The mole voles of eastern Europe and the spiny rats of Japan have some species in which the Y chromosome and SRY (sex-determining region Y protein), have completely disappeared.

The X chromosome remains in a single or double dose in both sexes.

A team led by Hokkaido University biologist Asato Kuroiwa found how the change operates in the spiny rat – a group of three species on different Japanese islands, all endangered.

The team found a small bit of duplicated DNA that contains the switch that normally turns on SOX9 in response to SRY. The SOX9 protein plays a key role in the determination of sex before birth.

When the researchers introduced this duplication into mice, they found that it boosts SOX9 activity. So this change in DNA could allow SOX9 to work without SRY.