A captured retroviral envelope syncytin gene associated with the unique placental structure of higher ruminants: impact of retroviral gene capture on placental diversity
Résumé
Syncytins are envelope genes of retroviral origin that have been co-opted for a role in placentation. In mammals, five syncytins with a placenta-specific expression and a fusogenic activity have been independently "captured" in primates, rodents and lagomorphs (which are all Euarchontoglires mammals) and conserved in a functional state. In addition to these five genes, a sixth independent syncytin capture event has been identified recently in the carnivora suborder, which belong to the Laurasiatheria that diverged from the Euarchontoglires shortly after mammalian radiation more than 100 Mya, and whose placental structure is different from that of primates, rodents and lagomorphs. An in silico search for intact envelope genes within the Bos taurus genome led to the identification of a seventh syncytin, syncytin-rum1, within the suborder Ruminantia where the placenta lacks an extended syncytium layer but displays heterologous cell fusion between placental and uterine cells, a process unique among eutherians. As revealed by both qRT-PCR and in situ hybridization, syncytin-rum1 is specifically expressed in the placenta, at the level of the trophoblast binucleate cells, consistent with a role in the formation of the trinucleate cells of the cow and the syncytial plaques of the ewe. Syncytin-rum1 is conserved among 16 representatives of higher ruminants, with evidence for purifying selection and functional conservation of fusogenic activity over 30 My, correlating with the emergence of the unique placental structure in these species. Altogether, these data argue for syncytins to be a major driving force in the emergence and diversity of the placenta.