Suppressing estrogen signaling by a Y-linked nuclear receptor determines sex in Bombina toads

Every living creature with a backbone needs a way to decide whether to become male or female. In humans and many other animals, this decision is written into a pair of chromosomes that act like a genetic switch. But in the vast, ancient world of amphibians, this switch has remained a mystery. For over a century, scientists have known that frogs and toads have sex chromosomes, yet they have been unable to find the specific gene that flips the switch. This is partly because amphibian genomes are enormous and messy, and their chromosomes often look identical under a microscope, making it nearly impossible to spot the tiny genetic difference that determines sex. Without knowing the master switch, we cannot fully understand how these animals develop or how new species evolve.

A team of researchers has finally cracked this code for a group of European and Asian toads known as Bombina. By building high-quality maps of the toads' entire genetic code, they discovered that a specific gene on the male's Y chromosome acts as the master switch. This gene works by silencing a chemical signal that would otherwise turn the animal into a female. The discovery reveals a new way that nature can determine sex, one that relies on blocking a hormone pathway rather than activating a male one. This finding not only solves a long-standing puzzle in amphibian biology but also provides a powerful tool for studying how these toads split into different species.

The researchers focused on the fire-bellied toad and its close relatives, which are famous for living in distinct habitats and forming narrow zones where they interbreed. To find the sex-determining gene, the team first created a detailed reference genome for the fire-bellied toad, a massive task given the size of its DNA. They then compared the DNA of male and female toads from a large family tree, looking for any genetic markers that appeared only in males. They found a small region on chromosome two that was present in males but missing in females. This region contained a gene called err1Y, which is a copy of a gene found on a different chromosome in both sexes.

The original version of this gene, found on an ordinary chromosome, is involved in regulating energy and metabolism. However, the new copy, err1Y, had changed significantly. It was located on the Y chromosome, meaning it is passed down only from father to son. The researchers found that this new gene is active specifically in the developing testes of young male toads, just as they are beginning to differentiate. In females, this gene is completely absent. This pattern strongly suggests that err1Y is the master switch that tells the developing gonad to become a testis.

To understand how this gene works, the team looked closely at its structure and function. They discovered that the err1Y gene produces a shortened version of a protein. Unlike the original protein, which can bind to DNA and turn on genes, this shortened version lacks the part needed to attach to DNA. Instead, it acts as a blocker. In the body, there is a natural pathway driven by estrogen that encourages the development of ovaries. The shortened err1Y protein binds to the receptors that normally carry out this estrogen signal, effectively jamming the mechanism. By blocking the estrogen signal, the gene prevents the gonad from becoming female, allowing it to default to becoming a male.

The researchers confirmed this mechanism through a series of experiments. They showed that when they introduced the shortened protein into cells, it successfully stopped the estrogen pathway from working. They also mapped the gene across different species of Bombina toads. They found that the err1Y gene is present and functional in all the species with 24 chromosomes, including the fire-bellied toad and the yellow-bellied toad. However, in species with 28 chromosomes, the gene is broken and no longer functions as a sex switch. This indicates that the 24-chromosome toads share a common ancestor where this gene was first recruited to determine sex, and it has been passed down ever since.

This discovery is significant because it reveals a new route to sex determination in poikilothermic (cold-blooded) vertebrates. In many animals, a master gene works by turning on a male-specific program. In these toads, the master gene works by turning off the female program. It acts as a brake on the estrogen pathway, ensuring that the animal develops as a male. This mechanism represents a novel strategy for vertebrate sex determination through the re-purposing of a duplicated nuclear receptor, a route not previously seen in this specific form. The findings also provide a clear genetic marker that can be used to identify the sex of these toads at any stage of life, which is crucial for conservation efforts and for studying how these species interact in the wild.

The ability to identify the sex of these toads genetically opens new doors for understanding how species form. The Bombina toads are a classic example of speciation, where two distinct species live side by side and occasionally mix. By tracking the Y-linked gene, scientists can now study how sex chromosomes influence the process of speciation and whether they play a special role in keeping species separate. The discovery of err1Y transforms our understanding of amphibian biology, showing that even in the most complex and ancient genomes, nature can find simple and elegant solutions to the fundamental question of how to become male or female.