Y Chromosome Fading? New Sex Gene May Hold the Key to Male Survival

Humans and other mammals rely on a single gene on the Y chromosome to determine sex. However, this crucial chromosome is steadily shrinking and may disappear entirely in millions of years. This raises concerns about the future of our species, but new research offers a glimmer of hope.

In a recent study published in the Proceedings of the National Academy of Sciences, researchers reveal a groundbreaking discovery regarding the evolution of sex determination in mammals, particularly focusing on rodents. The findings shed light on the potential future of the human Y chromosome and its implications for our species.

The process through which the Y chromosome dictates human sex can be described as follows:
In humans, like in other mammals, females possess two X chromosomes, while males carry a single X chromosome paired with a smaller chromosome known as the Y. Despite their names, the designations are not indicative of their shapes; rather, the X chromosome was initially labeled 'unknown'.

The X chromosome boasts approximately 900 genes responsible for various functions unrelated to sex, whereas the Y chromosome contains a limited number of genes, around 55, along with extensive non-coding DNA—repetitive DNA sequences with unclear functions.

However, the Y chromosome plays a pivotal role due to its possession of a crucial gene that initiates male development during embryo formation. Around 12 weeks after conception, this essential gene activates others, orchestrating the development of testes in the embryo. These embryonic testes produce male hormones, such as testosterone and its derivatives, ensuring the male sexual differentiation of the fetus.

Identified in 1990, this pivotal gene is known as SRY (sex-determining region on the Y). It functions by initiating a genetic pathway that commences with the activation of a gene called SOX9. Despite not residing on sex chromosomes, SOX9 is pivotal for male determination across vertebrates.

The disappearing Y
The diminishing Y chromosome presents a notable phenomenon among most mammals, including humans, where the X chromosome is rich in genes while the Y chromosome contains the crucial SRY gene along with a few others. This setup poses challenges due to the unequal distribution of X genes between males and females. The intriguing revelation comes from the study of Australia's platypus, which possesses entirely different sex chromosomes more akin to those found in birds.

In platypuses, the XY pair represents a typical chromosome pair, with both members being equivalent. This suggests that the X and Y chromosomes in mammals were once ordinary chromosomes not too long ago.

Consequently, it implies that the Y chromosome has undergone a loss of active genes, ranging from 900 to 55, over the 166 million years of separate evolution between humans and platypuses. This equates to an average loss of about five genes per million years. At this pace, the remaining 55 genes could disappear within the next 11 million years.

The proposition of the eventual disappearance of the human Y chromosome sparked controversy, leading to ongoing debates and conflicting claims regarding its expected lifespan. Estimates vary widely, ranging from infinity to a few thousand years.

Rodents lacking a Y chromosome
The exciting development is that we have identified two lineages of rodents that have successfully lost their Y chromosomes and continue to thrive.

Among these are the mole voles found in eastern Europe and the spiny rats native to Japan, which each include species devoid of the Y chromosome and the SRY gene. In these species, the X chromosome persists, present in either a single or double dose in both males and females. While the mechanism through which mole voles determine sex without the SRY gene remains unclear, researchers, led by biologist Asato Kuroiwa from Hokkaido University, have made significant progress with the spiny rat population. This group consists of three species residing on various Japanese islands, all of which are currently endangered.

Kuroiwa's team made a notable breakthrough in 2022 with the publication of their findings in the Proceedings of the National Academy of Sciences (PNAS). By identifying specific sequences present in the genomes of male rats but absent in females, they refined their research and conducted thorough testing on each individual rat to confirm the presence of these sequences. This research has shed light on the process of sex determination in these rodent populations and the genetic adaptations that allow them to thrive in the absence of the Y chromosome and SRY gene.

What implications does this have for the future of males?
The impending disappearance of the human Y chromosome has sparked speculation regarding the future of men from an evolutionary perspective. While some species, such as certain lizards and snakes, have adapted to female-only reproduction through parthenogenesis, this phenomenon is not viable for humans or other mammals due to the presence of approximately 30 crucial "imprinted" genes that require paternal contribution via sperm.

The necessity for sperm and men in the reproductive process implies that the extinction of the Y chromosome could potentially lead to the demise of the human race. However, the discovery of alternative sex determining mechanisms, as evidenced by the recent findings in rodents, offers a glimmer of hope. This suggests that humans may have the capacity to evolve a new sex determining gene to ensure reproductive viability.

Nevertheless, the prospect of evolving a new sex determining gene presents its own set of challenges and risks. One concern is the possibility of multiple new systems emerging in different regions of the world, leading to genetic diversity and potential complications in reproductive biology. Despite the optimism surrounding the potential for adaptation, careful consideration and further research are essential to understand the implications and consequences of such evolutionary changes for the future of humanity.