It is remarkable to consider that more than 40% of the human genome is comprised of transposable elements (TEs) and their relics. TEs were long thought of as either 'selfish' or 'parasitic' DNA elements that were there not for the sake of the host organism, but for their own sake in an evolutionary sense; thus they were considered to be either neutral or deleterious to their hosts. However, it is becoming increasingly clear that there are more complex interactions between TEs and their hosts than strict parasitism; these elements produce changes that have a broad range of fitness values at an organismal level. Recent evidence indicates that these elements confer a fitness benefit to the host more frequently than previously recognized. RNA silencing is thought to have evolved as a form of nucleic-acid-based, and thus sequence-directed, immunity to block the action of viruses and TEs. Host-parasite interactions are typically associated with rapid evolution because of a permanent antagonistic relationship resembling an "arms race" in which parasite adaptations are countered by host adaptations. Complex interactions between TEs and RNA silencing machineries have been co-opted to regulate cellular genes.
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