On the last day of August 2011, two geneticists held a debate at an international conference in Manchester, England. While Jennifer Graves defended that the mammalian Y chromosome would be doomed to a gradual disappearance — so much so that some rodents have already disappeared from the map due to a continuous and rapid degeneration of this chromosome —, Jennifer Hughes, although agreeing with the degenerative history of the chromosome, argued that the phenomenon would already be stagnant in humans. A decade later, we are still uncertain about the fate of the Y chromosome, despite much research pointing to its continued deletion.
The sex of animals is determined by environmental factors such as temperature, or genetics, through the combination of specific chromosomes, such as X, Y, Z and W. Understanding all the aspects that govern sex determination is not simple. What we find in biology books is that in mammals, sex is established by sex chromosomes, one inherited from the mother (the X) and the other from the father (the X or the Y).
Although there are exceptions, in general women have a pair of X’s and men have an X and a Y. Millions of years ago, the X and Y of mammals were very similar in size and function, but over time the Y has fallen behind . Currently, the human X harbors about a thousand genes that encode proteins with different roles in the organism, from determination of the genitalia to brain development and cognition. The Y was left with only about 50 genes, including the SRYone of those responsible for commanding the development of the testicles.
But some mammals took a different path. Female platypuses, for example, have ten X chromosomes, and males, five pairs of X and five pairs of Y. On the other side of the spectrum, a species of mole from Europe permanently lost the Y chromosome over generations: males and females. females carry only one X, that is, other chromosomes must harbor genes responsible for sex determination.
In addition to this mole, it is known that another species of rodent, this one in Japan, no longer has the ancient Y, the mouse tokudaia osimensis — he no longer has the gene SRY, and the genes responsible for sperm production were translocated to a chromosome shared by males and females. The researchers found a difference between the sexes in the gene SOX9, which is duplicated in males. Humans also have SOX9which is activated by SRY and has a key role in male sex determination. How did the Japanese rats lose the Y and also the SRYthe hypothesis is that the doubling of the SOX9 be the bud that leads to the development of genitalia in males.
What future will the Y have among humans? Estimates that take into account the emergence of sex chromosomes 160 million years ago, and the time required for the Y to lose 95% of its content, suggest that the Y has a life expectancy of between 11 million and a few thousand years. The possibility of its demise is a fact, at which point alternative forms of sex determination would take its place. If that happens, another even bigger change may arise: if new systems evolve unevenly in different populations, different species of humans may emerge across the world…
While the role of the Y chromosome remains guaranteed at least in the current generation, recent evidence suggests that some genes located on it may be related to aspects of human health, influencing the immune and inflammatory response, and increasing or decreasing the risk for certain diseases. Thus, with the disappearance of the Y, other complicating factors would arise.
We don’t know if XY men will exist forever, but in the coming decades science should unravel part of this puzzle. New studies are already reinforcing the existence of genes related to the development of the ovaries and testes that are outside the sex chromosomes, proving that the sex determination mechanisms are much more complex and fascinating than previously thought.
Rossana Soletti has a PhD in morphological sciences and is a professor at the Federal University of Rio Grande do Sul.
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