What is the impact of transposable elements an host genome variability?

The spread of a transposable element family through a wild population may b e of astonishing rapidity. At least three families of transposable genetic elements have recently invaded Drosophila melanogaster worldwide, including the P element. The mechanism has been a process of effectively replicative transposition, and, for the P element, has occurred notwithstanding the st erility induced by unrestricted movement. This element's invasion into D. m elanogaster has been accompanied by the development of heterogeneity betwee n P sequences, most of which now have internal deletions. Increasing eviden ce suggests that some deleted elements can repress P transposition, thereby protecting the host from the harmful effects of complete elements. Such re pressing elements may rise to high frequencies in populations as a result o f selection at the level of the host. We here investigate selective sweeps invoked by the spread of P sequences in D. melanogaster populations. Numero us high-frequency sites have been identified on the X chromosome, which dif fer in frequency between populations, and which are associated with repress ion of P-element transposition. Unexpectedly sequences adjacent to high-fre quency P-element sites do not show reduced levels of genetic diversity, and DNA variability is in linkage equilibrium with the presence or absence of a P element at the adjacent selected site. This might be explained by multi ple insertions or through a selection for recombination analogous to that s een in 'hitchhiking'.