«Basically, these viruses appear to allow the mammalian genome to continuously evolve, but they can also bring instability," said Andrew Xiao of the Department of Genetics and Yale Stem Cell Center, senior author of the paper published online March 30 in the journal Nature. «Aside from the embryo, the only other places people have found this virus active is in tumors and neurons.»
Xiao and the Yale team discovered a novel mechanism by which the early embryo turns off this virus on the X chromosome, which ultimately determines the sex of an organism. If the level of this molecular marker is normal, X chromosomes remain active, and females and males will be born at an equal ratio. If this marker is overrepresented, X chromosomes will be silenced, and males will be born twice as often as females.
«Why mammalian sex ratios are determined by a remnant of ancient virus is a fascinating question," Xiao said.
Tens of millions of years ago viruses invaded genomes and duplicated themselves within the DNA of their hosts. Xiao estimated that more than 40% of the human genome is made up of such remnants of viral duplications. In most cases, these remnants remain inactive, but recently scientists have discovered they sometimes take on surprising roles in developing embryos and may even push mammalian evolution. Researchers found that the virus active in the mouse genome that influences sex ratios is relatively recent — in evolutionary terms — and is enriched on the X chromosome.
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Xiao said it is possible that this mechanism might be used to suppress cancer, which has been known to hijack the same virus to spread.
He also noted in other organisms, such as C elegans and the fruit fly Drosophila, this mechanism plays an entirely opposite role and activates genes, not suppresses them.
«Evolution often uses the same piece but for different purposes and that appears to be the case here," Xiao said.
Source: http://news.yale.edu/2016/03/30/sex-baby-ancient-virus-makes-call
