January 20, 2011

Amoebae Pack A Lunch Before They Travel

A species of amoeba -- Dictyostelium discoideum -- has shown primitive farming behavior as it travels along, according to a new study.

In results of the study, reported Jan. 19 in the journal Nature, evolutionary biologists Joan Strassmann and David Queller of Rice University show that the social amoebae (commonly known as slime molds) increase their odds of survival through a rudimentary form of agriculture.

Research by lead author Debra Brock, a graduate at Rice, found that some amoebae sequester their food -- particular types of bacteria -- for later use.

"We now know that primitively social slime molds have genetic variation in their ability to farm beneficial bacteria as a food source," said George Gilchrist, program director in the National Science Foundation's Division of Environmental Biology, which funded the research. "But the catch is that with the benefits of a portable food source, comes the cost of harboring harmful bacteria."

After these "farmer" amoebae aggregate into a slug, they migrate in search of nourishment--and form a "fruiting body", or a stalk of dead amoebae topped by a sorus, a structure containing fertile spores. They then release the bacteria-containing spores to the environment as feedback for continued growth.

Food management has been seen in animals including ants and snails, but never in organisms as simple as these.

The behavior falls short of the kind of "farming" that more advanced animals do. Ants, for example, nurture a single fungus species that no longer exists in the wild.

But the notion that an amoeba that spends much of its life a single-celled organism could hold short of consuming a food supply before decamping is rather astonishing.

More than just a snack for the journey of dispersal, the idea is that the bacteria that travel with the spores can "seed" a new bacterial colony, and thus a food source in case the new locale lacks in bacteria.

D. discoideum is already a rather amazing creature, having proven its "social" nature as it gathers together into a mobile, multicellular structure in which 20 percent of the individuals die, to the benefit of the ones that make it into the fruiting body.

Researchers, looking to study the amoebae further, happened across the never-before-seen, truly unique behavior -- discovered perhaps because the samples came from the wild, rather than grown in the laboratory.

"It was a bit of serendipity, really," said Brock. "I had previously worked with them, looking at developmental genes. Not many people work with wild clones but I had started in a new lab and my advisers had a large collection of them, and I came with a bit of a different perspective," she told BBC News.

Once Brock spotted the amoebas' fruiting bodies carrying bacteria, she measured how many of the spores were responsible, finding that about a third of them traveled with their bacterial seeds.

The behavior seemed to be genetically built-in. Clones of the "farmer" amoebas in turn developed into farmers, while clones of the "non-farmers" did not.

"To think of a single-celled amoeba performing something that you could consider farming, I think, is surprising," Brock told BBC's Jason Palmer. "Choices like that are generally costly, so there has to be a pretty large benefit for it to persist in nature."

That is to say, the amoebae, in choosing not to consume all of the bacteria around them, are forced to make smaller fruiting bodies that cannot travel as far when they disperse.

There is thus an evolutionary balance to be struck between the advantage gained by showing up with the beginnings of a crop and the cost of bringing it.

The finding was a surprising once, Jacobus Boomsma of the University of Copenhagen told Plamer. It gives insight that has been absent in farming creatures already known to science.

For example, all the individuals of a given ant or termite species farm particular species of fungus exclusively, and the "free" versions of the advanced farmed fungi no longer exist.

"Here, farming and non-farming [members of the species] coexist, so they look perfectly normal until you put them under the microscope and know what you're looking at; [the bacteria] don't assume specialized roles as crops like fungi that ants and termites rear," Boomsma told BBC News.

"In other farming systems that we see, they always lack this intermediate stage," he said.

Brock said that further study has already found other species of amoeba that "pack a lunch," but that D. discoideum carries more than just a snack.

"Bacteria generally provide huge resources that are really untapped," she said. "These amoebas carry bacteria that aren't just used for food, so that's what I'm looking into now."

The findings could have implications for treating disease. It could, for instance, provide clues to the way tuberculosis bacteria invade cells, infecting the host while resisting attempts to break them down, said Strassmann.

The results demonstrate the importance of working in natural environments with wild organisms whose complex ties to their living environment have not been broken.


Image Caption: Amoebae fruiting bodies, with spores and bacteria. Credit: Owen Gilbert


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