Octopuses Have a Hidden Sex Chromosome That Predates Dinosaurs

University of Oregon researchers have discovered a sex chromosome in the California two-spot octopus that likely dates back 480 million years—long before octopuses and nautiluses diverged. This makes it one of the oldest known animal sex chromosomes.

The finding confirms that octopuses and other cephalopods, including squid and nautiluses, use chromosomes to determine sex, solving a long-standing biological mystery.

Unlocking the Secrets of Cephalopod Sex Determination

“Cephalopods are already fascinating creatures, and we’re still uncovering so much about them—especially in neuroscience,” said Gabby Coffing, a UO doctoral student in biologist Andrew Kern’s lab. “Now, we’ve found another intriguing trait: their sex chromosomes are incredibly ancient.”

Coffing, Kern, and their team published their findings in Current Biology on February 3.

How Different Species Determine Sex

In most mammals, sex is determined by chromosomes, but animals use a wide range of mechanisms. Turtles rely on egg incubation temperature, while some fish use a single gene rather than a full chromosome. Even in humans, the X/Y system isn’t always clear-cut—gene mutations and extra sex chromosomes can complicate sex determination.

Because cephalopods aren’t common lab animals like mice or fruit flies, their genetics remain largely unexplored. Though scientists have sequenced a few octopus genomes, linking genes to traits remains difficult.

A Surprising Genetic Find

While sequencing a female California two-spot octopus, UO researchers noticed a chromosome with only half the usual genetic material—something not seen in previously sequenced male octopuses.

“This chromosome had half the sequencing data, meaning there was only one copy,” Coffing explained. “That led us to realize we had found a sex chromosome.”

To confirm, the team examined existing octopus genomic data. They found the same half-sized chromosome in another octopus species, as well as in squid, which diverged from octopuses 248–455 million years ago. Even more striking, they identified it in nautiluses, which split from octopuses about 480 million years ago.

This suggests their common ancestor already used this sex determination system. Unlike many sex chromosomes, which evolve rapidly due to reproductive pressures, cephalopods seem to have stuck with the same system for nearly half a billion years.

The researchers initially suspected octopuses might follow a system like birds and butterflies, where males are ZZ and females are ZW. However, they haven’t yet identified a W chromosome. It’s possible octopuses use a simpler system where males have two copies of the Z chromosome while females have only one.

For now, the octopus continues to keep some of its secrets.

Read Original Article: Scitechdailly

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