Some slime molds transport and farm the bacteria they eat
By Megan Scudellari, Faculty of 1000, The Scientist, the-scientist.com
[Published 19th January 2011 06:00 PM GMT]
Humans, ants and numerous other species farm their food, but no microoganism has been shown to participate in agriculture, until now. For the first time, researchers have discovered that a species of social amoeba — a slime mold — carries, seeds, and harvests a crop of their bacterial diet, researchers report in this week’s issue of Nature.
“This is an eye-opener,” said Jacobus Boomsma, an evolutionary biologist at the University of Copenhagen who studies insect farming societies and was not involved in the research. “I would never have imagined that things as simple as slime molds could do a primitive version of farming.”
When soil-dwelling Dictyostelium discoideum amoebas run out of nearby bacteria to eat, the social organisms group together into slugs — conglomerations of 100,000 individuals — and inch along to a new location. There, they produce a fruiting body filled with spores that are released to take up residence in different, hopefully more productive environments.
While collecting D. discoideum fruiting bodies in the wild, Debra Brock of Rice University in Houston, Texas, noticed that some appeared to contain bacteria in addition to spores. Analyzing 35 wild D. discoideum colonies, Brock and colleagues discovered that one-third of the colonies tested did not eat all of the available bacteria in their location, but instead incorporated some of them into their fruiting bodies to seed a new crop of bacteria in a new location. They christened the bacteria-harboring amoebas “farmers.” The other two-thirds of the amoebas, though the same species, were “non-farmers” — they ate all the bacteria in their current location, never transporting any along with the spores of the fruiting bodies.
“This is a bet-hedging strategy,” said Brock. “There is a definite fitness advantage in a couple of scenarios,” she said. First, in habitats with no bacteria to eat, farming amoebas that bring their bacteria with them are significantly more successful — they proliferate abundantly, while non-farmers barely survive. The farmers also thrive when grown on soil containing a variety of bacteria, suggesting that the transported bacteria were preferable to the bacteria in the soil. “It was quite surprising,” said Brock. “Just like we don?t like some foods,” the amoebas didn’t like the local bacteria, she noted.
Yet in situations with plentiful preferred bacteria to eat, non-farmers always outperform the farmers, Brock said, likely because they eat all the bacteria in the location and have more nutrients to proliferate and produce more spores. There may be another drawback to farming, the authors pointed out: Carrying bacteria may expose farmer amoebas to harmful bacteria.
The trade-offs of farming may be why some of the clones farm while others don’t, noted Boomsma. The actual genetic or molecular differences that cause some colonies to farm while others don’t are still unknown, said Brock.
The finding suggests that farming may be a far more widespread endeavor than previously suspected. Over the last ten years, for example, researchers have also identified primitive farming societies in snails and damselfish, noted Boomsma in an accompanying News & Views article in Nature. And now there’s an example of a farming microorganism. “I wouldn’t exclude that there are more hiding out there,” he added.
Brock, D.A., et al., “Primitive agriculture in a social amoeba,” Nature, 496: 393-6, 2011. doi:10.1038/nature09668.