Y Chromosomes In Chimps And Humans Evolving Faster Than Expected

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Y Chromosomes In Chimps And Humans Evolving Faster Than Expected

http://www.medicalnewstoday.com/articles/176015.php

Contrary to a widely held scientific theory that the mammalian Y chromosome is

slowly decaying or stagnating, new evidence suggests that in fact the Y is

actually evolving quite rapidly through continuous, wholesale renovation.

By conducting the first comprehensive interspecies comparison of Y chromosomes,

Whitehead Institute researchers have found considerable differences in the

genetic sequences of the human and chimpanzee Ys - an indication that these

chromosomes have evolved more quickly than the rest of their respective genomes

over the 6 million years since they emerged from a common ancestor. The findings

are published online this week in the journal Nature.

" The region of the Y that is evolving the fastest is the part that plays a role

in sperm production, " say , first author on the Nature paper and

a postdoctoral researcher in Whitehead Institute Director Page's lab. " The

rest of the Y is evolving more like the rest of the genome, only a little bit

faster. "

The chimp Y chromosome is only the second Y chromosome to be comprehensively

sequenced. The original chimp genome sequencing completed in 2005 largely

excluded the Y chromosome because its hundreds of repetitive sections typically

confound standard sequencing techniques. Working closely with the Genome Center

at Washington University, the Page lab managed to painstakingly sequence the

chimp Y chromosome, allowing for comparison with the human Y, which the Page lab

and the Genome Center at Washington University had sequenced successfully back

in 2003.

The results overturned the expectation that the chimp and human Y chromosomes

would be highly similar. Instead, they differ remarkably in their structure and

gene content. The chimp Y, for example, has lost one third to one half of the

human Y chromosome genes--a significant change in a relatively short period of

time. Page points out that this is not all about gene decay or loss. He likens

the Y chromosome changes to a home undergoing continual renovation.

" People are living in the house, but there's always some room that's being

demolished and reconstructed, " says Page, who is also a Medical

Institute investigator. " And this is not the norm for the genome as a whole. "

Wes Warren, Assistant Director of the Washington University Genome Center,

agrees. " This work clearly shows that the Y is pretty ingenious at using

different tools than the rest of the genome to maintain diversity of genes, " he

says. " These findings demonstrate that our knowledge of the Y chromosome is

still advancing. "

and Page theorize that the divergent evolution of the chimp and human Y

chromosomes may be due to several factors, including traits specific to Y

chromosomes and differences in mating behaviors.

Because multiple male chimpanzees may mate with a single female in rapid

succession, the males' sperm wind up in heated reproductive competition. If a

given male produces more sperm, that male would theoretically be more likely to

impregnate the female, thereby passing on his superior sperm production genes,

some of which may be residing on the Y chromosome, to the next generation.

Because selective pressure to pass on advantageous sperm production genes is so

high, those genes may also drag along detrimental genetic traits to the next

generation. Such transmission is allowed to occur because, unlike other

chromosomes, the Y has no partner with which to swap genes during cell division.

Swapping genes between chromosomal partners can eventually associate positive

gene versions with each other and eliminate detrimental gene versions. Without

this ability, the Y chromosome is treated by evolution as one large entity.

Either the entire chromosome is advantageous, or it is not.

In chimps, this potent combination of intense selective pressure on sperm

production genes and the inability to swap genes may have fueled the Y

chromosome's rapid evolution. Disadvantages from a less-than-ideal gene version

or even the deletion of a section of the chromosome may have been outweighed by

the advantage of improved sperm production, resulting in a Y chromosome with far

fewer genes than its human counterpart.

To determine whether this rapid rate of evolution affects Y chromosomes beyond

those of chimps and humans, the Page lab and the Washington University Genome

Center are now sequencing and examining the Y chromosomes of several other

mammals.

This research was funded by the National Institutes of Health (NIH) and the

Medical Institute (HHMI).