A slime-mould 'slug' formed of tens of thousands of amoebae may carry bacteria with it to seed pastures new. Credit: Carolina Biological/Visuals Unlimited/Corbis

A slime mould long thought to hunt bacteria as prey turns out to have unexpected abilities: according to researchers in Texas, some of the amoebae are actually farmers, husbanding their bacterial 'crops' much as some ant species farm fungus.

As social amoebae, slime moulds are bizarre creatures that live as individual singled-celled organisms while feeding, but congregate in groups of tens of thousands to form multicellular 'slugs' that migrate to new areas when food supplies dwindle. But now, a selection of the soil-dwelling species Dictyostelium discoideum have been shown to husband their bacterial food source. By prudently harvesting the bacteria and then migrating with them, the amoebae are able to seed a new food source at their destination.

The study, published today in Nature1, shows that this 'bacterial husbandry' is similar to the behaviour of other social animals, such as fungus-farming ants — although compared to the ants, which actively feed, nurture and defend their crops, the amoebae are relatively primitive farmers, with no active cultivation at the new site.

It was previously thought that slime moulds were strictly predators of bacteria, forming the multicellular slug when scant food supplies prompted a move to new hunting grounds. Debra Brock, a molecular biologist at Rice University in Houston, Texas, who led the study, attributes this gap in our knowledge to the fact that very few labs work on wild Dictyostelium. "Most labs use a clone that was discovered in the 1930s and developed to be a good lab organism. That particular clone just happened to be a non-farmer," she says.

Brock and her team collected a sample of 35 wild clones — genetically identical members within the species — and noticed that some of these seemed to have bacteria in their reproductive structures, alongside their spores. The team showed that the bacteria carried by the amoebae had the potential to initiate new populations in the lab, even after the amoebae had been stimulated to migrate en masse. The researchers dubbed the bacteria-carrying amoebae, which made up one-third of all clones in the wild population, 'farmers'.

Food on the move

When presented with a food source, farmer amoebae stop feeding earlier than their non-farming relatives, and save some food to take with them during their multicellular migration. If they then end up in an area that lacks sufficient edible bacteria, they can rely on their own supply to seed a new population for harvest.

Dictyostelium farmers seem sometimes to benefit from their husbandry skills even in natural soil that harbours a variety of bacteria, because they can bring along bacteria of their choice, just as humans plant seeds in areas with naturally growing vegetation.

But farming has its costs. When the researchers compared the success of the farmers and non-farmers in areas of abundant edible bacteria, they found that the former produced relatively fewer offspring. This is probably because farmers consistently relinquish nourishment so that they can save some to migrate with, which in turn reduces their ability to create offspring — a damaging sacrifice in times of plenty.

Another potential cost was that the farmer clones travelled shorter distances than non-farmers. But rather than being a problem, this could just be because carrying your own food source means that you don't have to travel as far to find lunch.

Michael Purugganan, a biologist at New York University, sees the husbandry practice of Dictyostelium as having intriguing parallels with human farming societies, owing to their tendency to become more settled. "Are they migrating less because now they can farm? This sounds like what happened to human societies when agriculture originated," he says.

However, "it would be interesting to know to what extent they allow the bacteria to proliferate at the new location. If the lag time was very short, it would be less like true farming and more like just bringing lunch along on the trip," says Purugganan. "Farmers don't eat their seed. They wait for the crop to grow."