Path-Dependent Evolution: Chitons’ Visual System Shaped by Shell Armor
Biologists have long been fascinated by the idea of rewinding the tape of life’s history to see if organisms would evolve in different ways. A recent study published in Science has shed light on this question by examining the evolution of vision in chitons, a group of mollusks. The researchers discovered that two types of eyes, eyespots and shell eyes, evolved independently in different lineages of chitons. What’s even more intriguing is that the type of eye evolved was determined by an unrelated feature: the number of slits in the chiton’s shell armor.
The researchers at the University of California, Santa Barbara stumbled upon this phenomenon while studying chitons. They found that a lineage could evolve either eyespots or shell eyes, but never both. This represents a real-world example of path-dependent evolution, where a lineage’s history shapes its future evolutionary trajectory. It’s like a one-way door, opening up some possibilities while closing off others for good.
Rebecca Varney, the lead author of the study, describes this as one of the first cases where path-dependent evolution has been observed in a natural system. While it has been seen in lab-grown bacteria, witnessing it in a real-world scenario is truly exciting for scientists.
Chitons are small mollusks with a body plan that has remained relatively stable for 300 million years. They have eight shell plates that act as protective armor. However, these shell plates are not just inert armor; they are decorated with sensory organs that help chitons detect potential threats.
There are three types of sensory organs in chitons: aesthetes, eyespots, and shell eyes. Aesthetes are all-in-one receptors that allow chitons to sense light, chemicals, and mechanical cues in their environment. Some chitons also have visual systems, either eyespots or shell eyes. Shell eyes are more complex and can capture rough images, allowing chitons to detect looming predators.
To understand how these different types of eyes evolved, the researchers sequenced DNA from over 100 chiton species. They observed that an increase in the density of aesthetes on the shell was the first step before evolving either shell eyes or eyespots. Interestingly, both eyespots and shell eyes evolved independently multiple times across the chiton phylogeny, representing instances of convergent evolution.
The researchers were surprised by these findings. They had expected a stepwise evolution from aesthetes to eyespots and then to shell eyes. However, they discovered that there are multiple paths toward vision in chitons.
During a six-hour drive, Varney and Todd Oakley developed a hypothesis that the number of slits in a chiton’s shell could be crucial to the evolution of its visual system. All light-sensing structures on the shell are attached to nerves that pass through the shell slits to connect to the body’s main nerves. More slits mean more openings for nerves to run through.
Varney noticed that lineages with 14 or more slits evolved eyespots, while lineages with 10 or fewer slits evolved shell eyes. The number of slits determined which type of eye could evolve in chitons. This suggests that the evolution of their visual systems is closely tied to the number of shell slits.
The researchers are now investigating why the number of slits constrains the type of eye that can evolve. They are studying the circuitry of the optic nerves and how they process signals from hundreds or thousands of eyes. Another possibility is that the relationship between eye type and the number of slits is driven by the way the shell plates develop and grow in different lineages.
While there is still much to learn about how chitons see the world, their eyes are poised to become a prime example of path-dependent evolution. This phenomenon, although common, is rarely demonstrated as clearly as in the case of chitons. The study’s findings have the potential to reshape our understanding of how evolution and development interact and may even find their way into future textbooks on evolution.
