New theory shows why Y chromosome may be more resilient than previously believed

The Y chromosome, which is relatively smaller than its counterpart, the X chromosome, has significantly reduced in size across 200 million years of evolution.

Even scientists who analyze the Y chromosome have used the term “wimpy” to denote it but despite this fact, it still continues to stick around, and added to this, sex chromosomes found in non-mammalian vertebrates experience quite a bit of evolutionarily changeover.

An opinion paper published in the Trends in Genetics journal on August 6^th, 2020, has outlined a novel concept, known as the “persistent Y hypothesis,” to elucidate why the Y chromosome might be stronger than it initially appears.

The Y chromosome is generally thought to be protected from extinction by having important functions in sex determination and sperm production, which, if moved to somewhere else in the genome, would signal its demise. However, we propose that the future of the Y chromosome is secure because it carries executioner genes that are critical for successful progression of male meiosis—and unlike other genes on the Y, these executioners self-regulate.”

Paul Waters, Study Co-Author and Professor, University of New South Wales

At the time of meiosis, haploid gametes (sperm and eggs) are formed by sexually reproducing organisms. Each of these gametes contains just a single copy of each chromosome. These organisms perform this activity through one round of genome replication and then through two successive rounds of the division of cells. This process of meiosis is closely controlled to prevent chromosome abnormalities and infertility.

In one meiotic step, both the X and Y chromosomes need to be silenced during a particular window.

Importantly, the Y chromosome bears genes that regulate this process, a feature that has been known for years now. We believe that bearing these genes is what protects the Y chromosome from extinction. The genes that regulate the silencing process, the Zfy genes, are called ‘executioner’ genes.”

Aurora Ruiz-Herrera, Study Co-Author and Professor, The Autonomous University of Barcelona

Ruiz-Herrera continued, “When these genes are turned on at the wrong time and at the wrong place during meiosis, they are toxic and execute the developing sperm cell. They essentially act as their own judge, jury, and executioner, and in doing so, protect the Y from being lost.”

The Y chromosome exists in all except a few numbers of mammalian species. By observing the rare mammals that do not follow the guidelines, scientists were able to gain a crucial understanding of the Y chromosome—for instance, a handful of rodent species.

Waters added, “I’ve always been a firm believer that the comparison of unusual systems is informative to other systems. Determining the common prerequisites for rare Y chromosome loss enabled us to build a hypothesis for how Y chromosomes persist in most species.”

The association between Waters and Ruiz-Herrera—who are based half a world apart—proved fruitful during the current COVID-19 pandemic.

Earlier this year, we put together a grant application to examine aspects of X chromosome silencing during meiosis. After the shutdown of our labs, we decided to massage our discussions into a review article. We had no idea we would essentially stumble onto such an intuitive mechanism to explain why the mammal Y chromosome has persisted in most species.”

Paul Waters, Study Co-Author and Professor, University of New South Wales

Both the scientists have now planned to closely study the evolution of the executioner genes and explore how they are controlled from functional and evolutionary viewpoints.

“The mammalian Y has been taken as a symbol of masculinity, not only in popular culture but also in the scientific community. Despite that, many have projected that, given enough time, it will be eventually lost. However, we propose the Y chromosome can escape this fatal fate. So our male colleagues can breathe easy: the Y will persist!,” Ruiz-Herrera concluded.