A groundbreaking study has revealed that octopuses may have the oldest known sex chromosomes in the animal kingdom, dating back more than 480 million years. The research, conducted by scientists at the University of Oregon and published in Current Biology, provides the first genetic evidence of sex determination in cephalopods—solving a long-standing biological mystery.

Unraveling a centuries-old question

Until now, scientists were uncertain whether octopuses even possessed sex chromosomes. Unlike mammals, which rely on X and Y chromosomes to determine sex, cephalopods had only been categorized based on observable traits: individuals that laid eggs were considered female, while those that produced sperm were classified as male. Some researchers even hypothesized that cephalopods might determine sex based on environmental factors, similar to certain reptiles and fish.

However, by sequencing the genome of the California two-spot octopus (Octopus bimaculoides)—the first cephalopod species to have its genome fully mapped—researchers identified a distinct chromosome pair, known as chromosome 17, that differs between males and females.

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A unique genetic discovery

Lead researcher Gabrielle Coffing and her team found that female octopuses possess only a single copy of chromosome 17, while males have a double-Z pair. This discovery marks a stark contrast to the human XY system, where females typically have two X chromosomes and males have one X and one Y.

Further analysis across multiple cephalopod species—including three octopuses, three squid, and a nautilus—confirmed that this Z chromosome system is deeply embedded in their evolutionary history. Notably, the chambered nautilus (Nautilus pompilius), which diverged from octopuses between 520 and 398 million years ago, also displayed signs of the same genetic marker.

The oldest sex chromosome in the animal kingdom?

The findings suggest that cephalopods may possess the longest-lasting sex chromosome system known to science. The study’s evidence indicates that these chromosomes predate even those found in insects, which are estimated to be around 450 million years old.

With this discovery, scientists are not only rewriting the genetic history of octopuses but also shedding light on the evolution of sex determination across the animal kingdom. As further genomic research continues, cephalopods may yet unlock even deeper secrets about their ancient origins and evolutionary resilience.