Have you ever looked at a flat fish like a plaice or a sole, with two eyes on one side of the head, and think, “How did that happen?”
You’re lucky. Rice University biologist Kory Evans has the answer.
“Flatfish are some of the strangest vertebrates on the planet, and they got very, very quickly weird by changing multiple traits at once over a short period of time,” said Evans, assistant professor of bioscience at Rice, specializing in the study of evolution. of fish over long time scales.
Of all mammals, reptiles, birds, amphibians, and fish, flatfish are by far the most asymmetrical. Evans, the corresponding author of a study on the evolution of flatfish in the Proceedings of the National Academy of Sciences, says it helps keep that in perspective.
“Imagine any other animal,” he says. “For example, let’s say you walk and you see a squirrel, and one eye is here and the other is there,” he said, pointing to two places on the same side of his face. “This squirrel is having a hard time. And there are 800 species of these fish that do.
“Perspective helps to understand how strange these animals are.”
In terms of evolution, the asymmetry of flatfish is not only a novelty, it is an innovation and a trait that distinguishes flatfish even from their closest relatives.
Evans said that the evolution of flatfish is particularly interesting because they started out as typical symmetrical fish. They began to evolve their current form, or morphology, about 65 million years ago, and by 3 million years they were largely done.
“We got all of this new morphospace colonization in 3 million years,” Evans said. “And look how long has passed since then. So there’s a very short, short period of time that all of these new forms and all of these crazy species evolved.”
In their study, Evans and his co-authors Olivier Larouche de Rice and Sara-Jane Watson of the New Mexico Institute of Mining and Technology found that a tight integration of genetic traits in flatfish leads to a sort of evolutionary cascade.
“Integration is where there is a high degree of correlation between traits, so if you change one trait another trait will be changed as well,” Evans said. “At macroevolutionary timescales, it gets really interesting, as traits then start to co-evolve with each other. So if you change one trait, you could end up changing several more.
He said traits can become more integrated if their morphological development is controlled or influenced by networks of shared genetic interactions.
“If the signaling networks expand to encompass more and more traits, then you can theoretically spread the changes throughout an organism using the same signaling network, and you can change very quickly,” he said. declared. “It’s like pushing a button and turning the whole animal over at the same time.”
Evans, Larouche, and Watson used several methods to reconstruct the evolutionary history of flatfish. One was a phylogenetic comparative method that tracks the evolutionary history of traits between and among species. Phylogenetic trees have branches that show where species diverge. “Typically, the tree is built using genetics,” Evans said. “So maybe we’ll have a bunch of genomes for all of these species. And we can use that to determine who is most closely related to whom. Then once the tree is built, I can see how the traits have changed over time using the branching diagram of the tree as a guide. “
The researchers also used a micro-CT scanner in Evans’ lab to perform 3D scans of the skulls of several species of flatfish. The scans were used to create 3D morphometric models that could be compared for differences in shape. But many species of flatfish are so dissimilar that it was not possible to “separate them with just form or just phylogeny alone,” Evans said.
The researchers therefore created complex mathematical models to track the degree of integration between different regions of the skull over the 65 million year history of flatfish and their parents.
“We found that flatfish were much more integrated than non-flatfish, and this meant that the evolution of asymmetry for flatfish ended up being an integrated process, essentially involving changes all over the head.” , did he declare. “As the eye migrated, a bunch of other things changed as well. And it has become additive. So, as the flatfish skull became more and more integrated, more things started to change, per unit of time, than a generation before.
As to why flatfish evolved to be asymmetrical, Evans said that wasn’t the only way to become flat.
“Other flatfish didn’t do that, like stingrays,” he said. “They just went flat like a pancake. But their eyes weren’t both on the same side. The remora (aka suckerfish) are also a flat-looking fish, and they didn’t.”
Since evolution is a competition for “survival of the fittest”, the evolutionary success of the flatfish begs the question: is asymmetry somehow beneficial?
“I’m not going to lie,” Evans said. “I don’t really know if there is a benefit. I think they did it because they could.”