Viruses have had a major impact on the evolutionary path of animals. They shape populations through infectious diseases, and, as recently discovered, through the more subtle process of endogenisation into the genome. Retroviruses are a class of virus – think HIV and CMV– that insert their genetic material into genome of the host, as part of their life cycle. About 8% of the human genome is derived from retroviral insertions- not an insignificant amount when you consider that only 1.5% of the genome codes for functioning proteins.
Syncytin is a mammalian gene that originates from an endogenous retrovirus (ERV). Syncytin began in an ancient virus, as a gene encoding a protein that assisted with viral fusion and entry into the host cell. It has been adapted by mammals, and is important in the formation of the placenta through initiating cell-cell fusion. Syncytin genes have undergone strong positive selection, suggesting that these genes have contributed at least partially towards the driving force behind the evolution of Eutherian (placental) mammals.
Omittin’ syncytin… what does it do?
Redelsperger et al made the unexpected discovery that syncytin has a dual function in mice, which leads to an observable sexual dimorphism between males and females. The cell-cell fusion function that syncytin conducts in the placenta is also vital for the formation of muscle, through assistance with myoblast (muscle precursor) fusions. Gene knockout studies of either syncytin A or syncytin B led to a 20% reduction in muscle mass…
…but only in male mice!
This could explain the apparent dimorphism between male and female mammals with regard to build and muscle generation. The loss of either of these syncytin genes also incurred less effective muscle regeneration. These tests were repeated in muscle cells of pigs, dogs, and humans, with each species providing a very similar result.
Why is this important?
Syncytin is one of the few known endogenous retroviruses that has an adapted function in its host. It also is the first known ERV to have different functions in males and females. Why syncytin only contributes to muscle generation in male mice is still unknown, and this will be the basis of a lot of future research. Even more unusually, syncytin in humans is derived from a completely different retroviral gene than in mice and dogs. This is indicative of a different virus inserting into a different part of the genome at a different stage during evolution. Some of the big questions surrounding this research are: How did these different genes co-evolve to have the same function? And does human synyctin have a similar- but not identical- function?
The paper: Redelsperger et al. Genetic evidence that captured retroviral envelope syncytins contribute to myoblast fusion and muscle sexual dimorphism in mice. 2016. PLOS Genetics. http://journals.plos.org/plosgenetics/article/related?id=10.1371/journal.pgen.1006289
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