Cross section of mouse muscle (in blue: labeling of nuclei; in green: labeling of muscle fiber membranes). Normal male mice display larger muscle fibers than those seen in mutant, syncytin knock-out mice.
Credit: François Redelsperger via ScienceDaily
I've previously talked on my page about how an ancient retrovirus that integrated into early mammals helped to drive our evolution in a number of ways, including being responsible for placenta formation. A retroviral envelope gene called syncytin helps the placenta form as it mediates cell-cell fusion. Previously researchers had observed an increased expression of this gene at the interface between the fetus and the mother which later gives rise to the placenta.
New research expands what we know on the topic and suggests that these integrated syncythins are responsible for the increased muscle mass that males have. When this gene was knocked out in mice, the muscle cells were smaller (see the picture above) and the mice had a 20% reduction of muscle mass. However, this reduction in muscle mass was only observed in males. The researchers then confirmed that knocking out this gene resulted in smaller muscle cells by knocking syncytin out in sheep, dog and human primary myoblasts. The cell sizes were reduced 20-40% when synctin was silenced. These results not only demonstrate that syncytin is involved in muscle cell fusion, but that it is required for the increased muscle mass seen in male mammals. This could help explain some of the sexual dimorphism seen between male and female mammals.
This work adds to what we know about ourselves and how viruses have played a role in making us what we are.