The Vanishing Y Chromosome: Could Humans Evolve a New Sex Gene?

The Future of Men: Could a New Sex Gene Replace the Vanishing Y Chromosome?

The sex of mammals, including humans, has long been determined by the Y chromosome, specifically by a male-determining gene called SRY (Sex-Determining Region Y). This gene is crucial in initiating male development by triggering the formation of testes and subsequent production of male hormones like testosterone. However, recent research suggests that the Y chromosome is slowly degenerating and could eventually disappear in a few million years, potentially threatening the future of men unless a new sex-determining system evolves.

How the Y Chromosome Determines Human Sex

In mammals, the Y chromosome is essential for male development. While females have two X chromosomes, males have one X and one smaller, less gene-rich Y chromosome. The Y chromosome carries around 55 genes—many of which are involved in male development and reproduction, including the SRY gene. This gene initiates the development of the testes during early embryonic stages, which then produce hormones like testosterone, guiding the fetus toward male physical traits. Without the SRY gene, the embryo would develop as a female.

While the X chromosome contains over 900 genes involved in various bodily functions, the Y chromosome is much smaller and contains mostly non-coding DNA, with only a few essential genes. SRY, however, is the most important gene on the Y chromosome, determining the sex of an individual by triggering male-specific pathways, including the activation of the SOX9 gene. SOX9, a critical gene for all vertebrates, continues to be vital in male development, even though it is not located on the sex chromosomes.

The Vanishing Y Chromosome: A Slow Process of Degeneration

Recent evolutionary studies have suggested that the Y chromosome is slowly disappearing. In fact, the Y chromosome of humans and other mammals has been gradually shrinking for millions of years. This is not just a theoretical concern but a real biological trend. In the case of the platypus, an Australian mammal, the X and Y chromosomes are structured much differently than in humans. This raises the possibility that the mammalian XY system was once a pair of ordinary chromosomes that evolved into sex chromosomes over time.

Studies suggest that the Y chromosome has lost approximately 900 genes over the last 166 million years, with the remaining 55 genes slowly diminishing. The estimate is that the last active genes could disappear within the next 11 million years, potentially leading to the extinction of the Y chromosome.

Despite these alarming projections, it's important to note that mammals, including humans, have not reached the brink of Y chromosome extinction. While some speculate that the Y chromosome may last indefinitely, others believe it could disappear in just a few thousand years.

Rodents Without Y Chromosomes

Interestingly, two species of rodents have already lost their Y chromosomes and continue to thrive. The mole voles in Eastern Europe and the spiny rats of Japan have both evolved without the Y chromosome. Despite this, these species continue to reproduce, which raises the question: Could humans also evolve a new method for determining sex?

The discovery that these rodents could survive without the Y chromosome has opened up new possibilities for understanding how mammals might adapt to the loss of this critical sex chromosome.

One of the most exciting recent breakthroughs comes from a study led by biologist Asato Kuroiwa at Hokkaido University. In 2022, Kuroiwa's team discovered how the spiny rat—a species found in three isolated Japanese islands—has replaced its Y chromosome with a new, previously unknown system for male determination. Their research, published in Proceedings of the National Academy of Sciences, found that the spiny rats have a small genetic duplication near the SOX9 gene. This duplication, located on chromosome 3, triggers the same male-determining processes as the SRY gene, thus allowing males to be determined without the Y chromosome.

What This Means for the Future of Humans

The research into the loss of the Y chromosome in rodents presents an intriguing possibility for humans: We may not be doomed to extinction if the Y chromosome disappears. Rather than being a dead-end for our species, the disappearance of the Y chromosome may simply be a part of evolutionary change. In fact, the discovery that spiny rats have developed an alternative male-determining system suggests that humans, too, might evolve a new way of determining sex.

However, there are significant risks. Evolution of a new sex-determining system could lead to the emergence of separate human species, similar to the way the mole voles and spiny rats have diverged into different species due to their differing sex determination mechanisms. This "war" of sex genes could lead to a reproductive barrier, eventually separating populations and resulting in speciation.

In an even more dramatic scenario, the disappearance of the Y chromosome might lead to a complete shift in the biological processes that currently define human reproduction. If that happens, humans may eventually evolve into a completely different species over millions of years.

Conclusion: The Future of Male Determination

While the Y chromosome's eventual disappearance is still a long way off, the recent discoveries in rodent species suggest that humans might not be as doomed as previously thought. The possibility of evolving a new sex-determining system provides hope for our future, but it also opens up a host of questions. As the study of the Y chromosome continues, we may find that human evolution has a far more flexible future than we ever imagined.

Learn more: https://www.pnas.org/doi/10.1073/pnas.2211574119