A good friend of Deskarati is writing a book describing the world as we know it and how it came to be. He has finished the first section from big bang to a time when Earth was first suitable to produce life. So as he begins to explain where we came from, Deskarati asks what might seem an obvious question – Why did almost all multicellular life begin as a single cell? – Deskarati –
Any multicellular animal, from a blue whale to a human being, poses a special difficulty for the theory of evolution. Most of the cells in its body will die without reproducing, and only a privileged few will pass their genes to the next generation. 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. 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. Dennis Welker of Utah State University had given them a genetically diverse collection of wild-caught clones (genetically identical amoebae). They mixed amoebae from two clones together and then examined the fruiting bodies to see where the clones ended up. Each fruiting body included cells from both clones, but some clones contributed disproportionately to the spore body. They had cheated. How can a blob of protoplasm cheat? The answer, it turns out, is many different ways.
“They might,” Queller says, “have a mutation that makes an adhesion molecule less sticky, for example, so that they slide to the back of the slug, the part that forms spores.”
“But there are tradeoffs,” Strassmann says, “because if you’re too slippery, you’ll fall off the slug and lose all the advantages of being part of group.”