The South American skink Mabuya mabouya
Mark Stevens from Warrington, UK CC BY 2.0
Placentome and paraplacentomal region in Mabuya sp.
From Cornelis et al. PNAS 2017 (here)
As explained in previous posts (e.g. here), syncytins are the products of endogenous retroviral genes. The envelope (env) genes of retroviruses function to promote fusion of the viral membrane with the plasma membrane of a host cell. Syncytins are derived from env genes and are expressed in the placenta, where they promote fusion of cytotrophoblasts with the syncytiotrophoblast. Hitherto they have been identified in six orders of eutherian mammals and in one marsupial (previous post).
Cornelis et al. first determined the transcriptome of Mabuya placenta and identified four env genes. One of these (named Mab-Env1) was highly expressed in placenta and with the highest expression of RNA and protein occurring at the fetal-maternal interface including in a maternal syncytial layer. Importantly, Mab-Env1 was fusogenic in an ex vivo assay, which is an essential criterion for designating the protein as a syncytin. The receptor for Mab-Env1 was also identified in this study.