Why the Y chromosome will survive
A new Y chromosome hypothesis explains why the ‘wimpy’ sex chromosome has evaded extinction – by being its own executioner.
The Y chromosome is present in only a handful of mammalian species, and is no stranger to the adjective ‘wimpy’. Given how much it has shrunk over 200 million years of evolution, it has been a mystery why the sex chromosome has avoided extinction. A new theory – the ‘persistent Y hypothesis’ – could explain this.
Previously, its continued survival was put down to it being important in sex determination and sperm production; however, these functions could be transposed elsewhere in the genome, which would have been its death knell. Instead, the new theory proposes that its survival is down to the presence of self-regulating executioner genes that are critical for male meiosis, and difficult to move.
Successful meiosis requires the silencing of both the X and Y chromosomes during a particular window of time. Genes on the Y chromosome regulate this process and could only be heritably transposed to the X chromosome – an unlikely evolutionary event. “We believe that bearing these genes is what protects the Y chromosome from extinction,” explained co-author Aurora Ruiz-Herrera (Universitat Autònoma de Barcelona, Spain). “The genes that regulate the silencing process, the ZFY genes, are called ‘executioner’ genes. 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.”
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The Y chromosome hypothesis was born of the pandemic – resulting from a decision to take advantage of lab shutdowns. “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,” revealed co-author Paul Waters (University of New South Wales, Sydney, Australia).
Next, the duo will delve further into the executioner genes, investigating how they evolved and are regulated.
“The mammalian Y has been taken as a symbol of masculinity, not only in popular culture but also in the scientific community,” commented Ruiz-Herrera. “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!”