Scoop up some dirt, and you’ll probably wind up with some slime mold. Many species go by the common name of slime mold, but the ones scientists know best belong to the genus Dictyostelium. They are amoebae, and for the most part they live the life of a rugged individualist. Each slime mold prowls through the soil, searching for bacteria which it engulfs and digests. After gorging itself sufficiently, it divides in two, and the new pair go their separate, bacteria-devouring ways. But if the Dictyostelium in a stamp-size plot of soil should eat their surroundings clean, they send each other alarm signals. They then use the signals to steer toward their neighbors, and as many as a million amoebae converge in a swirling mound. The mound itself begins to act as if it were a single organism. It stretches out into a bullet-shaped slug the size of a sand grain, slithers up toward the surface of the soil, probes specks of dirt, and turns around when it hits a dead end. Its movements are slow – it needs a day to travel an inch – but the deliberateness of the movements eerily evokes an it rather than a they.
After several hours, the Dictyostelium slug goes through another change. The back end catches up with the tip, and the slug turns into a blob. About 20 percent of the cells move to the top of the blob and produce a slender stalk. In order to keep the stalk from flopping over, these cells must produce rigid bundles of cellulose. Unfortunately, this cellulose also tears apart the amoebae that make it. The remaining amoebae in the blob then take advantage of the suicide of their slugmates. They slide up to the top and form a globe. Each amoeba in the globe covers itself in a cellulose coat and becomes a dormant spore. In this form the colony will wait until something – a drop of rainwater, a passing worm, the foot of a bird – picks up the spores and takes them to a bacteria-rich place where they can emerge from their shells and start their lives over.
The individual amoebae forming the stalk make the ultimate sacrifice so that other Dictyostelium may live and perhaps reproduce. These stalk-formers are not marked for death when they are born. When the amoebae mix together and the slug takes shape, the individuals that wind up in the front end of the slug will be the ones that form the stalk. In other words, they get a losing ticket in the Dictyostelium lottery. Aside from their rotten luck, they are indistinguishable from the amoebae that will survive as spores.
It is remarkable that stalk-forming amoebae should remain loyal to their fellow amoebae. Why should they willingly join a group of other amoebae when their loyalty will end in its and their death? Why shouldn’t amoebae just stay away from the group and try to tough it out on their own? Of course, just joining a group is not a guarantee of loyalty. It’s not hard to imagine amoebae finding a way to avoid the lottery of death. Actually, we don’t even have to imagine them: scientists have discovered that some Dictyostelium will cheat their fellow amoebae, thanks to genes that ensure that they will form spores rather than stalks.
The puzzle of loyal amoebae is, at its foundation, a puzzle about evolution. In each generation, the members of a population will vary in all sorts of ways – in their size, in their shape, and in their behavior. Depending on the environment in which the population lives, some of these variations will give certain members an edge when it comes to surviving and reproducing. Genes that make successful variations possible will become more common, while the unsuccessful genes will become less common.
Imagine that a Dictyostelium divides in two, and one of its offspring undergoes a mutation that makes it cheat. It escapes the stalk lottery, and is guaranteed to become a spore. Over generations, its descendants would become more common because none of them have to die making a stalk. Its cheating gene would become more common in the population as a result. Other individuals might also mutate into cheaters on their own, and their offspring would thrive as well. Meanwhile, genes that promote cooperation would become less common. It might be possible for Dictyostelium to continue organizing slugs and stalks if only a small fraction of amoebae cheated. But in time natural selection could produce so many cheaters that a slug would fail to produce a stalk, dooming the spores to death. As plausible as this scenario may be, scientists don’t see it happening in the real world. Dictyostelium is thriving happily in forests around the world. Clearly betrayal has not evolved to catastrophic levels. Why not?
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This female praying mantis is finishing up the last tasty bits of the male that just mated with her. In the lead article in tomorrow’s science section of the New York Times, I talk to scientists who study females of some species that sometimes devour their mates. Sexual cannibalism is not common, but it is revealing. The evolutionary forces that shape the sexes can drive them into some extreme conflicts, even turning one sex into a meal for the other. In some cases, males actually become partners in their own demise–passively or complicitly. A new study indicates that male praying mantises are not so willing. They can tell when females are hungry, and they take extra precautions.
Like many parasites, a species of bacteria called Wolbachia takes charge of its own fate. Wolbachia can only survive inside the cells of its hosts–invertebrates such as this lovely common eggfly. This way of life limits Wolbachia’s opportunities for long-term survival. If Wolbachia lives inside a female insect, it can infect her eggs. When those eggs hatch and mature into adult insects, they will be infected by Wolbachia as well. But if Wolbachia should find itself in a male, it has reached a dead end. It cannot infect sperm cells, and thus it has no escape from a male host. When a male host dies, Wolbachia dies as well.
