By RADHA CHITALE
ABC News Medical Unit
July 17, 2009
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.
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