An analysis of Y chromosome gene loss has suggested that men might not 'become extinct' after all.
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LONDON: The male-specific chromosome, the Y chromosome, is not decaying quite as fast as has been predicted.
The Y chromosome decays much faster than other types of chromosomes such as the X chromosome and the non-sex chromosome (autosome), which are found in equal numbers in both sexes, and many geneticists have predicted that the Y chromosome might some day disappear altogether.
New research published in Nature today has analysed Y chromosomes from ancient rhesus monkeys and compared them to the Y chromosomes of modern humans and chimpanzees to find that Y chromosomes have not changed in the past 25 million years.
"The rhesus monkey gives us a good overview of the last 25 million years of evolution," said lead author and biologist Jennifer Hughes from the Howard Hughes Medical Institute at Massachusetts Institute of Technology (MIT) in the U.S.. "We were quite surprised that only 1 of 30 ancestral genes was lost (in the last 25 million years), and that happened in the newest region of the chromosome."
The importance of being Y
The sex chromosomes (X and Y) evolved from the non-sex autosomes during the past 200 to 300 million years, and many of the genes in the X and Y can be found in the autosomes of animals without sex chromosomes, such as chickens and reptiles. These genes are known as ancestral genes and likely play a critical role in development.
The main purpose of sex, from an evolutionary perspective, is to exchange genetic information between the chromosomes - a process known as 'crossing over'. Since the Y chromosome is the only chromosome without a partner, it cannot cross over. Crossing over also allows for the repair of damaged DNA, so the partner-less Y decays faster than any of the other chromosomes.
The Y and X chromosomes used to be identical, but over time the Y chromosome has shrunk significantly. Compared to the 1,100 or so genes contained by the X chromosome, the Y chromosome contains only 200 genes. The X chromosome is unpaired in men, but women have two copies of the X so it can cross over and replenish itself whenever it is carried by women.
In 2002, Australian scientists published a paper in Nature suggesting that the Y chromosome is decaying so rapidly that it could completely disappear, saying, "[it] will self-destruct in around 10 million years."
Why men aren't going extinct
To examine the rate of Y chromosome decay, Hughes sequenced the Y chromosome from rhesus monkeys and compared that sequence with the Y chromosomes from humans and chimpanzees. Old World monkeys (or Cercopithecidae) such as the rhesus monkey diverged from the ape lineage (which includes humans and chimps) about 25 million years ago, so provides a means for investigating gene loss since this time.
When a region of DNA starts to decay, it loses the unimportant parts quite quickly, while the critical genes remain intact. The Y chromosome can be divided into five regions, each of which started to degenerate at a different time. The oldest region started to decay about 240 million years ago, while the youngest started its decline just 30 million years ago. The oldest regions contain the same genes in rhesus monkeys and in humans, while the youngest regions are quite different.
Despite rearrangements in different regions, the researchers reported that the older regions are stable and have not lost any genes in the past 25 million years, since the human and Old World monkey lineages diverged.
Geneticist Jenny Graves from La Trobe University in Melbourne, one of the authors of the 2002 Nature paper, still thinks that the Y could disappear, as has happened in the Japanese spiny rat and the Transcaucasian mole vole. In these species it appears that the critical genes have simply relocated to non-sex chromosomes before the Y disappeared.
Graves highlighted the long, repetitive regions in the human Y that are absent in the rhesus Y. The human Y chromosome "looks like a small Y that then went nuts. If I were to bet on a species with a stable Y, I'd bet on the rhesus monkey", she said. "But every time we sequence a new species we learn more about what the essential parts of the Y are."
