Can evolution explanations be found in slime?
Most of the aliens that come out of Hollywood don’t really look alien at all. They may have pizza-size eyes or roachlike antennae, but their oddities are draped on a familiar humanoid frame.
If you want to find life forms that truly seem otherworldly, your local forest is a much better place than your local cineplex. It is home to creatures that are immensely old, fundamentally bizarre and capable of startlingly sophisticated behavior. They are the slime molds.
Slime molds are a remarkable lineage of amoebas that live in soil. While they spend part of their life as ordinary single-celled creatures, they sometimes grow into truly alien forms. Some species gather by the thousands to form multicellular bodies that can crawl. Others develop into gigantic, pulsating networks of protoplasm.
While naturalists have known of slime molds for centuries, only now are scientists really starting to understand them. Lab experiments are revealing the complex choreography of signals in some species that allows 20,000 individuals to form a single sluglike body.
The pulsating networks that some slime molds form are giving other scientists clues to solving difficult mathematical problems. In 2000, Japanese researchers placed Physarum polycephalum — the name means “many-headed slime mold” — in a maze, along with two blocks of food. It extended its tendrils down the corridors of the maze, bending around curves, reaching dead ends and then backing out of them. After four hours, the slime mold was feasting on both blocks of food.
Andrew Adamatzky, a researcher at the University of West England, has been watching slime molds since 2006, finding inspirations in their growth for designing computer software. One of his favorite hobbies is challenging slime molds to build highway systems. In 2010 he and his colleagues placed a slime mold in the middle of a map of Spain and Portugal, with pieces of food on the largest cities. The slime mold grew a network of tentacles that was nearly identical to the actual highway system on the Iberian Peninsula.
“If some countries started to build highways from scratch, I would recommend to them to follow the slime mold routes,” Adamatzky said.
Despite their name, slime molds are not related to bread mold or the black mold that grows in damp houses. They belong to a separate lineage that evolved from ordinary soil amoebas.
By analyzing the DNA of different slime mold species, researchers are reconstructing their evolutionary history, which turns out to reach back about a billion years. Since all known slime molds live on land, that suggests that they were early pioneers, arriving hundreds of millions of years before animals or plants.
“They may be as old as the terrestrial ecosystem,” said Sandra Baldauf, an evolutionary biologist at Uppsala University in Sweden.
Slime molds first came to scientific fame in the mid-20th century with the work of the Princeton biologist John Tyler Bonner. Bonner learned of a North American species of slug-forming slime mold called Dictyostelium discoides and began to raise them in his lab, studying them as a simple analog of animal embryos.
Today, biologists no longer think of Dictyostelium as an embryo: It is more like a society of amoebas that come together for a common cause, for which some will sacrifice themselves.
The organisms respond to starvation by rushing together by the thousands into a single blob. The blob stretches out into a slug-shaped mass about one millimeter long (one twenty-fifth of an inch), which then crawls like a worm toward light.
Once it reaches the surface of the soil, the slug undergoes another transformation: Most of the cells turn into a stiff stalk, while the others crawl to the top and form a sticky ball of spores. They stick to the foot of an animal and travel to a hospitable place.
Inside the slug, about 1 percent of the amoebas turn into police. They crawl through the slug in search of infectious bacteria. When the amoebas find a pathogen, they devour it. These sentinels then drop away from the slug, taking the pathogen with it. They then die of the infection, while the slug remains healthy.
When the slug is ready to make a stalk, more amoebas must die so that others can live. They climb on top of one another and transform their insides into bundles of cellulose. Eighty percent of Dictyostelium cells die this way, allowing the survivors to climb up their lifeless bodies and become spores.
David Queller and Joan Strassmann, a husband-and-wife team of Dictyostelium experts at Washington University in St. Louis, have found that some strains of the slime mold are natural-born cheats. If they are mixed with other strains, they are more likely to end up as spores than as dead stalk cells.
“Clearly this is not just a weird thing,” Queller said. “Those mutations happen all the time.”
Research by Queller and Strassmann has revealed some reasons the slime-mold world has not been overwhelmed by these cheats. For one thing, most of the amoebas that form a slug are closely related to one another.
“They’re helping relatives,” Strassmann said. Even if the slime molds die to form a stalk, many of their genes are passed on to the next generation through their kin.
To help relatives, Dictyostelium needs a way to recognize them. Researchers at Baylor College of Medicine in Houston recently figured out part of the way the slime molds tell kin from strangers. The amoebas make a pair of proteins on the surface of their cells, which fit snugly together — like “patches of Velcro,” as one researcher, Gad Shaulsky, put it.
Shaulsky and his colleagues reported in July that if these proteins cannot link to each other, amoebas cannot fuse. “They completely ignore each other,” said Adam Kuspa, another Baylor biologist.
Dictyostelium belongs to one of the two great branches of slime molds. Its branch is known as the cellular slime molds, because its spore and stalk are made out of many cells.
By contrast, the so-called acellular slime molds do not form slugs. Instead, two cells merge, combining their DNA into a new single-celled organism that just keeps growing — extending tentacles that can extend as far as several yards. It pulsates to pump food from its extremities to its core, and it can even crawl to search for food.