her natal colony to engage in a nuptial flight and collect sperm from several males. After mating, the males die and the queen falls to the ground where she loses her wings and searches for a suitable underground lair in which to found her colony. “She is essentially a living sperm bank, keeping sperm alive inside of tiny sacs in her abdomen for decades and gradually using them to produce over 100 million offspring in her lifetime”, Shik explains. “Amazingly, a foundress queen also carries a tiny bit of fungus in her mouth from her natal nest. In this way, leafcutter ants vertically transmit their specialized fungal crop across generations—a process which has led to true crop domestication.”

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As the colony grows, the ants actively cultivate their fungus, feeding it with freshly cut plant fragments and protecting it from pests and parasites. Each leafcutter forager carries plant fragments many times its weight; and a single colony can forage across a hundred tree species to collect over 300 kg of leaves a year. Shik and his team identified the foraged plant species using DNA barcoding and then measured the nutritional content of the fragments to test whether colonies can attain nutritional blends that optimize the growth of their fungal cultivar. “We discovered that while the fungal cultivar has surprisingly narrow needs for certain nutrients such as protein, a broad tolerance for variation in other nutrients, for instance carbohydrates, appears to enable colonies to forage across nutritionally diverse plant species.” 

Crop domestication

The leafcutter ants are the only fungus-growing ant genus to have achieved large-scale farming, but ancestral forms of small-scale farming ant species have also remained very successful over evolutionary time. This might be because they evolved in an empty niche using detritus like insect feces as fungal compost, while later emerging leafcutter ants became herbivores. Shik’s group recently published about these evolutionary tradeoffs with crop domestication across this diverse farming lineage. “We found evidence that ant farmers have traded greater crop productivity for higher crop vulnerability outside of specialized cultivation conditions, in a manner analogous to some of the plant crops domesticated by human farmers over the past 10,000 years,” he explains. Shik and his team are currently using genomic sequencing and micro-scale bioimaging to understand how crop domestication has modified the tiny nutritional rewards produced by the fungal cultivars of farming ants. 
 “This is basic research: it won’t solve the problem of climate change”, Shik continues. Yet his research might uncover some lessons about the path to natural resilience and sustainability in domesticated crops. “Humans have rapidly scaled up their farming systems over thousands of years through artificial selection of cultivars and cultural transmission of optimized farming practices. In contrast, ants achieved domesticated agriculture by means of natural selection and have proven resilient across 60 million years of climate fluctuation. Their ecological success across diverse habitats from grasslands to rainforests suggests demonstrates that something about their farming ‘technology’ makes them really robust.”

BIO:
Jonathan Shik is an Associate Professor in the Department of Biology at the University of Copenhagen, and a Research Associate at the Smithsonian Tropical Research Institute in Panama. He came to Denmark from the United States in 2014 on a Marie Curie Incoming Postdoctoral Fellowship. In 2017, he received an ERC Starting Grant. Jonathan leads a research group in integrative evolutionary biology exploring how and why nutritional niches diversify.