PREPRINT: Path dependent evolution and chiton eyes

 
​Happy to finally share our preprint on path dependent evolution in visual systems in chitons! Check it out here: https://doi.org/10.1101/2022.12.20.520810

Path dependence (contingency) is the idea that specific key events can push lineages down one or another path across evolutionary time. Kind of like skipping lunch leads to eating an earlier dinner, previous events can determine whether or not future events happen.

Chitons build iron teeth, but they also make TWO totally different types of eyes! Some chitons have hundreds of shell eyes, with a lens and retina, that look a lot like your camera-type eyes. But other chitons have thousands of eyespots, small receptors that work together like the facets of a fly's compound eye. BOTH types of eyes can provide spatial vision.

So what's going on, chitons? Why eyespots some times, and shell eyes other times? Why never both? We built the largest chiton phylogeny ever, and then mapped the two visual systems back to it. And we were SURPRISED.

There are TWO SEPARATE ORIGINS of shell eyes, AND there are TWO SEPARATE ORIGINS of eyespots. FOUR INDEPENDENT ORIGINS of vision! WHAT?!

We investigated the morphology of both types of eyes, and found a difference in the shells on the chitons' backs. Shell eyes and eyespots are embedded in the upper layer of chiton shells, so the optic nerves have to travel to an edge where they pass through openings (slits) to get down to the rest of the chiton's body and join the big nerve cords. We found that chitons with eyespots have way more openings than chitons with shell eyes, or with no eyes. 

Why? Well, you need hundreds of shell eyes but THOUSANDS of eyespots, so we think that more openings work kind of like a cable organizer at the back of your desk, helping keep all those optic nerves straight. Because eyespots work together, chitons can't let them get all tangled up without losing 'sight' of which eye is where! ​

What does this mean for evolution? Well, we hypothesized that the slits act as a critical junction, a key determining factor that forces lineages down one or another evolutionary path. This would mean that on a morphospace, we should see a GAP between eyespots and shell eyes, because the two rely on different morphological set ups. We plotted it, and sure enough we found a gap right between! 

Demonstrating path dependent evolution in a natural system requires a unique suite of circumstances, so much so that some scientists claimed it was impossible. Chitons are perfect, because even though different lineages gained different types of visual systems, they all live in really similar places, and have across their fossil record. That also meant we could time the evolution of eyes; one lineage of chitons gained eyespots in only 6 million years. For the record, that's FAST.

This was a really rewarding paper to write, especially because it doesn't seem like anybody noticed the slits-eyespots link before now! Never underestimate the power of some good old fashioned morphology, even alongside the latest and greatest 'omics.