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New Theory Could Explain How Animals Get Their Stripes

A male Ornate Boxfish (Aracana ornata)
A male Ornate Boxfish (Aracana ornata). Credit: The Birch Aquarium, Scripps Institution of Oceanography. Benjamin Alessio / University of Colorado Boulder.
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The same physical process that helps to remove dirt from laundry could explain how tropical fish and other patterned animals get their spots, according to new research.

Published in Science Advances, the new paper proposes that diffusiophoresis – the movement of particles suspended in a fluid – plays a role in the formation of complex animal patterns.

Using computer simulations that accounted for diffusiophoresis, researchers were able to recreate the intricate bi-color hexagonal patterns seen on the skin of ornate boxfish.

“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, an undergraduate researcher at the Department of Chemical and Biological Engineering at the University of Colorado Boulder, and the paper’s first author. “Our work uses a simple physical and chemical mechanism to explain a complicated biological phenomenon.”

Animal patterns are more than genetics

Many animals have evolved to display colorful and complex patterns on their skin and coats. These displays can serve as camouflage in hostile environments or as a means to attract the opposite sex during mating season.

While genes do encode certain amounts of pattern information, like the color of a leopard’s spots, genetics alone cannot determine exactly where these spots develop.

In 1952, even before biologists discovered the double helix structure of DNA, a bold new theory on how animals get their patterns was proposed by none other than Alan Turing, the mathematician widely heralded as the father of modern computing.

Turing proposed that as tissues develop, they produce chemical agents that can diffuse through the tissue similarly to how milk diffuses in coffee. Some of these agents will react together and form spots, he hypothesized, while others will inhibit each other and prevent the spread and reaction of the agents, giving rise to the space between spots.

Instead of thinking about complex genetic processes, Turing’s model proposed that a simple reaction-diffusion model could be enough to explain the basics of biological pattern formation. But this theory has one big problem.

“Surely Turing’s mechanism can produce patterns, but diffusion doesn’t yield sharp patterns,” explained senior study author Ankur Gupta, an assistant professor at CU-Boulder.  

Earning your stripes

Gupta and Alessio became interested in the Turing model after Alessio visited the Birch Aquarium in San Diego, where he was particularly taken by the intricate hexagonal-like patterns of the ornate boxfish (Aracana ornata).

The patterns reminded Alessio of some computer simulations he had been working on, where he had seen particles form sharply defined stripes. Turing’s theory alone might not be able to explain animal pattern formation, but perhaps accounting for diffusiophoresis might.

Top: A male ornate boxfish (Aracana ornata). Bottom left: A close-up picture of the fish’s natural hexagonal pattern. Bottom center: Fish pattern simulation based on Turing’s reaction-diffusion theory. Bottom right: Diffusiophoresis-enhanced reaction-diffusion simulation. Credit: The Birch Aquarium, Scripps Institution of Oceanography, Benjamin Alessio / University of Colorado Boulder.

Diffusiophoresis happens when a molecule or particle moves through a liquid in response to some environmental change, such as a difference in concentrations, which accelerates the movement of other types of molecules in the same environment.

This is the same effect at play inside a washing machine. A recent study by researchers at Princeton University found that rinsing soap-soaked clothes in clean water removes the dirt faster than rinsing soap-soaked clothes in soapy water. This is because the soap molecules diffuse out of the fabric and into water with a lower soap concentration, with this movement of the molecules also drawing out the dirt.

Crucially, as Alessio and Gupta observed in their simulations, the movement of molecules during diffusiophoresis always follows a clear trajectory and so can give rise to patterns with sharp outlines.

Diffusiophoresis may explain unique animal patterns

To test their theory, Gupta and Alessio ran a computer simulation to try and recreate the purple and black hexagonal patterns seen on boxfish skin using only Turing equations. Then, they conducted the same calculation but this time with modifying equations that could incorporate molecular motion due to diffusiophoresis.

They found that with the Turing equations alone, the computer could only produce blurry purple dots with a faint black outline. However, the modified equations produced an image much more similar to the real-life boxfish: a bright and sharp bi-color hexagonal pattern.

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Based on these simulations, the team believe that a combination of Turing’s chemical agent theory and diffusiophoresis could explain how animal patterns form. They also suggest that this mechanism could explain other patterns in biology, such as the arrangement of hair follicles in mice and the ridges on the roof of the mouth in mammals.

“Our findings emphasize diffusiophoresis may have been underappreciated in the field of pattern formation. This work not only has the potential for applications in the fields of engineering and materials science but also opens up the opportunity to investigate the role of diffusiophoresis in biological processes, such as embryo formation and tumor formation,” Gupta said.

Reference: Alessio BM, Gupta A. Diffusiophoresis-enhanced Turing patterns. Sci Adv. 2023. doi: 10.1126/sciadv.adj2457

This article is a rework of a press release issued by the University of Colorado Boulder. Material has been edited for length and content.