What makes a man a man? Socially, that is a complicated question. Genetically, however, it is as simple as a single Y chromosome.
But guys, that chromosome is in trouble.
In a new study, researchers say there is a dramatic loss of genes from the human Y chromosome that eventually could lead to its complete disappearance -- in the next few millennia. While the Y chromosome's degeneration has been known to geneticists and evolutionary biologists for decades, the study sheds new light on some of the evolutionary processes that may have contributed to its demise and posits that, as the degeneration continues, the Y chromosome could disappear from our genetic repertoire entirely.
"It's certainly possible, but it's difficult to predict when it will happen," said Kateryna Makova, an associate professor of biology at Penn State University, who led the study, which was published Thursday in the journal PLoS Genetics.
Although geneticists and evolutionary biologists agree that the Y chromosome is degenerating -- and far more rapidly than its X counterpart -- they reject the idea of a world far in the future where men are obsolete.
"The idea that the Y chromosome has just bailed out of an airplane without a parachute simply doesn't fit the facts," said Dr. David Page, director of the Whitehead Institute for Biomedical Research in Cambridge, Mass., and a Y chromosome expert. "The evidence from studies on natural deletions of [genes on] the human Y chromosome shows there are consequences, especially for sperm production, that implies very strong natural selection against the loss of genes on the human Y chromosome."
Y Chromosomes Had Problems From the Start
Humans have 23 pairs of chromosomes packed with genes that dictate every aspect of our biological functioning. Of these pairs, the sex chromosomes are different; women have two X chromosomes and men have an X and a Y chromosome. The Y chromosome contains essential blueprints for the male reproductive system, in particular those for sperm development.
But the Y chromosome, which once contained as many genes as the X chromosome, has deteriorated over time and now contains less than 80 functional genes compared to its partner, which contains more than 1,000 genes. Geneticists and evolutionary biologists determined that the Y chromosome's deterioration is due to accumulated mutations, deletions and anomalies that have nowhere to go because the chromosome doesn't swap genes with the X chromosome like every other chromosomal pair in our cells do.
Y Chromosomes Are Rapidly Losing Genes
However, Melissa Wilson, lead author of the study and graduate research fellow at Penn State University, pointed out that if there is no difference between a male who has lost a particular gene and one who still retains it, especially if both are still fertile, then that gene must be nonessential.
"Because they can lose [a gene] ... we conclude that it's on its way to dying in humans," she said.
Yet the Y chromosome perseveres, despite its rapid rate of deterioration.
"The key flaw in the logic [of Y chromosome deterioration] is the assumption that the Y chromosome can only lose genes," Page said. "But the human Y chromosome has gained genes not even on the X chromosome. Men who lose those genes do not transmit their Y chromosome."
Y Chromosome Can Gain Genes
Page pointed out that, while the Y chromosome may not share genetic material with the X chromosome, it can swap genes with other chromosomes as well as keep multiple copies of functional genes to increase their number on the Y chromosome. Makova and Wilson said that the increased rate of mutation on the Y chromosome could give rise to new genes that may prove beneficial and, therefore, remain on the chromosome.
Genetic change, whether by mutation, environmental stressors or by swapping bits of chromosomes, is the natural course of evolution, and evolution is weighted towards survival. Perhaps most importantly, Y chromosomes with defective male-specific genes, especially those involved in sperm production, are unlikely to reproduce and pass on those genes to their sons, which knocks highly defective chromosomes out of the gene pool. Genetic changes that do not favor reproduction are likely to get weeded out of the system.
"The most fundamental [principle] to all evolution is reproduction," said Dr. Ronald Crystal, chairman of the Department of Genetic Medicine at Weil Cornell Medical College. "No one knows why the Y chromosome has more pressures to evolve. It may be that the genes are irrelevant. ... But evolution figures out a way to maintain reproduction."
Reproduction Is Still Paramount for Evolution
Even if the Y chromosome becomes obsolete, reproduction will continue, in some form. Makova and Wilson said that new sex chromosomes may rise from non-sex chromosomes or that essential genes might move to other chromosomes, which has happened in some species of deer.
"Presumably, we will have moved genes around," said Dr. Harry Ostrer, director of the Human Genetics program at the New York University School of Medicine. "But the reproductive structures will be well conserved."
In other words, men will not fade away, even if their Y chromosomes do.