Oldest Sex Chromosome Found in Octopus Ancestor, 248M Years

Scientists have discovered the oldest known sex chromosome in animals, the octopus Z chromosome, which first evolved in an ancient ancestor of octopuses 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 common. For example, humans have X and Y chromosomes. 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, which include soft-bodied animals such as squids and octopuses, as well as hard-shelled creatures called nautiluses, researchers have been unsure about how individuals become male or female. Scientists generally thought that environmental factors such as temperature play a part — as they do for some reptiles and fish.

How did scientists discover the octopus Z chromosome?

In 2015, researchers reported sequencing a cephalopod genome for the first time — that of a male California two-spot octopus, Octopus bimaculoides. In the latest study, published in Nature on March 4, 2024, researchers mapped the genome of a female California two-spot octopus. They discovered 29 pairs of chromosomes and one single chromosome, called chromosome 17. By contrast, the male octopus genome had two copies of chromosome 17. That difference led the researchers to hypothesize that chromosome 17 was a sex chromosome.

Sequencing the DNA of other O. bimaculoides octopuses confirmed the idea. Males always had two copies of chromosome 17, whereas females had one copy. Chromosome 17 also contained several genes similar to those that encode proteins in human reproductive tissues, including a protein found in sperm.

How does the octopus Z chromosome work?

In animals including birds and butterflies, males similarly have two Z sex chromosomes, whereas females have one Z and one W sex chromosome. “It very much looked like we were looking at a Z chromosome in O. bimaculoides,” said evolutionary geneticist Andrew Kern at the University of Oregon in Eugene, who led the study.

But the researchers failed to find a W chromosome in the female octopuses. That suggested that males have ZZ sex chromosomes, whereas females are ZO, with the O denoting the lack of a W chromosome.

How old is the octopus Z chromosome?

The team also found Z chromosomes in some other octopus and squid species — but not in a nautilus. By comparing the genomes of different cephalopods, they estimated that the Z chromosome first emerged in an ancient ancestor of octopuses between 455 million and 248 million years ago — making it the oldest known sex chromosome in animals.

Why is this discovery important?

The discovery sheds light on the evolution of sex determination in cephalopods and other animals, and opens up new avenues for research on how sex chromosomes influence development, behavior and adaptation.

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