Angelfish stripes: a possible explanation.

That stripes appear so frequently on the skins of animals bears testimony to their importance. Zebras, skunks, and raccoons, for example, evolved various kinds of stripes, highlighting each species' distinctive environmental needs.

Those on the angelfish Pomacanthus semicirculatus offer a particularly striking example of patterning because they maintain a constant width and spacing as the fish grows. To achieve this, the angelfish must introduce additional stripes onto its body rather than stretch out the three it started out with as a juvenile.

Shigeru Kondo and Rihito Asai, molecular biologists at Kyoto University in Japan, now offer a possible explanation for the mechanism underlying this peculiar coloring system: a chemical wave of interacting pigments.

"The stripes of Pomacanthus maintain the spaces between the lines by the continuous rearrangement of the patterns," Kondo and Asai note in the Aug. 31 Nature. The scientists simulated this continuous rearrangement with an algorithm--a mathematical recipe--for a "reaction-diffusion wave." This chemical system starts out with one homogeneous color, then reacts with itself to produce a pattern of alternating colors. First proposed in 1952 by the British mathematician Alan Turing, this model can account for periodic patterns in nonliving systems (SN: 5/9/92, p. 311).

Kondo and Asai believe that this model also explains how an angelfish's striped patterns transform as it grows. Although experimental evidence has never linked this system to an organism, the researchers contend that, when it comes to an angelfish, "a simulation program based on a Turing system can correctly predict future stripe patterns."

In fact, they say, "the striking similarity between the actual and simulated pattern rearrangement strongly suggests that a reaction-diffusion wave is a viable mechanism for the stripe pattern."

The algorithm simulates the behavior of two molecules: an "activator" and an "inhibitor," each of which affects the other's rate of synthesis as the fish grows. As the mock molecules interact, moving through 50,000 runs of the computer program, a pattern emerges that resembles what's observed on an angelfish after 90 days of growth.

"Various models have been put forward to explain stripe patterning," says Hans Meinhardt, a molecular biologist at Germany's Max Planck Institute for Evolutionary Biology in Tubingen. But these have been "undermined" by the finding that the pigment reactions directed by the animals' genes follow different rules from those that the computer obeyed, he notes in a commentary accompanying the Nature paper.

Nevertheless, he credits Kondo and Asai with demonstrating that a relatively simple chemical mechanism might in fact govern the behavior of the pigments that give rise to the tropical fish's stripes. Indeed, that the stripes of both the angelfish and the new computer simulation branch out in a similar way should prompt more study of the pattern's underlying rules, he told Science News.

Some of the regulatory features of stripe formation in fishes do parallel those observed in fruit fly embryos, Meinhardt says. Yet this proposed pattern-forming reaction in angelfish differs markedly from the action of the stripe-regulating genes known to exist in the fruit fly.

While he says that this model offers a "fresh look" at stripe formation in growing organisms, Meinhardt concludes that the stripes of the Pomacanthus "still call for more explanation."

Kondo and Asai point out that while they do not know which molecules spawn the angelfish's arrangement of stripes, their simulated molecules do mimic the behavior of proteins in water.

Viewing the angelfish's stripes from another perspective, they argue that study of the pigments in the animal's skin may produce a deeper understanding of the chemicals and mechanisms governing pattern formation. In fact, by combining data from fish with those from their simulation, the scientists believe "it should be possible to identify the molecules involved."