Oldest Animal Sex Chromosome Evolved in Octopuses 380 Million Years Ago, Study Reveals

Octopuses stand out among cephalopods and other invertebrates due to their strange characteristics. Their mating process is also unique, involving the spermatophore's deposition to the female body using the male's hectocotylus.

Studies on octopuses' structures and abilities also shed light on evolutionary changes in the animal kingdom. Recently, experts discovered these animals' role in understanding the ancient origins of sex chromosomes.

Sex Determination

In most animals, such as mammals and some insects, sex chromosomes determine whether an organism will become a male or a female. In humans, females have two X sex chromosomes, while males typically have one X and one Y sex chromosome.

For some groups of animals, sex is determined differently. For instance, scientists have been unsure how cephalopods, including soft-bodied animals (squids and octopuses) and hard-shelled creatures (nautilus), become male or female. Experts generally thought that environmental factors like temperature play a vital role, as they do for some fish and reptiles.

Revolutionary Discovery on Cephalopod Genome

In 2015, scientists sequenced a cephalopod genome for the first time using a male California two-spot octopus (Octopus bimaculoides). Led by Caroline B. Albertin of the University of Chicago, they tried to explore the molecular bases of cephalopod brain and body innovations. The result of their study is discussed in the paper "The Octopus Genome and the Evolution of Cephalopod Neural and Morphological Novelties."

Albertin and her colleagues identified hundreds of cephalopod-specific genes which showed elevated expression levels in specialized structures like the suckers, skin, and the nervous system. They also discovered evidence for large-scale genomic rearrangements closely related to transposable element expansions. The analysis of the researchers suggests that substantial expansion of gene families played a vital role in the evolution of cephalopod morphological changes.

In a separate study, Gabrielle C. Coffing from the University of Oregon led a team of researchers in mapping the genome of a female California two-spot octopus. They found 29 pairs of chromosomes and a single chromosome called chromosome 17. On the other hand, the male octopus genome only had two copies of chromosome 17. This led the researchers to conclude that chromosome 17 was a sex chromosome.

This assumption was confirmed when scientists sequenced the DNA of other Octopus bimaculoides. This means that males always had two copies of chromosome 17, while females had only one.

Chromosome 17 was also found to contain several genes similar to those that encode proteins in human reproductive tissues, such as a protein found in sperm. In animals like birds and butterflies, males have two Z sex chromosomes, while females have one Z and one W sex chromosome.

Since the researchers failed to find a W chromosome in the female octopuses, this suggests that males have ZZ sex chromosomes, while females have ZO, with the O indicating the lack of a W chromosome. The researchers also found Z chromosomes in other octopus and squid species but not in a nautilus.

The pattern suggests that the Z chromosome once evolved in the lineage, resulting in modern squid and octopuses after slitting off from hard-shelled nautiloids. This means the Z chromosome first appeared 450 and 250 million years ago and has been retained in modern times.