The Wilhelmine E. Key 1991 Invitational Lecture. The evolutionary history of the P family of transposable elements.

Similar to other transposable genetic elements, P elements occasionally exhibit non-Mendelian inheritance because of their ability to move, from one genomic site in their host species to another, during certain phases of their life cycle. The biological range of this capacity for transposition is almost always restricted to new sites within the same nuclear genome, but exceptionally it appears that interspecific horizontal transfer of P elements can occur. Although the P family appears to have had an ancient origin, its present natural distribution appears to be patchy and phylogenetically restricted to a limited number of Dipteran species. The most likely explanation of the observed restriction is the requirement of transposition for a host-encoded factor whose range is itself similarly restricted. Occasional horizontal transfer of P elements into a new host species, together with the normal mode of vertical transfer, may be the mechanism that ensures the survival of this, and other transposable element families, over long periods of evolutionary time.