A glimpse into the forgotten time capsules of evolution

Transposable elements (TEs) can be thought of as viruses of the genome. They are short stretches of DNA that multiply in genomes even if this activity causes diseases. Since TEs are part of our genomes they are transmitted from parent to offspring (vertical transmission). Occasionally, however, they are transmitted between species (horizontal transmission). As a result, the TE will gradually, over the next generations, invade the genomes of most specimens of the novel host species. It is a mystery how such a horizontal transfer happens. It is however thought that horizontal transfer is a rather rare event, happening all few million years per species and TE. Recently we showed that in the model organism Drosophila melanogaster 11 horizontal transfer of TEs happened in a mere 200 years. These 11 TEs rapidly spread in worldwide populations, thereby increasing the genome of the flies by about 1%. This high rate of invasions is likely not normal. Rather, we argue that the last 200 years experienced a highly elevated rate of genomic invasions. The habitat expansion of D. melanogaster into the Americas, which was probably mediated by human activity (e.g., global trade), likely triggered this elevated rate of invasions. Habitat expansions have been reported for thousands of species in the last decades, ranging from insects, over plants, to mammals. It is thus possible that these human mediated habitat expansions led as an inadvertent side effect to a massive TE contamination of the genomes of these species. In this project we will test three questions. First are other species also showing a high rate of recent TE invasions. Second can we identify risk factors for a high rate of recent invasions (e.g., habitat expansion). Third did the rate of invasions increase in the last centuries, simultaneously with the increase in global freight traffic. We will draw on three different sources of historical specimens: different Drosophila species collected at various times during the last century, specimens from museum collections with an age around 100 -200 years, and insect remains from waterlogged sites that may have an age up to 5000 years. In summary this project will test whether humans inadvertently triggered the contamination of the genomes of species by selfish DNA.