Just as recent metagenomic studies have demonstrated vast information about the bacteria that teem in and around us, there is a deep, mysterious well of ancient infections within our genomes Alexei Evsikov from Jackson Labs spoke this morning about the activity of so-called LTR retrotranposons during early embryonic development in mice. LTR retrotransposons are essentially viral remnants that have camped out at various places in the genome sometimes landing in areas that break up genes and sometimes brining promoter regions in close proximity to genes that would have otherwise gone unexpressed. Evsikov presented studies suggesting that these transposons become actively expressed and may even be working to intercalate themselves into new areas of the genome during the crucial developmental period when genes from sperm and egg meet to form a new genome of a new living creature. The brilliance of this strategy from the transposon’s perspective is that it would be one of the best ways to secure its place in the germline of the new individual. Equally brilliant, however, is the fact that the mouse embryo seems to use the activity of these infectious elements to drive the expression of certain genes needed for development. If you turn off the cellular machinery necessary for these retrotransposons to do their thing, it spells disaster for the embryo. The viral-like elements can have bad effects, of course. You don’t want genetic elements, especially elements with promoters jumping about your genome willy nilly, but as always with biology it’s a careful balance. His ideas met with a bit of scepticism, but Evsikov says, “I just speculate that we have to change our thinking about retrotranpsons being the ultimate parasites, to being … actually an extremely good source of variation.”