Groundbreaking research on one of nature’s most fascinating creatures

Large, scrambled genomes. Tragic deaths. Tragic deaths.

Changes in cholesterol production lead to octopus death spiral

BY MATT WOOD

For years, scientists have pointed to the octopus’ optic gland as the source of its bizarre maternal behavior: After a mother octopus lays a clutch of eggs, she quits eating and wastes away; by the time the eggs hatch, she is dead. Some females in captivity even seem to speed up this process intentionally, mutilating themselves and twisting their arms into a tangled mess.

Just how this gland triggered the gruesome death spiral was unclear, but a recent study by researchers from the University of Chicago, the University of Washington and the University of Illinois Chicago shows that the optic gland in maternal octopuses undergoes a massive shift in cholesterol metabolism, resulting in dramatic changes in the steroid hormones produced.

Squid and octopus genome studies reveal how cephalopods’ unique traits evolved

BY DIANA KENNEY, MARINE BIOLOGICAL LABORATORY

Researchers unlock secrets of the octopus.

PHOTO BY TOM KLEINDINST

Squid, octopus and cuttlefish—even to scientists who study them—are wonderfully weird creatures. Known as the soft-bodied or coleoid cephalopods, they have the largest nervous system of any invertebrate, complex behaviors such as instantaneous camouflage, arms studded with dexterous suckers and other evolutionarily unique traits.

Now, scientists have dug into the cephalopod genome to understand how these unusual animals came to be. Along the way, they discovered cephalopod genomes are as weird as the animals are. Scientists from the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts; the University of Chicago; and other institutions reported their findings in two new studies in Nature Communications.

Alterations in cholesterol metabolism in other animals, including humans, can have serious consequences for longevity and behavior, and the study’s authors believe this reveals important similarities in the functions of these steroids across the animal kingdom, in soft-bodied cephalopods and vertebrates alike.

“We know cholesterol is important from a dietary perspective and within different signaling systems in the body, too,” said Z. Yan Wang, PhD’18, Assistant Professor of Psychology and Biology at the University of Washington and lead author. “It’s involved in everything from the flexibility of cell membranes to production of stress hormones, but it was a big surprise to see it play a part in this life-cycle process as well.”

Self-destruct hormones

In 2018, Wang, then a graduate student, and Clifton Ragsdale, PhD, Professor in the Departments of Neurobiology and of Organismal Biology and Anatomy, sequenced the RNA transcriptome of the optic gland from several California two-spot octopuses at different stages of their maternal decline. As the animals began to fast and decline, there were higher levels of activity in genes that metabolize cholesterol and produce steroids, the first time the optic gland had been linked to something other than peptide hormones.

In the new paper, published in May in Current Biology, the researchers analyzed the chemicals produced by the maternal optic gland. They found the optic gland undergoes dramatic changes to produce more pregnenolone and progesterone, maternal cholestanoids, and 7-dehydrocholesterol (a precursor to cholesterol) during the stages of decline. While the pregnancy hormones are to be expected, this is the first time anything like the components for bile acids or cholesterol have been linked to the maternal octopus death spiral.

Wang’s next research focus is the lesser Pacific striped octopus, which doesn’t self-destruct after breeding. “The optic gland exists in all other soft-bodied cephalopods, and they have such divergent reproductive strategies,” she said. “It’s such a tiny gland and it’s underappreciated. I think it’s going to be exciting to explore how it contributes to such a great diversity of life history trajectories in cephalopods.”

New research finds remarkable similarities in steroid hormone biology across cephalopods, mice and humans that can have dire consequences when disrupted.

Z. Yan Wang, PhD’18, in the lab at the Marine Biological Laboratory, where she conducted research this past summer on a Grass Fellowship.

PHOTO COURTESY OF THE MARINE BIOLOGICAL LABORATORY

“Large and elaborate brains have evolved a couple of times,” said co-lead author Caroline Albertin, PhD’16, Hibbitt Early Career Fellow at the MBL. “One famous example is the vertebrates. The other is the soft-bodied cephalopods, which serve as a separate example for how a large and complicated nervous system can be put together. By understanding the cephalopod genome, we can gain insight into the genes that are important in setting up the nervous system, as well as into neuronal function.”

The team analyzed and compared the genomes of three cephalopod species—two squids (Doryteuthis pealeii and Euprymna scolopes) and an octopus (Octopus bimaculoides).

Sequencing these three cephalopod genomes, never mind comparing them, was a tour de force funded by the Grass Foundation that took place over several years in labs around the world. “Probably the greatest advance in this new work is providing chromosomal-level assemblies of no less than three cephalopod genomes, all of which are available for study at the MBL,” said co-author Clifton Ragsdale, PhD, Professor in the Departments of Neurobiology and of Organismal Biology and Anatomy at the University of Chicago.

In the end, comparing the genomes led the scientists to conclude that evolution of novel traits in soft-bodied cephalopods is mediated, in part, by three factors: ■ Massive reorganization of the cephalopod genome early in evolution. Strikingly, the cephalopod genome “is incredibly churned up,” Albertin said. ■ Expansion of particular gene families. ■ Large-scale editing of messenger

RNA molecules, especially in nervous system tissues.

Key insights into cephalopod genomes provided by the studies include:

They’re large. The genome of the squid studied is 1.5 times larger than the human genome, and the octopus genome is 90% the size of a human’s.

They’re scrambled. “Key events in vertebrate evolution, leading to humans, include two rounds of whole-genome duplication,” Ragsdale said. “With this new work, we now know that the evolution of soft-bodied cephalopods involved similarly massive genome changes. But the changes are not whole-genome duplications but rather immense genome rearrangements, as if the ancestral genomes were put in a blender.”

PHOTO COURTESY OF THE MARINE BIOLOGICAL LABORATORY

They contain novel gene families. The team identified hundreds of genes in novel gene families that are unique to cephalopods.

In a related study, the team explored how the highly reorganized genome in Euprymna scolopes affects gene expression.