Abstract
Drosophila simulans presents a large variation in copy number among various transposable elements (TEs) and among natural populations for a given element. Some elements are absent in most populations, except in one or two which have many copies. This suggests that some TEs are being awaken in some populations and are in the process of invading the species while it is colonizing the world. We present evidence for this hypothesis by a detailed analysis of copy number, structure of the copy sequences, and transcription rate of the retrotranspos-able element 412 in natural populations of the two sister species D. melanogaster and D. simulans. The data support the idea that colonization of new habitats by a species is associated with mobilization of TEs, which leads to an increase in genome size of the invading species.