The gender of human and other mammal babies is determined by a specific gene located on the Y chromosome. However, the human Y chromosome is slowly deteriorating and could potentially disappear in the future, leading to the extinction of our species unless we develop a new gene for determining sex. Fortunately, certain rodents have already lost their Y chromosome and continue to thrive. A recent study published in the Proceedings of the National Academy of Science demonstrates how the spiny rat has evolved a new gene that determines the male gender.
In humans and other mammals, females have two copies of the X chromosome, and males have one X and one Y chromosome. The names of these chromosomes have nothing to do with their shape; the X chromosome was originally named "unknown" because it was discovered before the Y chromosome. The X chromosome contains around 900 genes involved in various functions unrelated to sex.
On the other hand, the Y chromosome has fewer genes (around 55) and contains a large amount of non-coding DNA, which is simple repetitive DNA that does not seem to have any known function. However, the Y chromosome is important because it contains a key gene that plays a critical role in male development. During the early stages of fetal development, around 12 weeks after conception, this gene activates other genes that control the development of the testes. The testes produce male hormones such as testosterone, which ensures that the baby develops as a male.
SOX9 Gene Importance
The master sex gene responsible for initiating male development in the embryo was identified as SRY (sex region on the Y chromosome) back in 1990. It acts by activating a genetic pathway that begins with a gene called SOX9. SOX9 is important for determining male gender in all vertebrates, even though it is not located on a sex chromosome.
Most mammals have an X and Y chromosome similar to humans, with the X chromosome containing many genes and the Y chromosome containing SRY and a few other genes. This system can cause problems because males and females have different amounts of genes on the X chromosome. Surprisingly, the platypus, a mammal native to Australia, has completely different sex chromosomes that are more similar to those of birds. This raises the question of how this unusual system evolved.
In the platypus, the XY pair of chromosomes is like any other chromosome pair, with both members equal in size and containing the same number of genes. This suggests that the X and Y chromosomes in mammals were once a normal pair of chromosomes that have diverged over time. This also means that the Y chromosome in humans has lost approximately 900-55 active genes over the 166 million years that humans and platypuses have been evolving separately.
This equates to a loss of about five genes per million years. At this rate, the last 55 genes on the Y chromosome will disappear in about 11 million years. The idea that the human Y chromosome is deteriorating and could potentially disappear has generated a lot of controversies, and there are still ongoing debates about how long the Y chromosome is expected to last, with estimates ranging from infinity to a few thousand years.
Thriving Y-Chromosomes
There is good news, however, as two known rodent lineages have already lost their Y chromosome but are still surviving. These include the mole voles of Eastern Europe and the spiny rats of Japan, some species which no longer have the Y chromosome or the SRY gene. Both males and females have a single or double dose of the X chromosome in these species. It is not yet clear how mole voles determine sex without the SRY gene, but a team led by Hokkaido University biologist Asato Kuroiwa has made progress in understanding the spiny rat, a group of three species found on different Japanese islands that are all endangered.
Kuroiwa's team discovered that most of the genes on the Y chromosome of spiny rats had been moved to other chromosomes, but they could not find any trace of SRY or the gene that substitutes for it.
A team of researchers has recently published a successful identification of a new male-determining gene in the spiny rat in the Proceedings of the National Academy of Science (PNAS). The team searched for sequences in the genome that were present in males but not in females and then refined these sequences and tested for their presence in individual rats. They discovered a slight difference near the key sex gene SOX9, located on chromosome 3 of the spiny rat.
This difference was a small duplication of DNA (only 17,000 base pairs out of more than 3 billion) present in all males but not in any females. The researchers suggest that this small bit of duplicated DNA may contain the switch that typically activates SOX9 in response to SRY. When they introduced this duplication into mice, they found that it increased SOX9 activity, which suggests that this change may allow SOX9 to function without SRY.
Development of the Human Species
The possibility that the human Y chromosome may eventually disappear has led to speculation about the future of our species. Some lizards and snakes can reproduce through a process called parthenogenesis, in which they can produce eggs using only their genes. However, this is not possible in humans or other mammals because we have at least 30 "imprinted" genes that can only function if inherited from the father through sperm.
This means that the end of the Y chromosome could potentially lead to the extinction of the human race because we need sperm and men to reproduce. The new finding that the spiny rat has evolved a new sex-determining gene supports the possibility that humans could also evolve a new way of determining sex. However, this process comes with risks. For example, what if more than one new system evolves in different parts of the world?
This could potentially lead to the separation of new species through a "war" of the sex genes, as has happened with mole voles and spiny rats. Therefore, if someone were to visit Earth in 11 million years, they might find no humans or several different human species that are kept apart by their different sex determination systems.
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