|Gray Short-tailed Opossum (Monodelphis domestica)|
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Syncytins are endogenous retroviral proteins that promote fusion of trophoblast to form a syncytium. They have been identified in several orders of placental mammals including ruminants and rodents. One marsupial, the Gray Short-tailed Opossum, has syncytial trophoblast. This led Guillaume Cornelis and colleagues to ask if it also had a syncytin. Their findings just appeared in PNAS (here). In brief, they did find a syncytin, which was named syn-Opo1; it was expressed in the placenta and able to promote cell fusion. The gene was present in most but not all members of the genus Monodelphis and absent in other marsupials. Thus it represents a relatively recent capture of a retroviral envelope (env) gene.
Even more intriguing was the presence of an env gene that was expressed in the placenta but unable to promote cell fusion. This gene was present in the genome of the Tammar Wallaby (Macropus eugenii), the Tasmanian Devil (Sarcophilus harisii) and 23 other marsupials. The gene was named pan-Mar-env2. Because it has been conserved (and is under purifying selection) it likely plays an important role in placentation that as yet is undefined.