First farmers


By Alison Motluk FUNGUS-growing ants are far more sophisticated and enterprising farmers than previously thought. After analysing fungi from hundreds of nests, researchers have concluded that ants continually introduce new varieties of fungus into their colonies. Ants became farmers about 50 million years before humans. They can’t digest the cellulose in leaves, but fungi can. So some ant species encourage a fungus to grow on the leaves they collect and feed on it instead. Over time, their farming system has become extremely complex—they grow fungi in their nests, secrete antibiotics to control other fungi and spread their waste as fertiliser to maximise their yields. Until now, scientists have assumed that one ancient type of fungus co-evolved closely with its ant hosts. But Ulrich Mueller of the University of Maryland at College Park and his colleagues have found that farmer ants are more creative than we have given them credit for. Mueller’s team genetically screened 862 types of fungi taken from nests, as well as related wild fungi. “What they cultivate is a surprisingly complex array of fungi,” he says. Their analysis shows that ants have repeatedly domesticated new species, some quite recently. DNA sequences from some ant crops, for instance, exactly matched the wild-type fungi, which strongly suggests recent introduction, says Mueller. Rather than a single domestication long ago, as entomologists previously assumed, the genetic evidence suggests at least six independent domestications (Science, vol 281, p 2034), but there may have been many more. The researchers also noticed that some fungi were found in geographically surprising patterns. They think that ants may swap crops with their neighbours. The DNA analyses suggest that cultivar sharing occurred at least seven times between ants of the same genus and four times between more distantly related ants. “Clearly in ants, the introduction of agriculture was unconscious,” says animal behaviourist Jared Diamond of the University of California School of Medicine in Los Angeles. “Five ants didn’t sit down and say: `We’re sick of being picked off by warblers, let’s farm mushrooms’.” However,
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