The cooperative, semi-suicidal blob lurking beneath the cow pasture

Aggregating Dictyostelium discoideum; photo from a series available at Wikimedia Commons

Amoebae gone to pasture

Timeline, 2009: Lurking beneath the soil and cow patties of a southeast Texas cow pasture, the cellular slime mold cloned itself and cloned itself again until it was about 40 feet across, billions of cells dense. The thing is, this enormous creature was really neither a slime nor a mold, but a group of social microbes, amoebas that were all exactly identical to one another.

It’s a cell, it’s a colony, it’s…a multicellular organism!

No one is quite sure why some microbes are social. But the cellular slime mold, Dictyostelium discoideum, makes a great model for figuring out why. This organism, a eukaryote, can start out life as a single cell, one that hangs out waiting for a hapless bacterium to wander by so it can have dinner. But if bacteria are in short supply or the soil dries up, slime molds can do some marvelous things. They can start socializing with each other, forming colonies—apparently quite large colonies. But they also can shift from being single-celled organisms or single-celled colonial organisms to becoming a multicellular organism. In the really bad times, these cells can signal to one another to differentiate into two different tissues, officially becoming multicellular.

In addition, these brainless blobs also exhibit altruism, a kind known as suicidal altruism in which some cells in a colony commit cell suicide so that cells elsewhere in the colony can thrive. And as the discovery in the Texas cow pasture shows, the organisms can exist in enormous clonal colonies, a quivering gelatinous mass just under the soil, waiting for conditions to improve.

An affinity for dung

How did the researchers even find this slime mold? They had to get themselves into the “mind” of the slime mold and figure out where this species might be most prone to going clonal, a response to an uninviting environment. Given that slime molds are fond of deep, humid forests, the researchers opted to search for the clonal variety in a dry-ish, open pasture at the edge of such forests. Why would they even expect D. discoideum to be there? Because the amoebae also happen to have an affinity for dung, as well as soil.

After careful sampling from 18 local fields, which involved inserting straws into cow patties and surrounding soil, the research team tested their samples for the presence of amoebae. Once the dishes showed evidence of the slime mold, the investigators then sampled each group and sequenced the genome. That’s when they realized that 40 feet of one cow pasture was one solid mass of amoebae clones.

The winner clones it all?

These findings raise intriguing questions for microbial ecologists. One problem they’re tackling is what determines which clone gets to reproduce so wildly at the edges of acceptable amoeba territory. Is it just one clone that arrives and gets to work, or do several show up, have a competition, and the winner clones all? They tested this idea in the lab and found that none of the clones they tested seemed to have any particular competitive advantage, so the selection process for which amoeba gets to clone at the edges remains a mystery.

Another question that intrigues the ecologists is why these single-celled organisms cooperate with one another in the first place. Such clonal existence is not unknown in nature—aspen trees do it, and so do sea anemones. And researchers have identified as many as 100 genes that seem to be associated with cooperative behavior in the slime mold. One explanation for cooperating even to the extreme of individual suicide is the clonal nature of the colony: the more identical the genes, the more the species derives from sacrificing for other members of the species. They ensure that the same genes survive, just in a different individual.

Machiavellian amoebae: cheaters among the altruists

Not all amoebae are altruistic, and some apparently can be quite Machiavellian. Some individual amoeba carry cooperation genes that have undergone mutations, leading them to be the cheaters among the altruists and take advantage of the altruistic suicide to further their own survival. What remains unclear is why cooperation continues to be the advantageous route, while the mutant cheats keep getting winnowed out.

Amoebae gone to pasture

By Emily Willingham

Word count: 672

 

Lurking beneath the soil and cow patties of a southeast Texas cow pasture, the cellular slime mold cloned itself and cloned itself again until it was about 40 feet across, billions of cells dense. The thing is, this enormous creature was really neither a slime nor a mold, but a group of social microbes, amoebas that were all exactly identical to one another.

No one is quite sure why some microbes are social. But the cellular slime mold, Dictyostelium discoideum, makes a great model for figuring out why. This organism, a eukaryote, can start out life as a single cell, one that hangs out waiting for a hapless bacterium to wander by so it can have dinner. But if bacteria are in short supply or the soil dries up, slime molds can do some marvelous things. They can start socializing with each other, forming colonies—apparently quite large colonies. But they also can shift from being single-celled organisms or single-celled colonial organisms to becoming a multicellular organism. In the really bad times, these cells can signal to one another to differentiate into two different tissues, officially becoming multicellular.

In addition, these brainless blobs also exhibit altruism, a kind known as suicidal altruism in which some cells in a colony commit cell suicide so that cells elsewhere in the colony can thrive. And as the discovery in the Texas cow pasture shows, the organisms can exist in enormous clonal colonies, a quivering gelatinous mass just under the soil, waiting for conditions to improve.

How did the researchers even find this slime mold? They had to get themselves into the “mind” of the slime mold and figure out where this species might be most prone to going clonal, a response to an uninviting environment. Given that slime molds are fond of deep, humid forests, the researchers opted to search for the clonal variety in a dry-ish, open pasture at the edge of such forests. Why would they even expect D. discoideum to be there? Because the amoebae also happen to have an affinity for dung, as well as soil.

After careful sampling from 18 local fields, which involved inserting straws into cow patties and surrounding soil, the research team tested their samples for the presence of amoebae. Once the dishes showed evidence of the slime mold, the investigators then sampled each group and sequenced the genome. That’s when they realized that 40 feet of one cow pasture was one solid mass of amoebae clones.

These findings raise intriguing questions for microbial ecologists. One problem they’re tackling is what determines which clone gets to reproduce so wildly at the edges of acceptable amoeba territory. Is it just one clone that arrives and gets to work, or do several show up, have a competition, and the winner clones all? They tested this idea in the lab and found that none of the clones they tested seemed to have any particular competitive advantage, so the selection process for which amoeba gets to clone at the edges remains a mystery.

Another question that intrigues the ecologists is why these single-celled organisms cooperate with one another in the first place. Such clonal existence is not unknown in nature—aspen trees do it, and so do sea anemones. And researchers have identified as many as 100 genes that seem to be associated with cooperative behavior in the slime mold. One explanation for cooperating even to the extreme of individual suicide is the clonal nature of the colony: the more identical the genes, the more the species derives from sacrificing for other members of the species. They ensure that the same genes survive, just in a different individual.

Not all amoebae are altruistic, and some apparently can be quite Machiavellian. Some individual amoeba carry cooperation genes that have undergone mutations, leading them to be the cheaters among the altruists and take advantage of the altruistic suicide to further their own survival. What remains unclear is why cooperation continues to be the advantageous route, while the mutant cheats keep getting winnowed out.

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About ejwillingham
Sciwriter/editor/autism-ADHD parent. SciMaven @ http://doublexscience.blogspot.com/. I speak my pieces @ http://daisymayfattypants.blogspot.com/ & @ http://thebiologyfiles.blogspot.com/

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