Generating chemical spots and stripes.

The distinctive patterns on the coats of such mammals as leopards, zebras and giraffes have inspired both folk tales and scientific investigations. Now laboratory experiments, supported by theoretical studies, have provided the first steps toward the possibility of linking a single pattern-forming mechanism - originally proposed 40 years ago - with biological patterns.

In 1952, mathematician Alan M. Turing suggested that biological forms mirror patterns in the concentrations of hypothetical chemicals called morphogens. He postulated that, under appropriate conditions, the reaction of these chemicals and the subsequent diffusion of their reaction products combine to create distinctive patterns from an initially uniform distribution of morphogens. He encapsulated this mechanism in a simple mathematical formula.

Experimental evidence that such a mechanism could govern a chemical system didn't emerge until 1990, when Patrick De Kepper and his co-workers at the University of Bordeaux in France produced a stationary pattern of spots in a thin gel continuously fed a fresh solution - containing malonic acid and chlorite and iodide ions-in a special chemical reactor (SN: 8/11/90, p.88).

"The crucial step, in my view, was the development of a reactor with which one could look for sustained patterns," says physicist Harry L. Swinney of the University of Texas at Austin, who led the effort to develop the apparatus both teams used for viewing Turing patterns. "People hadn't appreciated that if you wanted to look for [transitions from uniform states to stationary patterns], you had to maintain the system far from equilibrium."

Swinney and colleague Qi Ouyang then used this apparatus to demonstrate how adjusting the temperature or concentration of one or more of the reacting chemicals could abruptly produce a distinctive, stationary pattern of concentrations - made visible by a chemical indicator, which changes color in the presence of certain substances. In some cases, they could alternately raise and lower the temperature to create, then erase, the pattern.

In such experiments, described by Ouyang at last month's American Physical Society meeting in Washington, D.C., the researchers could start with a system showing no spatial concentration variations and, by adjusting the concentration of one component, produce distinctive patterns of spots or stripes.

Moreover, "if you keep going from the stationary patterns, you eventually get spatially chaotic patterns - states of chemical turbulence," Swinney says.

"These chemical systems are the first clear evidence that the Turing mechanism does actually occur in nature," says chemist Irving R. Epstein of Brandeis University in Waltham, Mass.

Now researchers are trying to find other combinations of chemicals that display Turing patterns. In the May 1 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, Epstein and colleague Istvan Lengyel propose a systematic approach for finding such examples.

They suggest that Turing patterns could arise in combinations of chemicals that under somewhat different conditions produce the swirling, spiral patterns or waves seen in oscillating chemical reactions. To get stationary rather than moving patterns would involve confining the reactions to a gel and ensuring that reaction-inhibiting molecules diffuse more rapidly through the gel than the initial reactants, or "activator" molecules. For example, chemically tying activator molecules to much larger, slow-moving molecules could produce the necessary effect.

"A key question for biologists and biochemists is whether they can find a biological system where they can identify the activator and the inhibitor [molecules] and really show that the Turing mechanism is active in the system," Epstein says.

Swinney notes: "To make a connection between the chemical patterns, which at this point are demonstrably Turing patterns, and actual biological patterns is an important leap that has yet to be made."