Death of Y may spawn new human species

ABC Science Online



The pending demise of the Y chromosome could give rise to a whole new species of human, a professor of comparative genomics says.



Scientists have been speculating about the demise of the Y chromosome for some years now but Professor Jenny Graves of the Australian National University in Canberra has come up with a bold new twist on the theory.



Graves, who has been working on sex chromosomes in marsupials, will present her theory at the 11th International Congress of Genetics in Brisbane today.



She will tell the conference that new 'male making' genes on other chromosomes could step up to do the job of the Y chromosome's SRY gene, which is the key to making males male.



But this could mean men without Y chromosomes would split off from those with, eventually evolving into a new species of hominid.



"It's quite possible that you could make new hominid species that way," she says.



When two populations become two species

Graves says men without a Y chromosome would be largely infertile. But a small number would reproduce and pass the new sex determining gene to their children.



Eventually the group with the new gene would separate from the Y gene group, potentially evolving into a new species, she says.



"[The two groups] couldn't mate with each other so they'd get gradually different, just like chimpanzees and humans gradually became different 5 million years ago," she says.



"When two populations become two species there's generally there's some sort of wedge driven between them so they can't mate with each other.



"It might be a mountain range ... but it might be something fundamental like the way they determine sex has flipped to some new way."



15 million years and counting

Graves says there are only 45 working genes left on the Y chromosome from "a grand total" of 1400.



It also contains a lot of 'pseudo genes', which look like they should work but don't, suggesting they've recently become defunct.



According to her projections the Y chromosome will disappear altogether in 15 million years.



This will occur because unlike the other coupled genes, the single Y chromosome can't recombine with a matching partner and is less able to refresh itself.



Mutations will build up and the mutated genes will eventually drop off the chromosome because they no longer perform any useful function.



Graves says this has already happened in the case of the mole vole, an aggressive little rodent that appears male and is able to reproduce despite having lost its Y chromosome.



XX men

Australian researcher Professor Andrew Sinclair, of Murdoch Childrens Research Institute, is researching so-called XX men, or the roughly one in 150,000 men who are born without a Y chromosome.



"What it's pointing to is the presence of new genes we haven't yet discovered to replace the ones on the Y chromosome," Sinclair says.



Alternately, the "volume" of previously existing genes may have been "turned up" in the absence of the Y genes, he says.



Sinclair's team is the first in the world to use new high-density gene chips to examine XX men in the hope of finding out which genes these are.



About 10% of affected men also have a tiny portion of the Y chromosome stuck on their X chromosome which carries across the testis determining gene, he says.



Sinclair says Grave's theory about a new human species could make sense "in a theoretical way" but is unlikely in reality.



"I don't know about a whole new species of human but if you lost the Y chromosome completely males would have to evolve in some way to deal with it," he says.



"If you have males without a Y chromosome I don't think I'd go as far as calling them a new species, but a new type of individual."

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