A new map of the octopus’ visual system provides clues to brain evolution

Summary: A new map of the octopus’ visual system classifies different types of neurons in a part of the brain dedicated to vision and sheds new light on the development of the brain and the wider visual systems.

Source: University of Oregon

It’s hard for the octopus to decide on just one party trick. It swims using jet propulsion, blasts inky chemicals at its enemies, and can change its skin in seconds to blend in with its surroundings.

A team of researchers from the University of Oregon is studying another defining characteristic of this eight-armed sea creature: its outstanding visual abilities.

In a new paper, they present a detailed map of the octopus’ visual system and classify different types of neurons in a part of the brain dedicated to vision. The map is a resource for other neuroscientists and provides details that could guide future experiments. And it could also teach us something about the development of brains and visual systems in general.

The team will report its findings on October 31 in Current Biology.

Cris Niell’s lab at UO studies vision, primarily in mice. But a few years ago, postdoc Judit Pungor brought a new species to the lab—the California two-spotted octopus.

Although this cephalopod is not traditionally used as a study object in the laboratory, it quickly attracted the interest of UO neuroscientists. Unlike mice, which aren’t known for having good eyesight, “squid have an amazing visual system, and a large part of their brain is devoted to visual processing,” Niell said. “You have an eye that is remarkably similar to the human eye, but after that the brain is completely different.

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

“Seeing how the octopus eye has evolved convergently, similar to ours, it’s cool to think about how the octopus’ visual system could be a model for the broader understanding of brain complexity,” said Mea Songco-Casey, a graduate student at Niells Laboratory and first author on the paper. “For example, are there basic cell types that are needed for this very intelligent, complex brain?”

Here, the team used genetic techniques to identify different types of neurons in the octopus’ visual lobe, the part of the brain dedicated to vision.

They selected six main classes of neurons, differentiated based on the chemical signals they send. A look at the activity of specific genes in these neurons then revealed other subtypes that indicate more specific roles.

In some cases, the researchers located specific groups of neurons in different spatial arrangements — for example, a ring of neurons around the visual lobe, all signaling with a molecule called octopamine. Fruit flies use this adrenaline-like molecule to increase visual processing when the fly is active. So maybe it could play a similar role in squid.

“Now that we know that this very specific cell type exists, we can start to go in and figure out what it’s doing,” Niell said.

About a third of the neurons in the data looked underdeveloped. The octopus brain grows, adding new neurons over the animal’s lifespan. These immature neurons, not yet integrated into brain circuitry, were a sign that the brain was expanding!

However, the map did not reveal sets of neurons that were clearly transmitted from human or other mammalian brains, as the researchers thought.

“At the apparent level, the neurons are not mapped to each other – they use different neurotransmitters,” Niell said. “But maybe they’re doing the same calculations, just in a different way.”

To dig deeper, you also need to get a better handle on cephalopod genetics. Because the octopus has not traditionally been used as a laboratory animal, many of the tools used for precise genetic manipulation in fruit flies or mice don’t yet exist for the octopus, said Gabby Coffing, a graduate student in Andrew Kern’s lab who worked on the study to have.

“There are many genes that we have no idea what their function is 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 function of specific neurons.

Niell’s team rises to the challenge. They are now working to map the octopus brain beyond the visual lobe to see how some of the genes they focused on in this study show up elsewhere in the brain. They also pick up from neurons in the visual lobe to determine how to process the visual scene.

Over time, their research could make these mysterious sea creatures a little less murky — and shed some light on our own evolution, too.

About this brain mapping and news from visual neuroscientific research

Author: Laurel Hamers
Source: University of Oregon
Contact: Laurel Hamers – University of Oregon
Picture: The image is credited to Niell Lab

Original research: Open access.
“Cell Types and Molecular Architecture of the Visual System of Octopus bimaculoides” by Cris Niell et al. Current Biology

abstract

See also

Cell types and molecular architecture of the visual system of Octopus bimaculoides

highlights

• scRNA-seq and FISH identified molecular cell types in the octopus visual system
• Cell types defined by functional and developmental markers reveal sublayer organization
• Immature neurons form transcriptional subsets that correspond to mature cell types
• This atlas is a foundation for the study of visual function and development in cephalopods

summary

Cephalopods have remarkable visual systems, with a camera-like eye and high visual acuity, which they use for a variety of sophisticated visually-driven behaviors.

However, the cephalopod brain is organized dramatically differently from that of vertebrates and invertebrates, and little is known about the cell types and molecular determinants of their visual systems organization beyond neuroanatomical descriptions.

Here we present a comprehensive single-cell molecular atlas of the octopus optic valve, which is the primary visual processing structure in the cephalopod brain.

We combined single cell RNA sequencing with RNA fluorescence on site Hybridization to both identify putative molecular cell types and to determine their anatomical and spatial organization within the optic lobe.

Our results show six major neuronal cell classes identified using neurotransmitters/neuropeptides, in addition to non-neuronal and immature neuronal populations.

We find that additional markers divide these neuronal classes into subtypes with distinct anatomical locations, revealing further diversity and detailed laminar organization within the optic lobe.

We also subdivide the immature neurons within this continuously growing tissue into subtypes defined by evolutionarily conserved developmental genes as well as new cephalopod- and octopus-specific genes.

Together, these results outline the organizational logic of the octopus visual system based on functional determinants, laminar identity, and developmental markers/pathways.

The resulting atlas presented here contains the “parts list” for neural circuits used for vision in octopuses and provides a platform for studies of the development and function of the octopus visual system and the evolution of visual processing.