Male Mice Born Without Y Chromosomes Can Still Reproduce

Wednesday, 03 February 2016 - 3:29PM
Genetic Engineering
Wednesday, 03 February 2016 - 3:29PM
Male Mice Born Without Y Chromosomes Can Still Reproduce
Will the Y chromosome someday be rendered obsolete? According to a new study, mice bred without their Y chromosomes can still breed as males (although not organically). Considering the gradual degradation and potential disappearance of the human Y chromosome, this study could have major implications for the future of the human race.

For the study, Monika Ward, a genetics researcher at the University of Hawaii, and a team of researchers isolated two Y chromosome genes in male mice that allowed for the development of the testes and the production of sperm, respectively. They then bred mice without any sign of the Y chromosome, and instead engineered the mice to develop testes and produce sperm in response to X chromosome genes that are involved in both processes. For the development of the testes, they replaced the Y gene with an X gene that is "next in line," or activated by the Y gene, and for sperm production, they over-expressed a gene that works in tandem with the Y gene.

From this process, healthy, Y chromosome-less mice were born, with some caveats. Their testes were smaller than normal, and although they were able to produce viable sperm, the sperm didn't mature enough to grow tails. As a result, the mice were only able to breed through in vitro fertilization, but they were still able to breed. Their male offspring were infertile, but the female offspring were perfectly healthy, and even produced fertile sons. 

This research could have greater implications for humans, as the Y chromosome has been degrading at a rapid rate, losing approximately 10 genes every million years. After starting out with approximately 1,600 genes, like the X chromosome, it has a fraction of that number, and may someday disappear altogether.

Opening quote
"The X bears about 1,600 genes with varied functions. But the Y has hardly any genes; maybe 50, and only 27 of these are in the male-specific part of the Y," geneticist Jenny Graves from La Trobe University in Australia wrote in The Conversation. "Many are present in multiple copies, most of them inactive, lying in giant loops of DNA. Most of the Y is made of repetitive 'junk DNA'. Thus the human Y shows all the signs of a degraded chromosome near the end of its life."
Closing quote

Some researchers claim that the Y chromosome has stabilized, now that it has been stripped down only to essential genes that are necessarily for fitness. But many contend that it is still rapidly disappearing, and will be eradicated altogether in as little as 4.5 million years.

So what does this mean? It does not, of course, mean the "end of men," although that would make for a catchy headline. Although female-only species have evolved in the animal kingdom--such as the whiptail lizard, which reproduces through parthenogenesis--researchers believe this couldn't happen to humans, as we have 30 genes that are only active if they are inherited through the sperm. 

It does, however, mean that Y chromosomes may not be a necessary part of the definition of "maleness," and that another sex determining gene will develop, as has already occurred in two rodent species. Interestingly, it would likely be impossible for an XX woman to produce fertile offspring with a man with a new type of sex gene, so if humans are still alive in 4.5 million years, then the disappearance of the Y chromosome may splinter our species into two or more humanoid groups.
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Genetic Engineering

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