A groundbreaking study has uncovered that octopuses may possess the oldest known sex chromosomes among animals, dating back over 480 million years. The research, conducted by researchers at the University of Oregon and published in Current Biology, provides the first genetic evidence of sex determination in cephalopods.

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Longstanding mystery  

Until now, researchers were uncertain whether octopuses even had sex chromosomes. Unlike mammals, whose X and Y chromosomes determine sex, scientists had previously relied on observational methods—distinguishing between individuals that laid eggs (females) and those that produced sperm (males). Some even speculated that cephalopods might determine sex based on environmental factors, similar to certain fish and reptiles.  

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

Unique sex-determination system  

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Lead researcher Gabrielle Coffing and her team discovered that female octopuses possess only a single copy of this chromosome, while males have a double-Z pair. This contrasts with the human XY system, where females typically have two X chromosomes, and males have one X and one Y.  

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Further analysis across multiple cephalopod species, including three octopuses, three squid, and a nautilus, confirmed that this Z chromosome system is deeply rooted in their evolutionary lineage. The chambered nautilus, which diverged from octopuses between 520 and 398 million years ago, also exhibited signs of the same genetic marker. This suggests that cephalopods may have the longest-lasting sex chromosome known in the animal kingdom—predating even insects, whose sex chromosomes are estimated to be around 450 million years old.