New research disputes the idea that the male Y chromosome will one day disappear.
The researchers looked at the genetic makeup of eight African and eight European men, a University of California-Berkeley news release reported.
"The Y chromosome has lost 90 percent of the genes it once shared with the X chromosome, and some scientists have speculated that the Y chromosome will disappear in less than 5 million years," evolutionary biologist Melissa A. Wilson Sayres, a Miller Postdoctoral Fellow in the Department of Integrative Biology at the University of California, Berkeley, and lead author of the new analysis, said.
A number of mammals have lost their Y chromosomes but are still able to produce healthy males. Researchers also successfully created mice that lacked Y chromosomes in a lab by shuffling around their genes; the rodents were still able to reproduce normally.
"Our study demonstrates that the genes that have been maintained, and those that migrated from the X to the Y, are important, and the human Y is going to stick around for a long while," Wilson Sayres said.
The team's observations revealed the Y chromosomes' "patterns of variation" were consistent with natural selection working to maintain the chromosome rather than eliminate it. The chromosome is tiny (containing only 27 unique genes compared to thousands in other chromosomes) which suggests it has only the most essential genes.
"Melissa's results are quite stunning. They show that because there is so much natural selection working on the Y chromosome, there has to be a lot more function on the chromosome than people previously thought," coauthor Rasmus Nielsen, UC Berkeley professor of integrative biology, said.
The evolution of the Y chromosome is also used to study the history of humans and how they migrated around the world. The findings could help make this research more productive.
The Y chromosome has degraded over the past 200 million years. Early versions of both the X and Y chromosomes were similar to others; they would swap a few genes creating offspring that was a mix between the two. At some point the X and Y chromosomes stopped swapping genes and started developing independently from one-another; this may have been because some "male-specific" genes, such as those affecting testes and sperm, could be harmful to females.
"Now the X and Y do not swap DNA over most of their lengths, which means that the Y cannot efficiently fix mistakes, so it has degraded over time," Wilson Sayres said. "In XX females, the X still has a partner to swap with and fix mistakes, which is why we think the X hasn't also degraded."
"Y chromosomes are more similar to each other than we expect," Wilson Sayres said. "There has been some debate about whether this is because there are fewer males contributing to the next generation, or whether natural selection is acting to remove variation."
The team determined that if their estimations of low variability were correct, only one in four males would have passed down their Y chromosome throughout history.
Since this scenario is unlikely, the team suggested natural selection could have caused the low variability.
"We show that a model of purifying selection acting on the Y chromosome to remove harmful mutations, in combination with a moderate reduction in the number of males that are passing on their Y chromosomes, can explain low Y diversity," Wilson Sayres said.
The team hope this conclusion will help us learn more about our ancestors.
"Melissa has shown that this strong negative selection -- natural selection to remove deleterious genes -- tends to make us think the dates are older than they actually are, which gives quite different estimates of our ancestors' history," Nielsen said.
