Slime Molds and Evolution

Great Carl Zimmer piece in the Times about slime molds. These creatures are not related to the molds that ruin your bread. They live in forests and spend most of their lives as independent amoebae:

[slime molds] 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.

Some biologists think this is how multicellular life first appeared. Among other things they wonder why more cells don't try to become spores, and why they sacrifice themselves willingly:

most of the amoebas that form a slug are closely related to one another.

“They’re helping relatives,” Dr. 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.

Dr. 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.

Very cool stuff.