It is known that genes inherited from ancient retroviruses are essential to the placenta in mammals, a finding to which scientists in the Laboratoire Physiologie et Pathologie Moleacuteculaires des Retrovirus Endogenes et Infectieux (CNRS/Universite Paris-Sud) contributed. Today, the same scientists reveal a new chapter in this story: these genes of viral origin may also be responsible for the more developed muscle mass seen in males. The new findings were published online on September 2, 2016 in the open-access journal PLOS Genetics. The article is titled “Genetic Evidence That Captured Retroviral Envelope Syncytins Contribute to Myoblast Fusion and Muscle Sexual Dimorphism in Mice.” Retroviruses carry proteins on their surface that are able to mediate fusion of their envelope with the membrane of a target cell. Once released inside that cell, the retrovirus’s genetic material becomes integrated into the host's chromosomes. In the rare cases where the infected cell is involved in reproduction, the viral genes may be transmitted to progeny. Thus nearly 8% of the mammalian genome is made up of vestiges of retroviruses, or "endogenous" retroviruses. Most of them are inactive, but some remain capable of producing proteins. Such is the case with syncytins, proteins that are present in all mammals and encoded by genes inherited from retroviruses "captured" by their ancestors. A little more than five years ago, and thanks to inactivation of these genes in mice, the team led by Thierry Heidmann, Ph.D., demonstrated that syncytins contribute to formation of the placenta. Because of their ancestral ability to mediate cell-cell fusion they give rise to the syncytiotrophoblast, a tissue formed by the fusion of a large number of cells derived from the embryo, at the fetomaternal interface.
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