Viruses in the genome help sheep with pregnancy
So-called endogenous viruses present in the genome contribute to successful reproduction in sheep. Scientists at Texas A&M University in College Station discovered that proteins made by a specific retrovirus in the animals' genome are necessary for regular placental development.
Researchers led by Thomas Spencer silenced genes from the endogenous Jaagsiekte sheep retrovirus that encode viral envelope proteins in pregnant sheep. To do this, they injected a molecule into the uterus of the animals that specifically binds to the genes for the envelope proteins, enJSRVs, and thus prevents their production.
On the 16th day after conception, the fetuses and the tissues surrounding them were smaller than normal. Furthermore, the supply of cells that later fuse with the uterine mucosa to form the placenta was ten to twelve times lower. Due to this defective placenta development, the embryos could not nest in the uterine lining. After 20 days, only one in five fetuses was still alive, but it was severely growth-retarded. In contrast, in two control experiments, five out of five and five out of six embryos developed well.
The retroviruses got into the genome of the animals by infecting germ cells a very long time ago and thus spreading to all cells of the following generations. Most of these viruses, which make up about eight to ten percent of the mammalian genome, are harmless because genetic modifications have made them no longer infectious. However, the host organisms, like the sheep here, have made use of some of the foreign proteins in the course of evolution and can no longer do without them. A similar viral protein, HERV-W, has been suggested to play a similar role in human placenta development.