A living relict of an ancient species of farmer ants has startled biologists by cultivating a fancy, modern food crop that didn’t arise until more than 30 million years after the ants themselves. The surprising discovery is providing a new look at how symbiotic species evolve.
“It’s like a lost tribe of Neandertals growing a GMO crop,” says Ted Schultz of the Smithsonian Institution in Washington, D.C.
Ant colonies that grow their own specialty fungus for food can die when coaxed into trying to farm unfamiliar strains, lab tests show. Yet Apterostigma megacephala — the oldest knownspecies of farmer ant with only the simplest agricultural techniques — has somehow switched to grow a kind of fungus that’s been around for just 2 million to 8 million years, Schultz and his colleagues report in the May American Naturalist. Scientists thought this highly specialized fungus could only survive in the huge, sophisticated farms of leaf-cutting ants.
More than 240 ant species, all native to the Western hemisphere, grow some kind of fungus for food. Biologists divide these ant species into five groups, ranging from the simplest family farmers to industrial-scale producers. Each ant ag cluster has its companion cluster of crop fungus species.
Young queens carry a bit of mom’s fungal strain in their mouths when they fly out to start their own farms. When an ant species switches its specialty food, it’s usually a small change to a fungus in the same cluster as the original fungus.
Not so with the extreme crop switch by the new study’s A. megacephala ants, which Schultz describes as the “oddball of all oddballs.” Until 2009, the species was known only from four museum specimens. Its unusual combination of physical traits meant its ancestors split off on their own evolutionary path some 39 million years ago. Schultz and colleagues searched for living colonies of the ants in Peru and Brazil for 10 years before researchers found some in a not-very-exotic location: along the sides of a service road in a regional zoo.
There, small colonies with roughly 20 workers tend a baseball-sized, spongy mass of fungus riddled with passageways where farmhands and nursemaids scurry to their tasks. In a simple form of farming, the ants collect fresh insect droppings, flower parts and other small tidbits to feed the fungus but otherwise don’t process this debris.
So Schultz’s team was stunned when DNA analysis of the fungus (Leucoagaricus gongylophorus) showed that it was actually a relatively young species and the one farmed by the most highly evolved of farming ants, the leaf-cutting species. These ants snip greenery to make special fodder for their crop, which is so dependent on the ants that it can’t live without them. Yet somehow this pampered, domesticated fungus species also grows in the rough circumstances of the old-style farmer nests.
“Understanding better what governs such switches might let us understand how pairings evolve and how they change over time,” says Michael Thomas-Poulsen of the University of Copenhagen, who has worked with both ant and termite farms but who wasn’t involved in the new paper.
Schultz has ideas about which factors permit (or prevent) such a big crop switch, such as gut microbes and other microscopic members of the farm community. That’s a hypothesis that evolutionary biologist Duur Aanen of Wageningen Universityin the Netherlands is already testing among termites that farm fungi. So far, gut microbes of termites and the fungi themselves seem to divide the biochemical labor of breaking down plant material, a sign that the microbial compatibility — not just farming technique — matters in farmer-crop partnerships.