How animals get their stripes and spots

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

Nature has no shortage of patterns, from spots on leopards to stripes on zebras and hexagons on boxfish. But a full explanation for how these patterns form has remained elusive.

Now engineers  have shown that the same physical process that helps remove dirt from laundry could play a role in how tropical fish get their colorful stripes and spots. Their findings were published Nov. 8 in the journal Science Advances.

Biologists have previously shown that many animals evolved to have coat patterns to camouflage themselves or attract mates. While genes encode pattern information like the color of a leopard’s spots, genetics alone do not explain where exactly the spots will develop, for example.

In 1952, before biologists discovered the double helix structure of DNA, Alan Turing, the mathematician who invented modern computing, proposed a bold theory of how animals got their patterns.

Turing hypothesized that as tissues develop, they produce chemical agents. These agents diffuse through tissue in a process similar to adding milk to coffee. Some of the agents react with each other, forming spots. Others inhibit the spread and reaction of the agents, forming space between spots. Turing’s theory suggested that instead of complex genetic processes, this simple reaction-diffusion model could be enough to explain the basics of biological pattern formation.

Surely Turing’s mechanism can produce patterns, but diffusion doesn’t yield sharp patterns.

Where particles  form sharply defined stripes,  the process known as diffusiophoresis plays a role in nature’s pattern formation.

Diffusiophoresis happens when a molecule moves through liquid in response to changes, such as differences in concentrations, and accelerates the movement of other types of molecules in the same environment. While it may seem like an obscure concept to non-scientists, it’s actually how laundry gets clean.

One recent study showed that rinsing soap-soaked clothes in clean water removes the dirt faster than rinsing soap-soaked clothes in soapy water. This is because when soap diffuses out of the fabric into water with lower soap concentration, the movement of soap molecules draws out the dirt. When the clothes are put in soapy water, the lack of a difference in soap concentration causes the dirt to stay in place.

The movement of molecules during diffusiophoresis, as researchers observed in their simulations, always follows a clear trajectory and gives rise to patterns with sharp outlines. To see if it may play a role in giving animals their vivid patterns, researchers ran a simulation of the purple and black hexagonal pattern seen on the ornate boxfish skin using only the Turing equations. The computer produced a picture of blurry purple dots with a faint black outline. Then the team modified the equations to incorporate diffusiophoresis. The result turned out to be much more similar to the bright and sharp bi-color hexagonal pattern seen on the fish.

The research team’s theory suggests that when chemical agents diffuse through tissue as Turing described, they also drag pigment-producing cells with them through diffusiophoresis—just like soap pulls dirt out of laundry. These pigment cells form spots and stripes with a much sharper outline.

 Benjamin Alessio et al, Diffusiophoresis-Enhanced Turing Patterns, Science Advances (2023). DOI: 10.1126/sciadv.adj2457. www.science.org/doi/10.1126/sciadv.adj2457