What makes a male a male? That’s a loaded question, but the most basic answer is the presence of a Y chromosome. At conception, the presence of two X chromosomes means that a baby is female; when a baby has an X and a Y chromosome, the baby is male.
Being that only they have Y chromosomes, the weight of sex selection falls on the father. Males can either contribute their X or Y chromosome to their kin, and on the Y chromosome is the SRY gene, which causes male genitalia to develop. If the dad doesn’t fork over his Y chromosome, none of this takes place, and the baby will be female.
Yet researchers are now suggesting that the Y chromosome is in danger of disappearing. This naturally spurs fears that males will one day cease to exist.
See, there are concerns the degenerative changes the chromosome has experienced over time. It’s believed that millions of years ago, the Y chromosome was in really great shape and had a lot to offer, but for the last 6 million years or so, that appears to no longer be the case. What has happened since Y’s inception, and what does the future hold? That’s a question science is still trying to answer.
Hampered from the Start
The Y chromosome, though once genetically identical to the X chromosome, has always been at an evolutionary disadvantage because of the way it is (or isn’t) passed from parent to child.
It makes sense: While males don’t always pass on their Y chromosome (their X is a contender, too, remember), females pass down an X chromosome every time. Everyone gets an X chromosome; not everyone gets a Y.
Honey, have you seen my genes?
Aside from the obvious, many scientists believe that evolution has had a largely negative effect on the Y chromosome. In its heyday, roughly 300 million years ago, they believe that this structure used to carry a ton of genes—1,438 genes, according to research published in the journal Reproduction, Fertility, and Development.
But it’s been a rough several million years for the Y chromosome. These days, it’s down to a measly 45 genes, and some scientists speculate that it will only take 10 million years for those 45 stragglers to disappear.
Its remaining genes largely contribute to sex selection, while the 1,000-plus genes on the X chromosome serve a wide variety of purposes. Other researchers, however, have suggested that the degeneration of the Y chromosome came to a halt six to seven million years ago, which is believed to be the divergence of the genetic timelines of humans and chimpanzees.
Your genes don’t even fit!
“Survival of the fittest” has become the tagline for evolution. Genetic mutations that benefit species have a habit of surviving, while less beneficial mutations die off—often because their hosts did. What is interesting (and alarming) about the Y chromosome is that it is believed to be the exception of that rule. When it comes to the mutation of this structure, it simply doesn’t follow the norm.
Instead of the fittest Y chromosome being selected, it appears that its genes frequently mutate in ways that aren’t in the best interest of the chromosome’s survival, according to an article published by the journal Cell. This is believed to be the reason the chromosome appears to be experiencing degeneration over time.
Yet the chromosome survives anyway. “The Y chromosome seems to be a product of a perverse evolutionary process that does not select the fittest Y,” reads the Cell article, “which may cause its degradation and ultimate extinction.”
Guilty by Association
If it’s not clear by now, there are big concerns about what the history of the Y chromosome means for its future, considering its propensity for taking it on the chin. Why does the Y chromosome so often run into trouble?
Well, a process called genetic hitchhiking may be partly responsible for the degradation of the Y chromosome, according to an article published in the journal Genetics. Genetic hitchhiking is when mutations affect not just the “intended” gene, but innocent bystanders on the same DNA chain. Some mutations are advantageous, but others are deleterious, which the National Cancer Institute defined as a genetic alteration that “increases an individual’s susceptibility or predisposition to a certain disease or disorder.”
The Y chromosome, it turns out, is particularly attractive to hitchhikers.
“Sexual recombination facilitates the elimination of deleterious mutations … and the lack of recombination on the Y sex chromosome may have contributed to its degeneration through the accumulation of deleterious mutations,” reads a 2011 PLOS Genetics study.
What does all this mean for the future the male sex?
From the Y chromosome’s rapid loss of genes to the unique disadvantage it has always come up against, it’s easy to wonder if males are on borrowed time. If the Y chromosome is disappearing, what does that mean for males—and ultimately the human race?
Well, take a deep breath. “… even if the Y chromosome in humans does disappear, it does not necessarily mean that males themselves are on their way out,” argued geneticist Darren Griffin and biologist Peter Ellis in an article for The Conversation. In species that have lost their Y chromosomes, the SRY gene “has moved to a different chromosome, meaning that these species produce males without needing a Y chromosome.”
As for the Y chromosome, there are some encouraging signs that it will stick around.
First of all, we’re not sure if the Y chromosome is still losing genes. As mentioned above, there are many who believe that loss of genes came to hard stop with the divergence of chimpanzee and human genes. There might not be anything to worry about in that regard, but only time—lots of time—will tell.
Additionally, more recent research indicates that the Y chromosome has some way of pushing back against some of the processes that have contributed to its degeneration.
One way this is being accomplished is through a process known as gene amplification. This occurs when one very specific sequence of a DNA molecule is copied multiple times during one cell cycle. This is a process that has been observed in the sequencing of the mouse Y chromosome and is speculated to be partly fueled by the antagonistic relationship between the X and Y chromosomes, according to a study in the journal Cell.
Similarly, in a recent study male gene-sequencing study published in the journal PLOS Genetics, gene amplification was observed. Gene amplification is a way for the Y chromosome to “fight back” against gene loss and encourage the production of healthy reproductive cells.
The big question: Is it enough?
Although it does appear the the Y chromosome is starting to rally, the remaining debate is whether or not the process of gene amplification is enough to mitigate the damage already done. Can rapid copying of healthy sequences of DNA stop what is believed to the more rapid gene loss and degeneration?
There isn’t a simply answer to this question. There are two sides to this debate, and both make great points. Jenny Graves, a well known geneticist from Australia, supports the theory that the Y chromosome will eventually disappear. She argues that the Y chromosome is lacking significantly when compared to the X chromosome and that this is a result of millions of years of degeneration. She believes that the degeneration is the proof that the Y chromosome can’t survive and that evolution will eventually weed out these genetic structures, according to a debate summarized in Chromosome Research.
On the opposite side of the argument is Jenn Hughes, an evolutionary geneticist and biologist, who believes that the Y chromosome and the defense mechanisms it has in place are enough to rescue it from completely disappearing. She believes that instead of viewing the history of the Y chromosome as an indication of its doom, we should see it as an indication that it can survive simply because it has managed to do so for hundreds of millions of years already. She also points out that the Y chromosome hasn’t exclusively experienced loss, as it has also added eight genes in the process.
The outcome of the debate was put up to the audience, and the vote was a 50/50 split.
Again, only time will tell if the Y chromosome will wilt or if gene amplification is enough to prevent the demise of the gene that is so essential to the human race.
Oh, and if Y loses out and the SRY gene does move to another chromosome? According to Griffin and Ellis, “… the new sex-determining chromosome … should then start the process of degeneration all over again due to the same lack of recombination that doomed their previous Y chromosome.”