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New clues to brain evolution from a map of the octopus visual system

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Fluorescence image of an octopus brain showing the location of different types of neurons Credit: Niell Lab

Octopuses have a hard time choosing just one party trick. This gorgeous creature floats with the help of a jet engine, shoots inky chemicals at its enemies, and can change its skin in seconds to blend in with its surroundings.

Now a team of researchers from the University of Oregon (UO) has investigated another distinguishing feature of this eight-armed marine animal: its excellent visual capabilities.

They laid out a detailed map of the visual system of the octopus in a new scientific paper. On the map, they classify different types of neurons in the part of the brain dedicated to vision. As a result, it is a valuable resource for other neurologists, providing detailed information that can guide future experiments. In addition, it may teach us something about the evolution of the brain and the visual system more broadly.

The team reports their findings today (October 31) in the journal Modern biology.

Chris Niel’s lab at the UO studies vision, mostly in mice. But a few years ago, postdoctoral researcher Judith Pungor brought a new species to the laboratory – the California two-spotted octopus.

Although not traditionally used as a research subject in the laboratory, this cephalopod quickly caught the interest of UO neuropathologists. Unlike mice, which are not known for their good vision, “octopuses have an amazing visual system, and a large part of their brain is dedicated to visual processing,” Niel said. “They have eyes that are remarkably human-like, but after that the brain is completely different.

The last common ancestor of octopuses and humans was 500 million years ago, and since then the species have evolved in very different contexts. So scientists didn’t know whether parallels in visual systems extend beyond the eyes, or whether the octopus instead uses entirely different types of neurons and brain circuits to achieve similar results.

“Seeing how the octopus eye convergently evolved like ours, it’s cool to think about how the octopus visual system can be a model for a more general understanding of the complexity of the brain,” said Mea Songka-Casey, a graduate student in Neela’s lab and first author on the paper . “For example, are there basic cell types required for this highly intelligent, complex brain?”

Here, the team used genetic techniques to identify different types of neurons in the octopus’s optic lobe, the part of the brain responsible for vision.

They identified six main classes of neurons that differ based on the chemical signals they send. Examining the activity of specific genes in these neurons revealed additional subtypes, providing clues to more specific roles.

In some cases, scientists have identified specific groups of neurons in a distinctive spatial arrangement — for example, a ring of neurons around the lobe of the optic nerve that signal with a molecule called octopamine. Fruit flies use this adrenaline-like molecule to increase visual processing when the fly is active. So perhaps it could have a similar role in octopuses.

“Now that we know there’s this very specific type of cell, we can go ahead and figure out what it does,” Neal said.

About a third of the neurons in the data did not appear fully developed. An octopus’s brain continues to grow and add new neurons throughout the animal’s life. These immature neurons, not yet integrated into the brain’s circuits, were a sign that the brain was in the process of expanding!

However, the map did not reveal sets of neurons that clearly migrated from human or other mammalian brains, as the researchers had assumed.

“On an obvious level, neurons don’t map to each other—they use different neurotransmitters,” Neal said. “But maybe they’re doing the same kinds of calculations, just in a different way.”

Digging deeper will also require a better understanding of cephalopod genetics. Because the octopus has not traditionally been used as a laboratory animal, many of the tools used for precise genetic manipulation of fruit flies and mice do not yet exist for octopuses, said Gabby Koffing, a graduate student in Andrew Kern’s lab. who worked on the study.

“There are a lot of genes that we don’t know what they do because we haven’t sequenced the genomes of many cephalopods,” Pungor said. Without genetic data from related species as a point of comparison, it is more difficult to infer the functions of specific neurons.

Niel’s team is ready to take on this challenge. They are now working to map the octopus brain beyond the optic lobe, seeing how some of the genes they focused on in this study are expressed in other parts of the brain. They also record from neurons in the visual lobe to determine how they process a visual scene.

In time, their research may make these mysterious sea creatures a little less obscure—and shed light on our own evolution, too.

Reference: “Cell types and molecular architecture of the visual system of Octopus bimaculoides” 31 Oct 2022. Modern biology.
DOI: 10.1016/j.cub.2022.10.015

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