Why do female moles grow testicles?

LiveScience answers that question: because a female mole’s underground life is brutal:

Just like a more typical mammalian ovary, ovotestes nurture and release eggs for fertilization. They also happen to have a lump of testicular tissue stuck to one side.

While it’s not capable of making sperm cells, it does have what are known as Leydig cells for churning out a masculine-sized serving of androgens, or male sex hormones.

“We hypothesized that in moles, there are not only changes in the genes themselves, but particularly in the regulatory regions belonging to these genes,” says geneticist Stefan Mundlos from the Max Planck Institute for Molecular Genetics.

To test this, Mundlos and his colleagues pulled out all of the stops to map out the chromosomal remodeling the Iberian mole (Talpa occidentalis) underwent to modify their ovaries into testosterone factories.

The result is a better understanding of how the mole’s genome has been shuffled around over time in order to deliver a perfectly timed dose of regulatory growth factors.

Specifically they found a region involved with testicular development is flipped, adding extra code to a region that activates the pro-testicular growth factor gene FGF9.

They also found two extra copies of a gene that controls for androgen synthesis

“The triplication appends additional regulatory sequences to the gene – which ultimately leads to an increased production of male sex hormones in the ovotestes of female moles, especially more testosterone,” says lead author, Francisca Martinez Real from the Institute for Medical Genetics and Human Genetics in Germany.

Testing out these changes in transgenic mice resulted in females with similar amounts of androgens as the males, supporting the researchers’ hypothesis of large scale genomic changes being responsible for the testosterone surge.

“Our findings are a good example of how important the three-dimensional organization of the genome is for evolution,” says [Max Planck geneticist Darío] Lupiáñez.

“Nature makes use of the existing toolbox of developmental genes and merely rearranges them to create a characteristic such as intersexuality. In the process, other organ systems and development are not affected.”

You can read the original research in Science.