Human and mammal sex is determined by the gene in the Y chromosome that determines males. However, the Y chromosome in humans has been degenerating and could even disappear in the next million years.
The Role of the Y Chromosome
Males have a pair of one X and Y chromosome, with their female counterparts having a pair of X chromosomes. The names of these chromosomes are not connected with their shape, with X simply signifying "unknown."
The X chromosome consists of roughly 900 genes that are responsible for several functions that are not associated with X. On the other hand, the Y chromosome consists of around 55 genes and several non-coding DNA, which is a simple repetitive DNA that apparently does not do anything.
However, the Y chromosome plays a crucial role as it contains the gene that determines male development within an embryo. Roughly 12 weeks post-conception, this gene switches to others that regulate testis development. Such embryonic testis produces male hormones that make sure that the baby grows as a male.
This gene was found to be the SRY (sex region on the Y) gene in 1990. It functions by enabling a genetic pathway beginning with the SOX9 gene, which is crucial for male determination for every vertebrate.
Vanishing Y Chromosome
The majority of mammals have Y and X chromosomes that are similar to humans. However, the system comes with several issues due to the imbalance of X genes across females and males.
The evolution of such an odd system could be attributed to Australia's platypus, which has totally different sex chromosomes that are more similar to those of birds. In this species, the XY pair is just a typical chromosome with equal members. This posits that, not that long ago, mammal Y and X were just an ordinary chromosomal pair.
This also means that the Y ended up losing 900-55 active genes in the last 166 million years of separate evolution of both platypuses and humans. This loss is equivalent to five genes per million years. At such a rate, at least 55 genes may end up vanishing in the next 11 million years.
New Sex Gene For Determining Males
While this may sound concerning, the good news is that there are already two lineages of rodents that have lost their Y chromosome. Despite this, they are still surviving.
Eastern Europe's mole voles and Japan's spiny rats are two species with fully vanished SRY. Their X chromosome remains in either a single or double dose for both sexes.
A team led by Asato Kuroiwa, a biologist from Hokkaido University, looked into the spiny rat. The team found that most of the genes within the rats' Y chromosome had moved to other chromosomes. However, no SRY or substitute gene signs were found.
Findings were noted in the "Turnover of mammal sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9" study. The team observed sequences that were present in male genomes and absent in their female counterparts. They then refined these and examined the sequence for each rat.
They observed a small difference close to the SOX9 key sex gene on the spiny rat's chromosome 3. There was a small duplication present across every male and no female.
With this, the researchers suggest that this small duplicated DNA piece has the switch that typically enables the SOX9 in response to the SRY gene. When this duplication was introduced to mice, the researchers observed that SOX9 activity was boosted. Hence, the change allowed SOX9 to function without SRY.
These findings offer an alternative possibility for human males, suggesting the possibility of evolving a new gene for determining sex. However, this may come with risks, as there is a possibility that more than one system could evolve in different regions of the world. A "sex gene war" could result in a new species, which is what took place for spiny rats and mole voles.
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