How the Boxfish Got Its Spots

Boxfish

Zebras are striped, leopards are spotted, and cows are flecked. How these patterns arise has not had a complete explanation in science until now. Researchers at the University of Colorado Boulder, in the journal Science Advances, present an approach using the example of the boxfish: it could be the same physical process that helps remove dirt from laundry.

Many biological questions are fundamentally the same question: How do organisms develop complicated patterns and shapes when everything starts off from a spherical clump of cells,” said Benjamin Alessio, the study’s lead author and a student in the Department of Chemical and Biological Engineering, according to a statement. “Our work uses a simple physical and chemical mechanism to explain a complicated biological phenomenon.”

An important element in the explanation came from the mathematician Alan Turing in the 1950s. He hypothesized that tissues produce chemical substances during their development. These substances diffuse through the tissue. One can imagine this being like pouring milk into black coffee. Some of the substances would react with each other, forming spots. Others would inhibit the spread and reaction of the substances, creating spaces between the spots.

Surely Turing’s mechanism can produce patterns, but diffusion doesn’t yield sharp patterns,” said lead author Ankur Gupta. For example, when milk diffuses in coffee, it flows with blurred outlines in all directions. A visit to a museum aquarium gave him an idea, according to the statement. The complex pattern of a boxfish reminded him of a computer simulation he had created, where particles formed sharply defined stripes. Diffusiophoresis: that’s the technical term for this process.

boxfish
Boxfish: close-up of the natural hexagonal pattern, simulation of the pattern based on Turing’s reaction-diffusion theory, simulation with diffusiophoresis Photo: Benjamin Alessio / University of Colorado Boulder / The Birch Aquarium / Scripps Institution of Oceanography.

It occurs when a molecule, in response to changes such as concentration differences, moves through a liquid and accelerates the movement of other types of molecules in the same environment. The same principle applies when laundry gets clean. A recent study showed that rinsing soap-soaked clothes in clean water removes dirt faster than rinsing soap-soaked clothes in soapy water. This is because the soap diffuses from the fabric into water with a lower soap concentration, pulling out the dirt. When clothes are placed in soapy water, dirt remains due to the lack of a difference in soap concentration.

Using the approach of mathematician Turing, the study authors simulated the violet-black, hexagonal pattern on the adorned skin of the boxfish (see image). The result was a blurry image of violet dots and a faint black outline. Then they adjusted the equation with diffusiophoresis. The result this time was two-colored, hexagonal patterns that were bright and sharp. In this case, the pattern of the fish was much more similar.

When chemical substances, as described by Turing, diffuse through the tissue, they also pull pigment-producing cells with them through diffusiophoresis—just like soap pulls dirt from laundry, the authors conclude. These pigment cells form spots and stripes with much sharper contours. The theory could also explain other patterns, such as the arrangement of hair follicles in mice.

Gupta now hopes that the study could inspire further research. “This work also opens up the opportunity to investigate the role of diffusiophoresis in biological processes, such as embryo formation and tumor formation,” he said, according to the statement.

Featured Image: Flickr.