Y chromosome losing all genes, but men won't become extinct

August 23, 2012

By KAYOKO GEJI/ Staff Writer

Men fear not.

For many years, the Y chromosome, carried only by men, has been losing genes and shrinking. Some say it will disappear altogether, but it turns out the chromosome is surprisingly resilient. In fact, even if it does go the way of the dodo in the distant future, it seems that the male sex will still survive.

Comparing the Y and X chromosomes in humans, the Y chromosome, a symbol of the male sex, is passed on generation by generation from father to son. But its size pales in comparison to the queenly X chromosome, standing feebly in her shadow. While the X chromosome contains 1,098 genes, the Y has a mere 78.

There are many ways animals determine gender. Snakes and birds use chromosomes other than the X and Y. A crocodile's gender depends on the temperature when it hatches from an egg. The first mammals, our distant ancestors, split off from the reptiles around 300 million years ago, heading down the path of determining sex with the X and Y chromosomes.

The human Y chromosome continues to decay. Scientists say it has lost a thousand genes since about 300 million years ago. At this rate, it will have no genes in another 14 million years or so and disappear. ... At least, that's the story according to the "Y chromosome extinction theory," published a decade ago in a paper by an Australian researcher.

A TRANSFORMATION TAKING MORE THAN 100 MILLION YEARS

So, just how did the Y chromosome arrive at its present-day form?

A human cell has 46 chromosomes arranged in 23 pairs. Twenty-two of these pairs are "autosomes," found in both males and females. The remaining pair, comprising two "allosomes," is different in each gender. Females have two X chromosomes in this pair, while males have one X and one Y.

According to a theory postulated in 1967 by the late Dr. Susumu Ono, a geneticist, the male X and Y chromosomes were both originally autosomes of the same pair. Then, about 300 million years ago, male-specific genes began to appear in one member of the pair. Typically, when a mutation damages a chromosome, it repairs itself with the help of the other member of its pair. But the appearance of the male-specific genes in the one chromosome created a Y chromosome that is different from the X, rendering the Y unable to repair its mutated segments.

Thereafter, the Y chromosome underwent a series of major structural changes that inverted portions of it, erasing most of what it had in common with the X chromosome. Thus the damage to the Y built up. Although the damaged genes survived by obtaining new structures and functions, many ceased to function at all.

The gene SRY is well-known among the surviving genes because of the role it plays in growing the testicles. It was once a different gene that was also found in the X chromosome, but it accumulated damage over a period of at least 100 million years, starting around 300 million years ago. This miraculously allowed SRY to obtain a new role, transforming it into the human male's most essential gene.

However, most genes that lose their functions end up disappearing. Others remain but do nothing. We know now that more than half the genes in the Y chromosome are broken and do not function. Ever since the Y chromosome extinction theory came along, some scientists have made frightening predictions that these crippled genes will die off and men will disappear, and that the human race will become incapable of producing offspring and become extinct.

SMALL ELITE FORCE REMAINS RESILIENT

This past February, a team of American researchers published a paper that blows a hole in this theory. They found that the Y chromosome's decay has not been steady. Rather, it has occurred in stages through a process that has weeded out the weaker genes and left the stronger ones, ensuring the chromosome's survival.

They analyzed the Y chromosome of the rhesus macaque, a species of monkey that separated from a common ancestor of ours 25 million years ago. After the split, a region of genes in one of the monkey's chromosomes mutated over a long period of time, turning it into something other than the older X. This new Y chromosome then stabilized and retained its new form--as had happened to the human SRY.

However, some genes did not remain stable. These mutated relatively recently, becoming different from the genes in the X chromosome. The remaining, stronger crew of elite genes stubbornly survived, playing the roles of male-specific genes, such as the creation of sperm.

It seems men won't die out anytime soon. But nobody knows what may happen after hundreds of millions of years have passed. So what would occur if, hypothetically speaking, the Y chromosome were to disappear?

The Ryukyu spiny rats that live on the islands of Tokunoshima and Amami-Oshima in the Amami Islands, south of the island of Kyushu, provide some food for thought. Although they have no Y chromosome, this species still has males and females. In the opinion of Asato Kuroiwa, an associate professor at Hokkaido University, "A new gene formed within an autosome, replaced SRY and became the switch that determines gender."

Meanwhile, the male Okinawa spiny rat has an X and Y chromosome, just like in humans, but the Y is the larger one, rather than the X, as in humans. Kuroiwa believes this happened because an autosome attached to the Y chromosome, preventing it from dying out.

Yoichi Matsuda, a professor of molecular cytogenetics at Nagoya University who studies the evolution of chromosomes, has this to say: "Even when a species loses its Y chromosome, there are cases in which it will avoid extinction by acquiring a new system to determine gender, as the Ryukyu spiny rats have done. Gender is not such a simple thing that will vanish just from the loss of an allosome."

By KAYOKO GEJI/ Staff Writer
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