Transcaucasian mole vole. Image Courtesy of Heike HimmelreichOne of my favorites is a little burrowing mammal called a Transcaucasian mole vole. These guys live in the Caucasus Mountains of Armenia, Iran, Turkey, and Azerbaijan. There they are born, live, have babies and die. All without a Y chromosome.

This is really bizarre. In most mammals, two X chromosomes usually means that the animal is female and an X and a Y means the animal is male. All mole voles have a single X chromosome. So technically, there shouldn’t be any males running around. And yet, clearly, there are.

So what distinguishes a boy mole vole from a girl mole vole genetically? No one really knows.

In most mammals, the Y chromosome causes a fertilized egg to turn into a male because of the SRY gene. This gene starts a cascade of events that eventually results in a male.

One possibility would be if the SRY gene happened to move to another chromosome. There are certainly cases of this happening even in humans.

If this were the case, then maybe a different chromosome has the SRY gene in mole voles. Maybe there are versions of the gene that work and versions that don't. Now we have a gene no different than an eye or hair color gene.

Good model but it isn't true. Scientists have looked but it appears that these little guys don't have an SRY gene. They make the male/female decision in a completely different way.

Most likely somewhere along the way a gene mutated so that it could now determine the sex of these mammals. When this happened, the loss of the Y didn't matter much and so it was lost. The mole vole evolved into a Y-less mammal.

Of course, if any chromosome had to go it would be the Y. It has been under constant attack ever since it distinguished itself from the X chromosome 200 or 300 million years ago. It has gone from being one of the biggest chromosomes with 900-1400 genes to a bit of DNA with around 80 genes.

There are even active discussions about whether the Y is on a death spiral in all mammals. Soon we may all be mole voles. Or be gone. Some of my recent posts elsewhere on this topic:

Males going extinct?
Fish that change gender