Transposable elements and the evolution of heterochromatin.

Drosophila P elements were shown to insert frequently into telomeric and centromeric heterochromatin, and to prefer a region associated with efficient copy number regulation. Upon excision, P elements frequently altered the number of repeats in a tandem array of heterochromatic sequences, by inducing unequal gene conversion. These studies suggest that a flux of transposable element insertions and excisions has the capacity to rapidly and nonrandomly modify heterochromatic sequences dispersed at multiple chromosomal sites. We propose that transposable elements maintain genomic heterochromatin in a state of dynamic equilibrium and drive its rapid evolution.