Did you know that ancient viruses hide within our own genome? It’s been common knowledge among biologists that about 8 percent of our DNA is actually composed of viral fragments, and recent research revealed that there are at least two complete viral genomes – essentially, entire viruses – concealed within a large number of people.
The functions of these viral fragments or genomes are heavily debated, although there has been some evidence that one assists pregnant women in developing a “shield” for the fetus, protecting it from toxins present in the mother’s own body. Now, a new Nature study reveals that the sex of a developing fetus in most mammals, including perhaps humans, may also be determined by one of these ancient viral fragments.
Our chromosomes are thread-like structures that reside within the nucleus of each cell, and each contains DNA coiled around proteins that support it. Within humans and most other mammals, the sex cells – the eggs and sperm – contain different types of chromosomes: the former only has X chromosomes, while the sperm carries either an X or a Y chromosome.
The baby’s eventual sex is determined by which sperm fertilizes the egg. An X-carrying sperm will lead to an XX combination, which produces a girl; alternatively, a Y-carrying sperm leads to an XY combination – an eventual boy. This mechanism has been known for a very long time, but a team of biologists at Yale University have suggested that a virus has a say in what happens to the X chromosome during fertilization.
Does this viral remnant linger and behave in the same way in humans? Creations/Shutterstock
While examining embryonic stem cells of mice, the type of highly primitive stem cell that can become any type of specialized cell, the team noticed that a viral fragment was relatively abundant on the X chromosome. If this viral remnant was present at “normal” levels, the X chromosome would remain active, and females and males would be born at an equal rate.
Alternatively, if the remnant was “silenced,” the X chromosome becomes suppressed. This means that XY pairings during fertilization will be up to twice as more likely to occur, leading to an increase in the number of male babies. In order to suppress the virus, a particular chemical compound – a methyl bond – is manufactured by the host and added to the structure of DNA. This allows it to “disable” specific genes, including the virus’s, without directly altering the genetic sequence.
This particular viral remnant was estimated to have entered the genomes of mammals across the world around 1.5 million years ago, which is relatively recent in evolutionary terms, considering that viruses are likely to be billions of years old. Although it’s not clear if this viral-dependent sex determination mechanism operates in humans and other primates – rather than just mice – the researchers think that, with so much human DNA already known to be comprised of viral fragments, it’s more than likely.
“Why mammalian sex ratios are determined by a remnant of ancient virus is a fascinating question,” Andrew Xiao, the study’s coordinator and a molecular biologist at Yale University, said in a statement. Until further research can be carried out, it seems that, for the moment, this will remain a question without an answer.