The factors that influence sexual development across the animal kingdom come in many different forms. Sometimes temperature plays a determining role. Other times it’s chromosomes, whose interactions can be far more complex than the relatively simple combinations of X and Y typical in mammals. Now, scientists studying the octopus genome have finally shed some light on when chromosomes became a determining factor for sex in animals. In 2015, researchers reported that male California two-spot octopuses possess two copies of a chromosome known as the Z chromosome, whereas females have a single copy. In a preprint published last month, a different team found that the Z chromosome originated between 455 million and 248 million years ago. Previously, sturgeons were thought to be the animal with the oldest known animal sex chromosome, but the new discovery suggests octopuses have them beat by at least 70 million years. These sex-determining chromosomes may not just be the oldest known in the animal kingdom—they might also be the oldest among plants, too, Nature reports.
The male two-spot has 29 pairs of chromosomes (no, having six more than humans does not mean one for each extra arm). However, the female had only one copy of chromosome 17 rather than two. Expanding the sample confirmed the team had not accidentally picked an unusual specimen – female O. bimaculoides only have one copy of chromosome 17.
If this wasn’t a big enough hint that this is the octopus’s sex chromosome, some of the proteins coded for by genes on chromosome 17 resembled those in human sperm and other parts of the mammalian reproductive system.
While in mammals (monotremes aside) it is females that have two copies of the same sex chromosome – the X – birds, certain reptiles, and some insects reverse this. In these animals, males get two Z chromosomes, while females get a Z and a W.
“It very much looked like we were looking at a Z chromosome in O. bimaculoides,” Kern said, but there was no W to go with it in females. Such a system is not unprecedented – butterflies and moths have something similar, as do some plants. Geneticists refer to it as ZO sex determination.
The question the Oregon University team wanted to answer was whether this was something specific to two-spot octopuses, and if not, how far it is spread through the cephalopod class. They tested three other species of octopus and each also used ZO sex determination. So, it turns out, do three squid species, indicating it is very old, dating back at least to the last common ancestor of these two orders. Vampire squid and cuttlefish were not included, but given the timing of their divergences from the studied species, it seems likely they use the same system.
This makes the system at least 248 million years old. On the other hand, chambered nautiluses, who broke away 455 million years ago, don’t carry this Z chromosome, so it probably evolved more recently than that. The oldest surviving sex determination system previously found is in sturgeon fish, thought to have appeared 180 million years ago.
Sex determination might be expected to be one of the most stable parts of the genome. After all, if one member of a population changes, perhaps by mutation, there are obvious problems in reproducing with those using the old system. Despite this, we know from differences between closely related species that some animal families change system quite often. Cephalopods, however, seem to be happy with what they’ve got – or in the case of the females, what they haven’t.
