The octopus sex chromosome appears to have been maintained over hundreds of millions of years, making it the most ancient of such chromosomes in animals

Sex is determined in a variety of different ways throughout the animal kingdom. In mammals, it is determined genetically, not by temperature, as it can be in reptiles. Typically, females have two X chromosomes while males have an X and a Y. In birds and butterflies, males have two Z chromosomes, while females have one Z and one W chromosome.

Now researchers have discovered that octopuses determine sex with Z chromosomes: males have two Z chromosomes, and females have just one. Astonishingly, this cephalopod Z chromosome originated more than 480 million years ago, making it the most ancient animal sex chromosome known. The findings were posted on the preprint server bioRxiv.