Cellular Cooperation

The higher plants and animals, including man, are populous "nations" of closely cooperating cells. But back when life was new, the earth was inhabited by one-celled creatures only. Their achievement of cooperation was the greatest forward step in evolution.

How did the lonely cells first league together, subordinating their individual interests to the common good? Such questions fascinate biologists, for the mysterious force which makes cells cooperate is the basic law of multicellular life.

At the Marine Biological Laboratory at Woods Hole, Mass, (where the lecture-hall pointer is a fishing rod), a young Harvard biologist, Dr. John T. Bonner, is getting some of the answers. He works with a curious "slime mold," Dictyostelium discoideum, of the order Acrasiales, whose cells live alone and like it, but can also organize into a multicelled creature.

Ancient Way. When living alone, the cells are amoebae, and impossible to distinguish from other amoebae which have never learned to cooperate. They crawl slowly at random, grazing on bacteria. When one grows large, it divides in two by the cheerless, asexual mode of multiplication for which amoebae are famous. This is the ancient way of life, before cooperation.

But Dictyostelium discoideum is no ordinary amoeba; its cells have hidden capabilities. Thousands of them will be grazing peacefully, paying no attention to one another. Then a few will drift together, forming a little clump. All the amoebae for microns* around stop their feeding and dividing. Like city people running to the scene of an accident, they swarm toward the growing center (see cut). Some join end to end and stream in gay little chains. By thousands and tens of thousands they pile up in a heap.

The amoebae are still intact and outwardly unchanged, but something bigger than themselves has taken charge of their lives. When all the volunteers have arrived, the cell mass pokes up in a blunt spire, then falls on its side and forms a sausage-like "slug." As soon as the slug is formed, it acts like a multicelled animal, crawls with comparative rapidity and good coordination. It even has senses of a sort, for it is attracted by light.

Greener Pastures. At the end of the "migrating phase," the slug contracts to a blob, and rises into the air on a long, slender stalk. After this "culmination," the mass breaks up, and about 60% of the cells resume their solitary lives; the rest die. The entire cycle takes about four days. Dr. Bonner believes that the process has some "survival value," perhaps allowing the cells to dodge inimical conditions, or helping them migrate to greener bacterial pastures.

Many other biologists have studied Dictyostelium discoideum and related Acrasiales.* One slime-mold expert, Dr. K. B. Raper, of the Department of Agriculture, discovered (among other things) that the ultimate fate of the individual amoeba depends on how quickly it joins the aggregation. Latecomers form parts of the disc which supports the stalk; they die at the final breakup. The early birds form parts of the stalk itself; they die too. Only the middle-of-the-roaders, who arrive neither late nor early, live to continue the race.

Union Now. Dr. Bonner's most recent contribution to the understanding of slime molds comes close to their innermost secret: what force makes the amoebae abandon their individuality and join in temporary union? In a series of infinitely delicate experiments, he proved that the rallying signal sent out by the central mass cannot be electricity, magnetism or any form of radiation. Neither is there any structural connection between the separate amoebae. The only remaining possibility is a chemical substance secreted by the central mass.

To prove this, Dr. Bonner made a slow current of water flow past a central mass which was actively attracting amoebae. At once, cells upstream stopped moving toward the rallying point; the chemical recruiting officer was no longer reaching them. Those downstream continued to join up.

Dr. Bonner has not yet isolated this powerfully attractive chemical (which he calls "acrasin"), or found out much about it. When he does, he may have the secret of how cells learned to cooperate, preparing the way for multicellular man.

* One micron equals .000039 in.

* Dr. Bonner is pleased to find that in Edmund Spenser's Faerie Queene there is an appropriate witch, Acrasia:

. . . the faire Witch her selfe now solacing With a new Lover, whom, through sorceree And witchcraft, she from farre did thither bring. . .

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