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Pigment pas de deux puts stripes on zebrafish

Researchers watch yellow and black cells dance into patterns on fish fins

COLOR DANCE  Zebrafish owe their stripes to the coordinated movement of at least two types of pigment-producing cells.

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Scientists have long puzzled over how animals’ stripes, spots, dots, blazes and other color patterns arise. One of the most popular theories was proposed in 1952 by British mathematician Alan Turing, who showed that two chemicals spreading across a surface could spontaneously react to create patterns. By varying how chemicals diffuse and react under different conditions, Turing could reproduce many patterns seen in nature (SN: 7/17/10, p. 28).

Researchers have tried to find diffusing chemical signals that might guide color patterns in animals. But research published January 21 in the Proceedings of the National Academy of Sciences suggests that interactions between yellow pigmented cells and black ones helps create the striped pattern that gives zebrafish (Danio rerio) their name.

“The long quest for the suggested long-range diffusible signals has not been as fruitful as originally expected,” says Enrique Salas Vidal of the National Autonomous University of Mexico in Cuernavaca. Instead of diffusing chemicals, new work by Hiroaki Yamanaka and Shigeru Kondo of Osaka University in Japan suggests up-close communication between cells causes patterns to form during development.

Yamanaka and Kondo extracted pigment cells from zebrafish fins and watched the cells interact under a microscope. Yellow pigment cells called xanthophores reach out toward black pigmented cells known as melanophores. The black cells recoil and move away. Then the yellow cells extend projections called pseudopodia and give chase. The cells usually dance circularly around each other in a counter-clockwise spiral.

Mutations in some genes create either wide, fuzzy stripes in “jaguar” mutant fish or spots in “leopard” mutant fish. The researchers found that in fish with jaguar mutations, black melanophores circle yellow pigmented cells but don’t move away from them. In leopard mutants, the yellow xanthophores reach out toward the black cells but don’t chase them. The black cells also don’t run away. Those observations provide evidence that cell movement is important for pattern development in fish fins.

Still, that doesn’t mean Turing was wrong, says Kondo. “If we assume that the cell projection mimics the diffusion, the mathematical concept of the mechanism we found is very similar to the Turing model,” he says.

The researchers make a few assumptions that need to be tested, says Christiane Nüsslein-Volhard of the Max Planck Institute for Developmental Biology  in Tübingen, Germany. For instance, the Japanese group says that the “run-and-chase” behavior of yellow and black pigmented cells accounts for stripes on zebrafish bodies as well as their fins, but she and other researchers have evidence that iridescent cells called iridophores are also important for body stripes. The new study did not examine those cells.

“Further, in their model they assume a random initial distribution of xanthophores and melanophores. Such a situation has not been shown to exist in the fish at any time point in the development of the stripes,” Nüsslein-Volhard says.

Despite the caveats, she says, Yamanaka and Kondo have made an important contribution by creating a method to study formation of color patterns in the lab.

Editor's Note: This story was updated January 24, 2014, to correct Christiane Nüsslein-Volhard's institutional affiliation.

TAG, YOU'RE IT  Pigment cells from zebrafish play a game of tag in which black melanophores shy away from yellow xanthophores. The movements eventually lead to stripes on fishes’ fins. Mutations that interfere with this choreography create spots or fuzzy, wide stripes. Credit: H. Yamanaka and S. Kondo, PNAS, 2014

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