Sex chromosome discovered in octopuses is 480 million years old

Octopuses have long fascinated scientists with their intelligence, camouflage abilities, and unique biology.

These remarkable creatures continue to surprise researchers, revealing new secrets about their evolutionary past. One of the most intriguing mysteries has been the question of how octopuses determine their sex.

Scientists at the University of Oregon recently identified a sex chromosome in the California two-spot octopus. This discovery confirms that cephalopods, a group that includes squid and nautiluses, rely on chromosomes for sex determination.

The research also suggests that this chromosome has existed for at least 480 million years, making it one of the oldest known sex chromosomes in the animal kingdom.

Sex chromosome in octopuses

“Cephalopods are already such interesting creatures, and there are so many things we’re still learning about them, especially in neuroscience,” said Gabby Coffing, a doctoral student at the University of Oregon.

“This is just showing one more interesting thing about them: They have really ancient sex chromosomes.”

The findings, published in the journal Current Biology, provide new insights into the genetic history of octopuses and their relatives. Understanding this ancient sex determination system could help scientists uncover more about how these intelligent creatures evolved.

Sex determination varies across species

Sex determination is not the same for all animals. While humans and most mammals rely on the X/Y chromosome system, other species follow different methods.

In turtles, sex depends on the temperature at which the eggs incubate. Some fish determine sex using a single gene rather than a whole chromosome.

Even in humans, sex chromosomes are not always straightforward. Gene mutations or extra sex chromosomes can lead to biological variations that do not fit into a simple male or female category.

Cephalopods have long posed a challenge for genetic research. They are not standard laboratory animals like mice or fruit flies, making it difficult for scientists to study their genetics in depth.

Although researchers have sequenced the genomes of some octopus species, they still struggle to link specific genes to particular traits. The discovery of an octopus sex chromosome marks a major breakthrough in understanding how these animals develop and reproduce.

Unique sex chromosome in female octopuses

The research team at the University of Oregon made an unexpected discovery when sequencing the DNA of a female California two-spot octopus.

Among its 30 chromosomes, one stood out because it contained only half the usual amount of genetic material. This anomaly had not appeared in previous studies of male octopus DNA.

“This particular chromosome had half the amount of sequencing data, which indicated there was only one copy,” Coffing explained. “Then as we explored that more, we reached the conclusion that we must have stumbled upon a sex chromosome.”

This finding suggested that female octopuses carry a unique chromosome that males do not. The scientists needed to confirm their discovery by comparing it with data from other cephalopods.

If this chromosome appeared in multiple species, it would indicate an ancient and consistent pattern of sex determination.

Sex chromosome exists in other cephalopods

The team analyzed genomic data from other octopus species and found similar evidence of this unique chromosome.

The experts also detected the sex chromosome in squid, a species that diverged from octopuses between 248 and 455 million years ago. More surprisingly, they identified traces of it in the nautilus, a mollusk that split from the octopus lineage approximately 480 million years ago.

“This indicates that their common ancestor had this similar sex determination system,” Coffing said.

Most sex chromosomes change rapidly over time because they play a crucial role in reproduction. However, this cephalopod sex chromosome appears to have remained stable for hundreds of millions of years.

This suggests that octopuses and their relatives have relied on this system without significant evolutionary changes.

Comparing ancient sex chromosomes

While the octopus sex chromosome is one of the oldest known, other ancient sex chromosomes exist in nature.

Some plants, like mosses and liverworts, evolved their sex chromosomes early in Earth’s history. Insects also have sex chromosomes that date back about 450 million years, though these have changed considerably over time.

The discovery in octopuses challenges the idea that sex chromosomes always evolve quickly. It suggests that some systems remain stable if they continue to work effectively over long periods. Cephalopods seem to have found a genetic formula for sex determination that has stood the test of time.

Mystery of the missing W chromosome

One question remains unanswered: how exactly does the sex determination system work in octopuses? Scientists initially suspected that octopuses might have a system similar to birds and butterflies, where males have two identical chromosomes (ZZ) and females have one of each (ZW).

Kern and his team searched for a W chromosome in female octopuses but didn’t find one. This leaves open the possibility that octopuses follow a different system in which males have two copies of a single chromosome (ZZ), while females have only one (Z).

Researchers still need more evidence to confirm how octopus sex chromosomes function. While this study has solved one piece of the puzzle, many questions remain.

Secrets of the octopus

Octopuses continue to be one of the most enigmatic creatures in the ocean. Their intelligence, problem-solving skills, and unique genetics set them apart from most other animals.

The discovery of an ancient sex chromosome adds another layer to their evolutionary story. For now, researchers know that this octopus sex chromosome has been around for nearly half a billion years. However, the exact mechanics of how it works remain uncertain.

As scientists continue to explore cephalopod genetics, they will uncover more about how these fascinating animals developed their extraordinary traits.

The octopus has revealed one of its secrets, but it still holds on to many more.

The study is published in the journal Current Biology.

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