One of the most intriguing news that has permeated popular science discourse is the supposed ongoing but gradual disappearance of the Y chromosome in humans. This is one of the two sex chromosomes. Its presence in combination with an X chromosome determines a male sex. This means only males have the Y chromosome. It is also passed directly from a male parent to a male offspring. A sudden disappearance of this sex-determining chromosome would result in a particular species producing only female offspring, the disappearance of the male sex, and the eventual extinction of that entire species. However, despite several valid claims that the Y chromosome is disappearing over time, declaring that it will soon disappear is an oversimplification, and concerns about possible human extinction are overblown and unwarranted.
Debunking and Explaining: Misconceptions and Realities About the Status of the Human Y Chromosome
Evolution and Degeneration
The Y chromosome evolved from an autosome. It is believed to appeared about 200 to 300 million years ago in ancient mammals because of evolutionary pressures that necessitated a more reliable mechanism for sex determination compared to environmental factors. Note that sex determinations in some reptiles are determined by external factors like temperature.
It is also worth mentioning that the Y chromosome is a feature of many mammal species. It did not appear in one single species since its existence is a product of long-term evolution in the entire mammalian lineage. This sex chromosome has also undergone significant changes over millions of years. These include gene loss and structural rearrangements.
There is an ongoing scientific debate about the long-term fate of the Y chromosome. It is true that it has been undergoing a process of gene loss for millions of years. This is called degeneration. Several researchers suggest that this sex chromosome could eventually disappear within the next roughly 4 to 11 million years if the current rate of gene loss continues.
Note that this generation stems from the fact that the Y chromosome, unlike most chromosome pairs, is unable to undergo recombination with another X chromosome or has very limited recombination with the Y chromosome to swap genetic materials. Recombination is a critical process for repairing DNA damage and eliminating harmful mutations.
The absence of frequent recombination resulted in the accumulation of deleterious mutations over time. Moreover, since it is only present in males, natural selection acts on the Y chromosome less effectively than on chromosomes present in both sexes. Its small size and limited gene content further make it more vulnerable to the effects of genetic drift like mutations.
Human Y Chromosome
Nevertheless, according to other researchers, the Y chromosome may have reached a point of stability based on studies in humans and other mammals. Furthermore, even with continued degeneration due to gene loss, some researchers have also argued that novel sex-determining and reproduction mechanisms could evolve and emerge to compensate for gene loss.
The human Y chromosome has lost about 1393 of its 1438 original genes over time. This translates to a rate loss of 4.6 genes per million years. It now has 80 functional genes and about 280 so-called pseudogenes that are either disrupted or nonfunctional. The current loss suggests that the Y human chromosome will lose complete function within the next 10 million years.
However, based on comparative genomic analyses, although most mammals have also experienced Y chromosome degeneration due to gene loss, the human Y chromosome has not lost a single gene since the divergence of humans and chimpanzees between 6 to 7 million years ago. Comparison with other primates showed very little gene loss over millions of years.
Specific findings from the undertakings of researchers like David Page of the Whitehead Institute for Biomedical Research noted that the Y chromosomes in mammals underwent a period of rapid degradation in their early years and later went through strict conservation. The Y chromosome in humans has only lost a single ancestral gene in the past 25 million years.
A few mammals have actually lost their Y chromosome and even the more specific SRY gene which is primarily responsible for determining male sex. These include the Tokudaia species of rodents found in Japan and several mole voles like the Ellobius talpinus and Ellobius lutescens. These mammals have developed alternative sex-determination systems.
Pointers and Takeaways: The Future of the Human Y Chromosome and Other Y Chromosomes in Mammals
The current scientific consensus is that the Y chromosome is not disappearing but is shrinking or degenerating due to gene losses that occurred hundreds of millions of years ago. More recent research indicates that this degeneration due to gene loss has stabilized. The Y chromosome is not actively vanishing and is likely to remain functional in the future. The human Y chromosome has also been relatively stable since humans and chimpanzees diverged from a common ancestor between 6 to 7 million years ago.
FURTHER READINGS AND REFERENCES
- Bachtrog, D. 2013. “Y-Chromosome Evolution: Emerging Insights Into Processes of Y-Chromosome Degeneration. In Nature Reviews Genetics. 14(2): 113-124. Springer Science and Business Media LLC. DOI: 1038/nrg3366
- Bellott, D. W., Hughes, J. F., Skaletsky, H., Brown, L. G., Pyntikova, T., Cho, T.-J., Koutseva, N., Zaghlul, S., Graves, T., Rock, S., Kremitzki, C., Fulton, R. S., Dugan, S., Ding, Y., Morton, D., Khan, Z., Lewis, L., Buhay, C., Wang, Q., … Page, D. C. 2014. Mammalian Y Chromosomes Retain Widely Expressed Dosage-Sensitive Regulators. In Nature. 508(7497): 4940-499. Springer Science and Business Media LLC. DOI: 1038/nature13206
- Hughes, J. F., Skaletsky, H., Brown, L. G., Pyntikova, T., Graves, T., Fulton, R. S., Dugan, S., Ding, Y., Buhay, C. J., Kremitzki, C., Wang, Q., Shen, H., Holder, M., Villasana, D., Nazareth, L. V., Cree, A., Courtney, L., Veizer, J., Kotkiewicz, H., … Page, D. C. 2012. “Strict Evolutionary Conservation Followed Rapid Gene Loss on Human and Rhesus Y Chromosomes. In Nature. 483(7387): 82–86). Springer Science and Business Media LLC. DOI: 1038/nature10843
- Wilson, J., Staley, J. M., and Wyckoff, G. J. 2020. “Extinction of Chromosomes Due to Specialization is a Universal Occurrence. In Scientific Reports. 10(1). Springer Science and Business Media LLC. DOI: 1038/s41598-020-58997-2