Smallest Farmers Found? Amoebas Carry, Plant "Seeds"

Social network allows single-celled life-forms to be migrant "farmers."

D. discoideum amoebas join to form fruiting bodies (pictured), which allow the amoebas to relocate.

Photograph courtesy Owen Gilbert

Mason Inman

for National Geographic News

Published January 19, 2011

In the developed world, the small farmer may be going extinct. But among some amoebas, small farming—really small farming—is still a viable survival strategy, a new study says.

Generally speaking, when Dictyostelium discoideum amoebas run short of bacteria to eat in a patch of soil—presumably because the bacteria themselves are starving—the single-celled life-forms "start 'talking' to each other, and they gather together," said lead study author Debra Brock.

"When there're about a hundred thousand amoebas gathered together, then they form a fruiting body."

The resulting stalk sticks up into the wind and releases spores carrying the amoebas—and, it turns out, a few bacterial "seeds" too.

Brock and her colleagues found that, rather than eating all the food before leaving the previous location, the amoebas had encased the last morsels in shells for traveling.

When the spores landed, the amoebas emerged and released the bacteria seeds, planting them in hopefully greener pastures, much as human farmers move from exhausted fields to fertile ones.

(Also see "Human Waste Used by 200 Million Farmers, Study Says.")

"They bring their preferred bacteria, and this allows them to prosper and flourish in the new area," said Brock, a biologist at Rice University in Houston, Texas. (See an award-winning amoeba picture.)

Passive Farming

The amoebas don't seem to tend the bacteria while they're growing, so it's a passive kind of farming, Brock noted. By contrast, some ants termites, and other social insects are known to cultivate fungi or "milk" other insects for sugary meals.

Nonetheless, nothing like the amoeba farming has been seen before in single-celled organisms. (See "All Species Evolved From Single Cell, Study Finds.")

Brock said her team has since spotted this kind of primitive migrant farming in several other amoeba species—and they all share a trait with other known farming species, including us.

"We think they're able to do this because they're social," she said. Solitary amoeba species, she explained, don't join to form the fruiting bodies that make travel possible for D. discoideum.

The new study appears today in the online edition of the journal Nature.