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Once food had been plentiful, but no longer. In the early days of the colony, the amoebas had feasted on a rich supply of bacteria. But as the generations passed and the population swelled, they had hunted out their food supply. Now starvation threatens. Their home-- a scrap of deer dung which once provided all their needs-- has become a trap which they must escape if they are to survive. At last, one amoeba sends out a cry for help.
The starving amoeba begins to emit a chemical signal in the form of cyclic adenosine monophosphate, or cAMP. Nearby individuals sprout new pseudopods and crawl toward the source. They also begin to give off cAMP themselves, amplifying the call until the signal spreads to the far reaches of the colony. Amoebas cannot concurrently detect and produce cAMP, so they alternate, and the cells trace out intricate spiral patterns as they surge forward in waves.
The amoebas pile on top of one another in growing numbers until so many of them have joined the heap that this pile of microscopic single-celled organisms becomes visible to the naked eye. At first their behavior might seem odd; to gather together in the face of starvation surely ought to end in cannibalism or death. Not so, for they are capable of an extraordinary and rare transformation. The amoebas set aside their lives as individuals and join ranks to form a new multicellular entity. Not all the amoebas will survive this cooperative venture, however. Some will sacrifice themselves to help the rest find a new life elsewhere.
These astonishing creatures are Dictyostelium discoideum, and they are a member of the slime mold family. They are also known as social amoebas. Aside from the novelty value of an organism that alternates between unicellular and multicellular existence, D. discoideum is highly useful in several areas of research. Among other things, this organism offers a stellar opportunity to study cell communication, cell differentiation, and the evolution of altruism.
In response to the cAMP distress call, up to one hundred thousand of the amoebas assemble. They first form a tower, which eventually topples over into an oblong blob about two millimeters long. The identical amoebas within this pseudoplasmodium-- or slug-- begin to differentiate and take on specialized roles.
The slug begins to seek out light, leaving a slimy trail behind. Some of the amoebas take on the difficult role of sentinel, or immune-like functions. They circulate through the slug, hunting for pathogens. If they find any, they will engulf them in a process similar to the feeding behavior they once displayed when in solitary form. The pseudoplasmodium periodically sloughs off the sentinels-- and any pathogens they have engulfed-- and abandons them in the trail of slime. More cells will then be tapped to fill their place.
Once the slug finds a suitably sunny location, the unlucky cells at the "head" of the slug form a stalk for the others to climb. These cells--which make up roughly a fifth of the total population--will sacrifice themselves in order to provide a path up for their comrades.
The remaining cells then climb the stalk and collect on its tip, eventually resulting in a structure resembling a ping-pong ball balanced on top of a floppy wire. This formation is known as a "fruiting body." They then form spores, which are carried away by wind or passing animals or insects. Once carried to a suitable location, the amoebas emerge from spore form and begin the cycle again.
So long as all the amoebas which make up the slug are related, this impressive display of self-sacrifice on the part of the stalk cells makes sense. Though they will perish in the act of creating the stalk, they will pass along their genetic legacy via their kin. In fact, when the amoebas reproduce by division, they create an ever-increasing pool of genetically identical clones. These clones suffer no genetic cost at all from sacrificing their lives for each other.
More familiar multicellular organisms pool resources in a similar way. For example, in a human being, a liver cell fills a very different role from a lung or skin cell, but all of them harbor the same chromosomes. The result is that the liver doesn't need to compete with the lungs concerning reproduction. So long as the germ cells get lucky, all of the cells can be (metaphorically) content knowing they will pass on their genetic legacy.
However, when the cAMP call goes out, it isn't only related amoebas that answer it. Those of differing strains will come together to form a single slug. If one strain could figure out a way to duck out of stalk and sentinel duty, it would be expected to reproduce faster than its nobler compatriots.
As is true with all organisms, some will evolve in such a way that they can-- and will-- benefit from the colony's resources without contributing anything back. In theory, such "leeches" could potentially have a survival and reproductive advantage, thereby undermining the cooperative Dictyostelid lifestyle. Such cheating does take place, but nonetheless D. discoideum has been around for millions of years with no signs of imminent extinction. Thus the mechanisms for keeping cheating under control must be effective.
For one thing, the amoebas prefer to unite with kin. The amoebas are able to recognize each other through molecular markers. They mingle with other strains only when populations are low. At such times, the ability to form a larger slug outweighs the risk of cooperating with strangers.
In addition, evidence suggests that some social amoebas have evolved to link reproductive genes with altruistic ones. In the case of D. discoideum, researchers created a mutant strain of cells which are "deaf" to the chemical signal to become a self-sacrificing stalk cell. They then watched to see if these cells would gain a reproductive advantage. Just the opposite took place. The "cheater" mutant cells did not join in stalk formation, yet they rarely made it up the stalk to become spores, and therefore they died out. The traits of self-sacrifice and reproduction had become genetically entangled, it seems, allowing only the altruistic amoebas to produce offspring.
Finally, opportunities for cheating simply aren't very common. In the wild, these creatures spend much of their lives reproducing via division, and surrounding themselves with identical copies. Outside of laboratory experiments, cases where social amoebas run across strangers to exploit are rare. Cheater genes peter out once the cheaters run out of nobler amoebas to sponge off of. When exploiting one's clone mates, greed doesn't pay.
In addition to studies of altruism, study of D. discoideum is shedding light on how cells communicate. D. discoideum uses many of the same signaling processes found in all multicellular creatures. But unlike fish or frogs, D. discoideum can be frozen, thawed, grown by the millions in a matter of days, and stored away for years if need be. A website called DictyBase offers an impressive list of breakthroughs which can be credited to the social amoeba.
The consistency with which these amoebas act in the common good might inspire admiration in many. Yet a more cynical observer might point out that the amoebas are moved not by love of family and friends, nor by moral scruples, but by the cold mechanics of natural selection. Amoebas behave altruistically only because natural selection has led to a stable state in which self-sacrifice is the best way for them to pass on their genes. But the end result is the same, regardless of the natural forces that have shaped it. Altruism triumphs, and through their mutual selflessness the amoebas arrive at a new patch of bacteria-laden dung to call home.