At the heart of mammalian male sex determination lies a gene on the Y chromosome. However, the Y chromosome in humans is deteriorating and might vanish within a few million years, potentially threatening human survival unless a new sex gene emerges.
Fortunately, two rodent lineages have already survived the loss of their Y chromosomes, providing hope for alternative solutions.
A study published in Proceedings of the National Academy of Sciences illustrates how the spiny rat has adapted a novel male-determining gene.
Human Sex Determination and the Role of the Y Chromosome
In both humans and other mammals, females typically possess two X chromosomes, while males have an X and a smaller Y chromosome. The size or shape difference didn’t influence the naming; instead, ‘X’ denoted ‘unknown.’
While the X chromosome boasts around 900 genes performing various functions, the Y chromosome contains only about 55 genes and is filled with repetitive non-coding DNA that appears inert.
Despite its smaller genetic load, the Y chromosome carries a critical gene that initiates male development early in the embryonic phase.
This gene, identified in 1990 as SRY (sex region on the Y), activates other genes that regulate the growth of testes at about 12 weeks after conception, triggering testosterone production and male development.
The Vanishing Chromosome
Most mammals have X and Y chromosomes functioning similarly to ours. However, the imbalance of X chromosome genes between males and females presents a problem.
The peculiar aspect of this system’s evolution is highlighted by the unique sex chromosomes found in Australia’s platypus, mirroring bird chromosomes more than mammalian ones.
This revelation implies that the mammalian Y chromosome was once an ordinary member of a chromosome pair that has degenerated over time. Given this rate of gene loss, the human Y chromosome’s complete decay is projected in about 11 million years.
Debates about the Y chromosome’s decline have fluctuated, with lifespan estimates ranging vastly.
Y-Less Rodents Shedding Light on the Issue
Both mole voles from Eastern Europe and spiny rats from Japan have species without a Y chromosome or SRY gene yet continue to thrive.
Researchers have made strides in understanding how spiny rats determine sex without the SRY gene. In 2022, a team found a small duplication near the important sex gene SOX9 on chromosome 3 of the spiny rat that was present in all males but no females, suggesting a possible workaround for the absence of SRY.
Implications for Human Evolution
The potential disappearance of the Y chromosome has incited various theories about humanity’s future.
While parthenogenesis occurs in some reptile species, it’s not viable for humans due to essential genes requiring paternal contribution.
The discovery of alternative sex determination methods in rodents offers hope that humans could similarly evolve a new sex gene. Nevertheless, this evolution could come with repercussions, such as the emergence of different human species with unique sex determination systems.
In 11 million years, Earth’s visitors might not find humans as we know them but instead several diverse human species defined by these systems.
Jenny Graves, Distinguished Professor of Genetics and Vice Chancellor’s Fellow, La Trobe University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
An earlier version of this article was published in December 2022.
FAQs about the Fate of the Y Chromosome and Future of Male Determination
- Why is the Y chromosome important?
- The Y chromosome is crucial as it carries the SRY gene, which triggers the pathway for male sex determination in the embryo, leading to the development of testes and the production of male hormones.
- How likely is it that the Y chromosome will disappear?
- While it is a possibility based on the rate of gene loss over millions of years, predictions about the exact timeline vary significantly. Some estimates suggest it could take 11 million years for the Y chromosome to entirely vanish in humans.
- Can humans survive without the Y chromosome?
- Recent discoveries in rodents who have lost their Y chromosome and developed alternative sex-determining mechanisms suggest that it might be possible for humans to adapt similarly, should the Y chromosome disappear.
- How do spiny rats determine sex without the Y chromosome?
- Scientists found a small genetic duplication near the SOX9 gene on chromosome 3 that was present only in male rats, suggesting that it could act as a substitute for the SRY gene in initiating male development.
- Could the loss of the Y chromosome lead to new human species?
- It’s a theoretical possibility that the evolution of distinct sex-determining systems could result in reproductive isolation and the emergence of different human species.
Conclusion
The disappearance of the Y chromosome presents a unique challenge to the future of male sex determination in humans. Yet, nature’s own experiments with rodents who have lost their Y chromosome give us a glimpse into the adaptability of life. The discovery of alternative male-determining genes in spiny rats opens the door to new possibilities, hinting that as long as there remains a way for essential genes to be passed on from both parents, the blueprint for human existence can perdure. As with all matters of evolution, however, unpredictability reigns, and humanity’s distant future remains an uncharted landscape of genetic mystery and potential speciation.