BBiologists have often wondered what would happen if they could go back in time and let evolution happen again. Would organisms evolve in different ways, or would they end up with the same eyes, wings, and other traits because of their past evolution?
A new paper published in Science this February describes a rare and important test case for that question, which is fundamental to understanding how evolution and development interact. A team of researchers at the University of California, Santa Barbara stumbled upon it while studying the evolution of vision in a little-known group of mollusks called chitons. In that group of animals, the researchers found that two types of eyes—eyespots and shell eyes—each evolved twice independently. A lineage could have one type of eye or the other, but not both.
Independently, chitons evolved eyes four times, which is really impressive.
Interestingly, the type of eye that a lineage had was determined by an apparently unrelated older feature: the number of slits in the chiton’s shell armor. This is an example of “path-dependent evolution,” where a lineage’s history shapes its future evolution. Certain points in a lineage's history open up certain possibilities while closing off others.
“This is one of the first cases [where] we’ve actually been able to see path-dependent evolution,” said Rebecca Varney, a postdoctoral fellow in Todd Oakley’s lab at UCSB and the lead author of the new paper. Although path-dependent evolution has been observed in some bacteria grown in labs, “showing that in a natural system was a really exciting thing to be able to do.”
“There’s always an impact of history on the future of a particular trait,” said Lauren Sumner-Rooney, who studies invertebrate visual systems at the Leibniz Institute for Evolution and Biodiversity Science and was not involved in the new study. “What’s particularly interesting and exciting about this example is that the authors seem to have pinpointed the moment in time where you get that split.”
For that reason, the chitons “are likely to enter future textbooks on evolution” as an example of path-dependent evolution, said Dan-Eric Nilsson, a visual ecologist at Lund University in Sweden who was not involved in the research.
Chitons, small mollusks that live on intertidal rocks and in the deep sea, are like little tanks protected by eight shell plates—a body plan that’s remained relatively stable for some 300 million years. Far from being inert armor, these shell plates are heavily decorated with sensory organs that enable the chitons to detect possible threats.
The sensory organs come in three types. All chitons have aesthetes, a wildly synesthetic all-in-one receptor that enables them to sense light as well as chemical and mechanical cues in the environment. Some chitons also have proper visual systems: either thousands of light-sensing eyespots or hundreds of more complex shell eyes, which have a lens and retina for capturing rough images. Animals with shell eyes can detect looming predators, in response to which they clamp themselves tightly onto the rock.
A team of researchers led by Varney used exome capture to sequence strategic sections of DNA from old specimens in the collection of Doug Eernisse, a chiton specialist at California State University, Fullerton. They sequenced DNA from over 100 chiton species to create the most comprehensive phylogeny for chitons to date. Then the researchers mapped the different eye types onto the phylogeny and observed that an increase in the density of aesthetes on the shell was the first step before evolving either shell eyes or eyespots. They found that eyespots and shell eyes each evolved two separate times across the phylogeny, showing two separate instances of convergent evolution.Chitons evolved eyes four times, and the researchers estimated that in the neotropical genus, the eyespots evolved within just 7 million years—a blink of an eye in evolutionary time.
The results surprised the researchers. Instead of a stepwise evolution in complexity, there are multiple paths toward vision.
Varney and Oakley developed the hypothesis that the number of slits in a chiton’s shell could be key to the evolution of chiton vision during a six-hour drive from a conference in Phoenix back to Santa Barbara. ChitonRebecca Varney and her team extracted DNA from chiton shells to build a comprehensive tree of life. They discovered that chitons evolved visual systems four separate times.
All light-sensing structures on the chiton shell are attached to nerves, which pass through the shell slits to connect to the body’s main nerves. The slits function as cable organizers, bundling sensory neurons together. More slits mean more openings through which nerves can run. Dan SpeiserThe number of slits locked into place which kind of eye type could evolve: A chiton with thousands of eyespots needs more slits, whereas a chiton with hundreds of shell eyes needs fewer. In short, the number of shell slits determined the evolution of the creatures’ visual systems.
The discoveries spark new questions. The researchers are actively looking into why the number of openings affects the type of eye that can develop. Addressing this will involve studying how the optic nerves work and process signals from hundreds or thousands of eyes.
This article was
originally published
Quanta Abstractions
As a chiton, “you might start life with 10 eyes and finish your life with 200 eyes.”
Lead image: The visual systems of chitons, a type of marine mollusk, represent a rare real-world example of path-dependent evolution—where a lineage’s history irrevocably shapes its future trajectory. Credit: Stefan Ziemendorff / Shutterstock
The visual systems of a group of mollusks reveal how future evolution depends on the past.
As a consequence, the growing edge of a shell plate has to leave holes for new eyes—many small holes for the eyespots, or fewer larger holes for the shell eyes. Too many or too-big holes could weaken a shell to its breaking point, so structural factors might limit which eyes are possible.
Much remains to be discovered about how chitons see the world, but in the meantime, their eyes are primed to become biologists’ new favorite example of path-dependent evolution, Nilsson said. “Examples of path dependence that can be really well demonstrated, as this case [is], are rare—even though the phenomenon is not only common, it’s the standard way things happen.”
This article was originally published on the Quanta Abstractions blog.
Lead image: The visual systems of chitons, a type of marine mollusk, represent a rare real-world example of path-dependent evolution—where a lineage’s history irrevocably shapes its future trajectory. Credit: Stefan Ziemendorff / Shutterstock
