The notion that evolution will eventually make the Y (male) chromosome extinct has been overturned by a group of scientists who say the Y chromosome has a long, healthy future. The scientists, from the Whitehead Institute, have effectively pooh-poohed the so-called "rotting Y theory" which is based on the fact that the Y has lost hundreds of genes over the past 300 million years. The rotting Y theorists have assumed this trend is ongoing, concluding that inevitably, the Y will one day be utterly devoid of its genetic content.
"For the past 10 years, the one dominant storyline in public discourse about the Y is that it is disappearing," said Whitehead Institute Director David Page. "I can't give a talk without being asked about the disappearing Y. This idea has been so pervasive that it has kept us from moving on to address the really important questions about the Y."
Page's team refuted the rotting Y theory by sequencing the Y chromosome of the rhesus macaque - an Old World monkey whose evolutionary path diverged from that of humans some 25 million years ago - and comparing it with the sequences of the human and chimpanzee Y chromosomes. The comparison, published this week in the journal Nature, reveals remarkable genetic stability on the rhesus and human Ys in the years since their evolutionary split.
Understanding this finding requires a bit of historical context. Before they became specialized sex chromosomes, the X and Y were once an ordinary, identical pair of autosomes like the other 22 pairs of chromosomes humans carry. To maintain genetic diversity and eliminate potentially harmful mutations, autosome pairs swap genes with each other in a process referred to as "crossing over."
Roughly 300 million years ago, a segment of the X stopped crossing over with the Y, causing rapid genetic decay on the Y. Over the next hundreds of millions of years, four more segments, or strata, of the X ceased crossing over with the Y. The resulting gene loss on the Y was so extensive that today, the human Y retains only 19 of the more than 600 genes it once shared with its ancestral autosomal partner.
"The Y was in free fall early on, and genes were lost at an incredibly rapid rate," says Page. "But then it leveled off, and it's been doing just fine since."
How fine? Page says the human Y has lost just one ancestral gene in the past 25 million years, and that loss occurred in a segment that comprises just 3 percent of the entire chromosome. The researchers describe the Y's evolution as one marked by periods of swift decay followed by strict conservation.
"Our empirical data fly in the face of the other theories out there. With no loss of genes on the rhesus Y and one gene lost on the human Y, it's clear the Y isn't going anywhere," concluded says Page lab researcher Jennifer Hughes.
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Source: Whitehead Institute for Biomedical Research
