Oldest Sex Chromosome: Found in Octopus Ancestor 248M

Scientists have discovered the oldest known sex chromosome in animals, which emerged in octopus and squid between 455 million and 248 million years ago. This finding reveals how some cephalopods determine their sex genetically, instead of environmentally.

What are sex chromosomes?

Sex chromosomes are special chromosomes that determine whether an individual becomes male or female. In many animals, including most mammals and some insects, sex chromosomes are X and Y. In humans, females usually have two X sex chromosomes, and males typically have one X and one Y sex chromosome.

However, for some animal groups, such as cephalopods, the soft-bodied mollusks that include squid and octopuses, it was unclear how they decided their sex. Some researchers thought that factors like temperature could play a role, as they do for some reptiles and fish.

How did scientists discover the octopus sex chromosome?

In 2015, researchers sequenced the first cephalopod genome, that of a male California two-spot octopus (Octopus bimaculoides). In the latest study, published in Nature on March 4, 2024 , researchers sequenced the genome of a female California two-spot octopus and found that she had one single chromosome, called chromosome 17, while the male had two copies of it. This suggested that chromosome 17 was a sex chromosome.

The researchers confirmed this hypothesis by sequencing more octopuses and finding that males always had two copies of chromosome 17, while females had one. They also found similar sex chromosomes in some other octopus and squid species, but not in nautiluses, which are more distantly related to octopuses.

What is special about the octopus sex chromosome?

The researchers named the sex chromosome Z, following the convention for birds and butterflies, where males have ZZ sex chromosomes and females have ZW sex chromosomes. However, in octopuses, females do not have a W chromosome, but rather an O chromosome, which denotes the absence of a W chromosome.

The researchers estimated that the Z chromosome emerged in an ancient ancestor of octopuses around 380 million years ago , making it the oldest known sex chromosome in animals. The previous record-holder was a fish sex chromosome that evolved around 200 million years ago .

The discovery of the Z chromosome in octopuses sheds light on the evolution of sex determination in animals and reveals the diversity of mechanisms that different groups use to reproduce.

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