Previous studies have identified two pairs of envelope (env) genes of retroviral origin that have been independently captured by their host for a role in placentation. In simians, syncytin-1 [1–3] and syncytin-2 [4, 5] entered the primate genome 25 and >40 million years (My) ago, respectively. They retained their coding capacity in all the subsequent branches; syncytin-1 and syncytin-2 display placenta-specific expression, are fusogenic in ex vivo cell-cell fusion assays, and one of them displays immunosuppressive activity . A pair of env genes from endogenous retroviruses (ERVs) were then identified in the mouse, named syncytin-A and -B, which share closely related functional properties although they have a completely distinct origin, showing a divergent sequence and a different genomic location compared to primate syncytins . As found for the latter, syncytin-A and -B have the status of bona fide genes. They have been conserved since their entry into the Muridae genome, approximately 20 million years (My) ago; they display placenta-specific expression, mediate cell-cell fusion in ex vivo assays , and one of them is immunosuppressive . Recently we have further unambiguously demonstrated via the generation of syncytin-A knockout mice that these genes are indeed essential for placentation, with a lack of cell-cell fusion observed in vivo at the level of the placenta of the knockout embryos, resulting in impaired maternal-fetal exchanges and death of the embryos at mid-gestation . Therefore, it appears that on some occasions in the course of mammalian evolution, env genes from endogenous retroviruses have been "co-opted" by their host to participate in the formation of the syncytiotrophoblast layer, at the maternal-fetal interface, by mediating the fusion of mononucleated cytotrophoblasts.
A further question that we wanted to answer was whether mammals belonging to orders other than rodents and primates but possessing a placenta with a related architecture, i.e. with a syncytiotrophoblast layer in direct contact with maternal blood at the maternal-fetal interface, have also "captured" retroviral env genes to generate, in a convergent manner, this specific placental structure. Among mammals whose placenta displays such a structural organization at the maternal-fetal interface, i.e. with a haemochorial placenta, the lagomorpha order was selected over the two other orders which also possess a haemochorial placenta - i.e. the Insectivora (hedgehog) and Chiroptera (bats) ; this was done because one of its representatives, the rabbit (Oryctolagus cuniculus), has an already sequenced genome, and can be reared and investigated easily at different stages of gestation, and has a placental physiology that has been appropriately described [9–11].
Here, by combining in silico search for env genes within the rabbit genome, RT-PCR assays for their in vivo transcriptional activity in a large panel of tissues including the placenta, cloning of the candidate genes, ex vivo assays for their fusogenicity and, ultimately, in situ hybridization of placenta sections, we identify a new fusogenic and placenta-specific endogenous env gene, displaying all the characteristic features of a bona fide syncytin gene, that we named syncytin-Ory1. Although we demonstrate that the syncytin-Ory1 protein shares the same ASCT2 receptor in common with human syncytin-1, it is divergent from all four syncytins previously identified in rodents and primates and must therefore have been captured independently from a distinct ancestral retrovirus. The occurrence in a third order of Mammals of a new syncytin gene that is specifically expressed at the maternal-fetal interface within the placental junctional zone provides strong support to the notion that ERVs have played a convergent role in the recurrent emergence of syncytiotrophoblast-containing haemochorial placentae in the course of evolution.