Of ties that bind

EVOLUTIONARY BIOLOGY

If you want to find life forms that truly seem otherworldly, step into your local forest, home to creatures that are immensely old, fundamentally bizarre and capable of startlingly sophisticated behaviour. They are slime molds, a remarkable lineage of amoebae 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 starting to understand them.

Experiments are revealing the complex choreography of signals in some species that allows 20,000 individuals to form a single sluglike body. The networks that some slime molds form are giving other scientists clues to solving difficult mathematical problems. In 2000, Japanese researchers placed Physarum polycephalum, (meaning “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.

Useful in software design
Andrew Adamatzky, a researcher at the University of West England, has been trying to find inspirations in slime mold growth since 2006, for designing computer software. One of his favourites 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 nearly identical to the actual highway system on the Iberian Peninsula. Despite their name, slime molds are not related to bread mold or the black mold in damp houses. They belong to a separate lineage that evolved from ordinary soil amoebae.

“They may be as old as the terrestrial ecosystem,” said Sandra Baldauf, an evolutionary biologist at Uppsala University in Sweden. It was Princeton biologist John Tyler Bonner who first 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. Biologists no longer think of Dictyostelium as an embryo: It is more like a society of amoebae that come together for a common cause, for which some will sacrifice themselves.

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 one per cent of the amoebae turn into police, crawling through the slug in search of infectious bacteria. When the amoebae 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 amoebae must die so that others can live. They transform their insides into bundles of cellulose.

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 likely to end up as spores than as dead stalk cells. Queller and Strassmann’s research has revealed reasons the slime-mold world has not been overwhelmed by these cheats. For one, most slug-forming amoebae are related to one another.

Even if the slime molds die to form a stalk, their genes are passed on to the next generation through their kin. Researchers at Houston’s Baylor College of Medicine figured out part of how the slime molds tell kin from strangers. The amoebae make a pair of proteins on the surface of their cells, which fit together, like “patches of Velcro,” as researcher Gad Shaulsky put it. Shaulsky and colleagues reported that if these proteins cannot link to each other, amoebae cannot fuse.

Cellular and acellular varieties
Dictyostelium belongs to the branch known as the cellular slime molds, because its spore and stalk are made out of many cells. By contrast, acellular slime molds do not form slugs.
Instead, two cells merge, combining their DNA into a new single-celled organism that extends tentacles stretching into several yards. It pulsates to pump food from its extremities to its core, and can even crawl to search for food. Acellular slime molds also make spores, producing tens of thousands of stalks.

Researcher Adamatzky and his colleagues grew a slime mold network of highways for Canada, then placed a crystal of sea salt – which repels slime molds – on the map where the Bruce nuclear power plant is located, on Lake Huron in Ontario. The slime mold abandoned its tendrils near the salt and grew a new highway pattern that rerouted food across Canada.

At the University of New South Wales in Australia, Madeleine Beekman and her colleagues documented the decision-making of slime molds by presenting Physarum with two different kinds of food: either rich in protein, or rich in carbohydrates.

The slime molds grew tendrils to both foods, but adjusted their sizes to get the best balance of protein and carbohydrates that allowed them to grow fastest. In another experiment, Beekman and her colleagues made the choice harder by putting food under bright lights, which Physarum tries to avoid. In the first trial, the scientists offered the slime mold food chunks containing three per cent oat flakes in the dark, and five per cent oat flakes in bright light.

The mold was just as likely to ooze toward either kind of food. But when the scientists added a one per cent chunk to the dark area, that was enough to tip the balance:
Even though there was still not as much oat in the dark, 80 per cent of the mold now oozed in that direction. This might seem an irrational switch, but it is one that humans make as well.

Global project
*Scientists know very little about many other slime molds on earth. In 2003, Stephenson and other experts embarked on The Global Eumycetozoan Project, based at the University of Arkansas, which has doubled the known species of slime molds.

*Biologists have found slime molds in Antarctica, in barren deserts, in the canopies of jungles and even on the leaves of household plants. There may be thousands of individual slime molds in a pinch of soil.

*Evolutionary biologist Baldauf says, “I think it’s the tip of the iceberg.” “They go to some incredible place like a mountain in Patagonia, and they take a tiny soil sample and bring it back. But who knows what’s a foot away?”