Retrotransposons: Genomic and Trans-Genomic Agents of Change

Genome structure in higher eukaryotes is highly dependent on the type and abundance of transposable elements, particularly retrotransposons , in their non-coding DNA. Retrotransposons are generally viewed as genomic parasites that must be suppressed in order to ensure genome integrity. This perception is based on the instances of retrotransposons having caused deleterious structural variation in genomes. Recent data are beginning to provide a more positive view of the impact of retrotransposons , particularly in mammals, where the evolution of the placenta has depended on the exaptation of a type of retrotransposon , endogenous retroviruses. Finally, exosome trafficking of retrotransposons between cells has been shown to induce the innate immune system gene expression, possibly indicative of a role for retrotransposons in the regulation of the innate immune system. It may be time for us to review the status of retrotransposons and reclassify them as symbionts rather than parasites.