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Retroviruses, which insert their genetic material into our genomes to copy themselves, have left behind genes that help to steer our immune systems and mold the development of embryos and the placenta.

Now researchers report in PLOS Genetics that syncytin, a viral protein that enables placenta formation, also helps to increase muscle mass in male mice. These results could partially explain a lingering mystery in biology: why the males of many mammalian species tend to be more muscular than females.

Cell-cell fusion is also pivotal for muscle fiber formation and repair, where the myotubes are formed from the fusion of mononucleated myoblasts into large multinucleated structures.
[...] [The greater than 20% reduction in muscle mass] is only observed in males, which subsequently show muscle quantitative traits more similar to those of females. In addition, [the authors] show that syncytins also contribute to muscle repair after cardiotoxin-induced injury, with again a male-specific effect on the rate and extent of regeneration.
[...] similar effects are observed with primary myoblasts from sheep, dog and human, with a 20–40% reduction upon addition of siRNA against the corresponding syncytins.