The gender of human and other mammal infants is determined by a gene found on the Y chromosome, known as the male-determining gene. However, the Y chromosome in humans is undergoing a degenerative process and may cease to exist within a few million years, posing a threat to our survival unless we undergo evolutionary changes to develop a new gene responsible for determining sex.
Fortunately, there is some promising news as two lineages of rodents have already experienced the loss of their Y chromosome and managed to adapt accordingly.
A recent study published in the Proceedings of the National Academy of Science sheds light on how the spiny rat species has undergone the evolution of a novel gene responsible for determining maleness.
How the Y Chromosome Determines Human Sex
In humans, and similarly in other mammals, females possess two X chromosomes, while males have a single X chromosome along with a smaller and less significant chromosome known as the Y chromosome. The names “X” and “Y” were not assigned based on their physical shape; rather, the “X” originally represented the unknown.
The X chromosome contains approximately 900 genes that perform various functions unrelated to sex. On the other hand, the Y chromosome carries only a few genes (around 55) and a significant amount of non-coding DNA consisting of repetitive sequences that seemingly lack any specific function.
However, the Y chromosome plays a crucial role due to its inclusion of a vital gene responsible for initiating male development in the embryo.
Around 12 weeks after conception, this pivotal gene activates other genes that control the development of the testes. The embryonic testes produce male hormones (such as testosterone and its derivatives), which ensure the proper development of the baby as a male.
The identification of this critical sex-determining gene as SRY (sex region on the Y) occurred in 1990. Its function involves initiating a genetic pathway that begins with a gene called SOX9, which plays a key role in male determination across all vertebrates, despite not being located on the sex chromosomes.
The Disappearing Y
Most mammals, including humans, possess a similar X and Y chromosome system, wherein the X chromosome carries numerous genes, while the Y chromosome contains the SRY gene along with a few others. However, this system poses challenges due to the unequal distribution of X genes in males and females.
The intriguing aspect is the evolution of this peculiar system. Surprisingly, the platypus found in Australia has entirely different sex chromosomes, resembling those observed in birds.
In platypus, the XY pair consists of two equivalent chromosomes without any discernible differences. This suggests that the mammalian X and Y chromosomes were once ordinary chromosome pairs not too long ago.
Consequently, it implies that the Y chromosome has lost approximately 900 to 55 active genes over the 166 million years during which humans and platypus have undergone separate evolutionary paths. This accounts for a loss of about five genes per million years. At this rate, the remaining 55 genes on the Y chromosome will vanish within 11 million years.
When we first presented the notion of the imminent disappearance of the human Y chromosome, it sparked a considerable controversy. Even today, there are varying assertions and counterarguments regarding the anticipated lifespan of our Y chromosome, with estimates ranging from infinity to a few thousand years.
Rodents With No Y Chromosome
The encouraging news is that we have evidence of two rodent lineages that have successfully lost their Y chromosome yet continue to thrive.
One such example is the mole voles found in eastern Europe, as well as the spiny rats native to Japan. In certain species of these rodents, the Y chromosome, along with the SRY gene, has completely vanished. However, the X chromosome remains intact, present in either a single or double dose in both males and females.
While the mechanism of sex determination in mole voles without the SRY gene is still not fully understood, a team led by biologist Asato Kuroiwa from Hokkaido University made significant progress with the spiny rat, a group of endangered species inhabiting different Japanese islands.
Kuroiwa’s team made a noteworthy discovery by observing that most of the genes typically found on the Y chromosome of spiny rats had relocated to other chromosomes. Surprisingly, they found no trace of SRY or any gene acting as a substitute for it.
In 2022, their successful identification was published in the Proceedings of the National Academy of Sciences (PNAS). The team identified specific DNA sequences present in the genomes of male rats but absent in females. They further refined these sequences and tested them in each individual rat.
Their findings revealed a small variation near the crucial sex gene SOX9, located on chromosome 3 of the spiny rat. This variation involved a tiny duplication of DNA, spanning only 17,000 base pairs out of over 3 billion. This duplication was present in all males and absent in females.
The researchers propose that this small duplicated segment of DNA contains the switch responsible for activating SOX9 in the absence of SRY. To support their hypothesis, they introduced this duplication into mice and observed an increase in SOX9 activity. This suggests that the duplication could enable SOX9 to function independently of SRY.
What This Means for the Future of Men
The impending disappearance of the human Y chromosome, from an evolutionary perspective, has sparked speculation about our future as a species.
While certain reptiles such as lizards and snakes can reproduce as female-only species through a process called parthenogenesis, where they generate eggs containing their own genes, this is not possible for humans or other mammals. This is due to the presence of around 30 crucial “imprinted” genes in humans that only function properly when inherited from the father through sperm.
For humans to reproduce, we rely on sperm and the involvement of men, implying that the extinction of the Y chromosome could potentially lead to the extinction of the human race.
However, a recent discovery offers a glimmer of hope by suggesting that humans have the potential to evolve a new sex determining gene, thereby avoiding such an outcome. Phew!
Nonetheless, the evolution of a new sex determining gene brings its own set of risks. What if multiple new systems evolve independently in different parts of the world?
A “war” of the sex genes could ensue, leading to the divergence and separation of new human species, similar to what has occurred with mole voles and spiny rats.
Therefore, in approximately 11 million years, if someone were to visit Earth, they might find either no humans at all or several distinct human species, each isolated by their respective sex determination systems.