How could the extreme degree of cooperation multicellular existence requires ever evolve? Why aren't all creatures unicellular individualists determined to pass on their own genes?
Joan Strassmann, PhD, and David Queller, PhD, a husband and wife team of evolutionary biologists at Washington University in St. Louis, provide an answer in the Dec. 16 issue of the journal Science. Experiments with amoebae that usually live as individuals but must also join with others to form multicellular bodies to complete their life cycles showed that cooperation depends on kinship.
If amoebae occur in well-mixed cosmopolitan groups, then cheaters will always be able to thrive by freeloading on their cooperative neighbors. But if groups derive from a single cell, cheaters will usually occur in all-cheater groups and will have no cooperators to exploit.
The only exceptions are brand new cheater mutants in all-cooperator groups, and these could pose a problem if the mutation rate is high enough and there are many cells in the group to mutate. In fact, the scientists calculated just how many times amoebae that arose from a single cell can safely divide before cooperation degenerates into a free-for-all.
The answer turns out to be 100 generations or more.
So population bottlenecks that kill off diversity and restart the population from a single cell are powerful stabilizers of cellular cooperation, the scientists conclude.
In other words our liver, blood and bone cells help our eggs and sperm pass on their genes because we passed through a single-cell bottleneck at the moment of conception.
The social amoebae
Queller, the Spencer T. Olin professor, and Strassmann, professor of biology, moved to WUSTL from Rice University this summer, bringing a truckload of frozen spores with them.
Although they worked for many years with wasps and stingless bees, Queller and Strassmann's current "lab rat" is the social amoeba Dictyostelium discoideum, known as Dicty for short.
The social amoebae can be found almost everywhere; in Antarctica, in deserts, in the canopies of tropical forests, and in Forest Park, the urban park that adjoins Washington University.
Experiments explain why almost all multicellular organisms begin life as a single cell
Enlarge
Some stages in the life cycles of a social amoeba. When bacteria are scarce, the amoebae send out a distress signal, rush together to form a loose aggregate (second row, right), then a tight aggregate (second row, middle) and then a finger (second row, left). The finger falls over and becomes a slug (front row, far left) that crawls toward heat and light. Once the slug finds a suitable spot, the back end spreads out, raising the front end in the air (the “Mexican hat” at far left). The front end elongates to form a stalk and the back end of the slug flows up the stalk, reorganizing itself at the top into a ball of spores (back row, right).
The amoebae spend most of their lives as tiny amorphous blobs of streaming protoplasm crawling through the soil looking for E. coli and other bacteria to eat.
Things become interesting when bacteria are scarce and the amoebae begin to starve. They then release chemicals that attract other amoebae, which follow this trail until they bump into one another.
A mound of some 10,000 amoebae forms and then elongates into a slug a few millimeters long that crawls forward (but never backward) toward heat and light.
The slug stops moving when it has reached a suitable place for dispersal, and then the front 20 percent of the amoebae die to produce a sturdy stalk that the remaining cells flow up and there become hardy spores.
Crucially, the 20 percent of the amoebae in the stalk sacrifice their genes so that the other 80 percent can pass theirs on.
When Strassmann and Queller began to work with Dicty in 1998, one of the first things they discovered was that the amoebae sometimes cheat.
http://www.physorg.com/news/2011-12-multicellular-life-cell.html
Joan Strassmann, PhD, and David Queller, PhD, a husband and wife team of evolutionary biologists at Washington University in St. Louis, provide an answer in the Dec. 16 issue of the journal Science. Experiments with amoebae that usually live as individuals but must also join with others to form multicellular bodies to complete their life cycles showed that cooperation depends on kinship.
If amoebae occur in well-mixed cosmopolitan groups, then cheaters will always be able to thrive by freeloading on their cooperative neighbors. But if groups derive from a single cell, cheaters will usually occur in all-cheater groups and will have no cooperators to exploit.
The only exceptions are brand new cheater mutants in all-cooperator groups, and these could pose a problem if the mutation rate is high enough and there are many cells in the group to mutate. In fact, the scientists calculated just how many times amoebae that arose from a single cell can safely divide before cooperation degenerates into a free-for-all.
The answer turns out to be 100 generations or more.
So population bottlenecks that kill off diversity and restart the population from a single cell are powerful stabilizers of cellular cooperation, the scientists conclude.
In other words our liver, blood and bone cells help our eggs and sperm pass on their genes because we passed through a single-cell bottleneck at the moment of conception.
The social amoebae
Queller, the Spencer T. Olin professor, and Strassmann, professor of biology, moved to WUSTL from Rice University this summer, bringing a truckload of frozen spores with them.
Although they worked for many years with wasps and stingless bees, Queller and Strassmann's current "lab rat" is the social amoeba Dictyostelium discoideum, known as Dicty for short.
The social amoebae can be found almost everywhere; in Antarctica, in deserts, in the canopies of tropical forests, and in Forest Park, the urban park that adjoins Washington University.
Experiments explain why almost all multicellular organisms begin life as a single cell
Enlarge
Some stages in the life cycles of a social amoeba. When bacteria are scarce, the amoebae send out a distress signal, rush together to form a loose aggregate (second row, right), then a tight aggregate (second row, middle) and then a finger (second row, left). The finger falls over and becomes a slug (front row, far left) that crawls toward heat and light. Once the slug finds a suitable spot, the back end spreads out, raising the front end in the air (the “Mexican hat” at far left). The front end elongates to form a stalk and the back end of the slug flows up the stalk, reorganizing itself at the top into a ball of spores (back row, right).
The amoebae spend most of their lives as tiny amorphous blobs of streaming protoplasm crawling through the soil looking for E. coli and other bacteria to eat.
Things become interesting when bacteria are scarce and the amoebae begin to starve. They then release chemicals that attract other amoebae, which follow this trail until they bump into one another.
A mound of some 10,000 amoebae forms and then elongates into a slug a few millimeters long that crawls forward (but never backward) toward heat and light.
The slug stops moving when it has reached a suitable place for dispersal, and then the front 20 percent of the amoebae die to produce a sturdy stalk that the remaining cells flow up and there become hardy spores.
Crucially, the 20 percent of the amoebae in the stalk sacrifice their genes so that the other 80 percent can pass theirs on.
When Strassmann and Queller began to work with Dicty in 1998, one of the first things they discovered was that the amoebae sometimes cheat.
http://www.physorg.com/news/2011-12-multicellular-life-cell.html
