There is a long-held assumption that in mammals, female development is the default option. In other words, embryos will develop into females unless certain male factors are present. In particular, an embryo must have the Sry gene, which is located on the Y chromosome present only in males. Females have two X chromosomes, whereas males have an X and a Y.
This still holds true for embryonic development, but a new study led by Mathias Treier at the European Molecular Biology Laboratory (EMBL) shows that the story is different in adults.
They discovered a gene (Foxl2) on a non-sex chromosome (not the X or Y) that is required to keep adult females from becoming males. If this gene is turned off in adult female mice, the male pathway is no longer suppressed and the mouse develops sperm and male hormone-producing cells.
The Foxl2 gene is present in most vertebrates, and thus predates the mammalian Y chromosome. Therefore, there may have been a time when male development was the default option.
You can watch Dr. Treier explain his findings here.
Or read Ed Yong’s description on his Not Exactly Rocket Science blog.