The Earliest Known Animal Sex Chromosome is 480 Million Years Old

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.