In findings published in the journal Science Advances, engineers at the University of Colorado have shown that the same physical process that helps remove dirt from laundry could play a role in how tropical marine fishes get their colourful stripes and spots.
In 1952, before biologists discovered the double helix structure of DNA, Alan Turing, the mathematician who invented modern computing, proposed a theory of how animals got their patterns. He hypothesized that developing tissues produce chemical agents that diffuse through tissues in a process similar to adding milk to coffee. Some of the agents react with each other, forming spots while others inhibit the spread and reaction of the agents, forming space between spots.
“Turing’s mechanism can produce patterns, but diffusion doesn’t yield sharp patterns,” said Ankur Gupta, an assistant professor in the Department of Chemical and Biological Engineering. For instance, when milk diffuses in coffee, it flows in all directions with a fuzzy outline.
A breakthrough came when Benjamin Alessio, the paper’s first author and an undergraduate researcher in the Department of Chemical and Biological Engineering, visited the Birch Aquarium in San Diego where he was impressed by the intricate pattern shown by the Ornate Boxfish Aracana ornata. The pattern reminded Alessio of computer simulations he had been conducting, where particles form sharply defined stripes. A member of the Gupta research group, Alessio wondered if 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.
To see if it may play a role in giving animals their vivid patterns, Gupta and Alessio 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-colour hexagonal pattern seen on the fish.
The 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.
“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.
[image: A male Ornate Boxfish (Aracana ornata). Inset 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]
More information: Benjamin Alessio et al, Diffusiophoresis-Enhanced Turing Patterns, Science Advances (2023). DOI: 10.1126/sciadv.adj2457. www.science.org/doi/10.1126/sciadv.adj2457
Journal information: Science Advances