17 July 2009
Male chromosome facing extinction
by Kate Melville
It has long been suspected that the sex chromosome that only males carry is deteriorating and could disappear entirely within a few million years, but until now, no one has understood the evolutionary processes that control the Y chromosome's demise. Now, a pair of Penn State scientists has discovered that the Y chromosome has evolved at a much more rapid pace than its partner chromosome, the X chromosome, leading to a dramatic loss of genes on the Y chromosome at a rate that, if maintained, eventually could lead to the Y chromosome's complete disappearance.
Humans have 23 pairs of chromosomes, which are the structures that hold our DNA, but just one pair of these chromosomes are sex chromosomes, while the others are referred to as non-sex chromosomes. "In eutherian mammals [humans and other non-marsupial mammals], the sex chromosomes contain an additional region of DNA whereas, in the egg-laying mammals and marsupials [such as opossums and platypuses], this additional region of DNA is located on the non-sex chromosomes," said Penn State's Kateryna Makova. "At first, bits of DNA within this additional region were readily swapped between the X and Y chromosomes, but some time between 80 and 130 million years ago, the region became two completely separate entities that no longer swapped DNA. One of the regions became specifically associated with the X chromosome and the other became specifically associated with the Y chromosome."
Writing in PLoS Genetics, Makova explains that by comparing the DNA of the X and Y chromosomes in eutherian mammals to the DNA of the non-sex chromosomes in the opossum and platypus, the researchers were able to go back in time to the point when the X and Y chromosomes were still swapping DNA, just like the non-sex chromosomes in the opossum and platypus. The scientists then were able to observe how the DNA of the X and Y chromosomes changed over time relative to the DNA of the non-sex chromosomes. "Our research revealed that the Y-specific DNA began to evolve rapidly at the time that the DNA region split into two entities, while the X-specific DNA maintained the same evolutionary rate as the non-sex chromosomes," said Makova.
After determining that the Y chromosome has been evolving more rapidly and has been losing more genes as a result, they wanted to find out why the Y chromosome has not already disappeared entirely. "Today, the human Y chromosome contains less than 200 genes, while the human X chromosome contains around 1,100 genes," said co-researcher Melissa Wilson. "We know that a few of the genes on the Y chromosome are important, such as the ones involved in the formation of sperm, but we also know that most of the genes were not important for survival because they were lost, which led to the very different numbers of genes we observe between the once-identical X and Y. Although there is evidence that the Y chromosome is still degrading, some of the surviving genes on the Y chromosome may be essential, which can be inferred because these genes have been maintained for so long."
Makova and Wilson then decided to test the hypothesis that some of the genes on the Y chromosome are being maintained because they are essential. The team's approach was to compare the expression and function of genes on the Y chromosome with analogous genes on the X chromosome. "If the genes' expressions and/or functions were different, then it would make sense that the genes on the Y chromosome would be maintained because they are doing something that the genes on the X chromosome can't do," explained Makova. "This hypothesis turned out to be correct."
Although some of the genes on the Y chromosome have been maintained, most of them have died, and the team found evidence that some others are on track to disappear, as well. "Even though some of the genes appear to be important, we still think there is a chance that the Y chromosome eventually could disappear," said Makova. "If this happens, it won't be the end of males. Instead, a new pair of non-sex chromosomes likely will start on the path to becoming sex chromosomes."
In the future, the team plans to use its newly generated data to create a computer model that tracks the degeneration of the Y chromosome. The scientists hope to determine how long it will take for the Y chromosome to disappear.
Source: Penn State University