The evolutionary life history of P transposons:: from horizontal invaders to domesticated neogenes

P elements, a family of DNA transposons, are known as aggressive intruders into the hitherto uninfected gene pool of Drosophila melanogaster. Invading through horizontal transmission from an external source they managed to spread rapidly through natural populations within a few decades. Owing to their propensity for rapid propagation within genomes as well as within populations, they are considered as the classic example of selfish DNA, causing havoc in a genomic environment permissive for transpositional activity. Tracing the fate of P transposons on an evolutionary scale we describe different stages in their evolutionary life history. Starting from horizontal transfer events, which now appear to be rather a common phenomenon, the initial transpositional burst in the new host is slowed down by the accumulation of defective copies as well as host-directed epigenetic silencing. This leads to the loss of mobility and, finally, to molecular erosion by random mutations. Possible escape routes from genomic extinction are the reactivation within the original host genome by recombination or suspension of the repressing regime. horizontal emigration to a virgin gene pool, or genomic integration and acquisition of a novel function as a domesticated host gene.