Discovery of sex chromosomes 248 million years ago in octopuses

Scientists have discovered the oldest known sex chromosomes in octopuses and squids from 455 million to 248 million years ago - 180 million years earlier than the previous record.

Ancient chromosomes have been found in octopuses and squids , suggesting they may have been among the first animals to determine sex through a genetic blueprint, rather than environmental signals.

Octopuses appear to have evolved sex chromosomes at least 248 million years ago. (Photo: Olga Visavi / Shutterstock).

Sex chromosomes are the norm in mammals. In humans, the sex chromosomes are X and Y. Males usually have X and Y chromosomes, while females have two X chromosomes, although some variations, such as XXX or XXY, have can cause many different effects without affecting anything.

For a long time, researchers were unsure whether molluscs, including squid and octopuses, determine their sex using chromosomes. Mollusks have different ways of handling reproduction, including hermaphroditism or sequential hermaphroditism, in which individuals swap sexes over time.

Octopuses have only one sex, but it's unclear whether genes or environmental signals determine that sex. In some reptiles and fish, factors such as temperature determine the sex of the offspring.

In 2015, researchers completed the first full genetic sequence of a cephalopod, the California two-spotted octopus (Octopus bimaculoides) . However, this evidence still needed further validation, so a team led by Andrew Kern, a biologist at the University of Oregon, set out to fill them in using highly precise methods.

They quickly noticed that one chromosome, chromosome 17, appeared to be less jam-packed with genes than the others in their series. Because they sequenced a female octopus, they compared their results with a previous male. In the case of males, chromosome 17 looks no less than other chromosomes in octopuses.

This is a clue that chromosome 17 may be involved in sex differences. To confirm, the team sequenced four more octopuses, two males and two females, and confirmed that females have only one copy of chromosome 17, while males have two. So they represent the sex chromosomes of the octopus which are not XY and XX like in humans but ZZ and Z0.

'This is an astonishingly long time for a sex chromosome to be conserved,' the researchers wrote .

Before this study, the oldest confirmed sex chromosomes were in sturgeon, according to Nature News, at about 180 million years old.