Anecdotal , Historical and Critical Commentaries on Genetics

S OMEWHERE in Latin America a single P-element copy found its way into the genome of Drosophila melanogaster from another insect species. Once there, these transposable elements made use of their new hosts’ DNA repair mechanism to increase their copy number while transposing to new genomic positions. Within the span of a few decades they spread worldwide to encompass nearly the entire species. The only populations to escape this invasion were the stocks that were maintained in the laboratories of early Drosophila geneticists and were thus reproductively isolated from the rest of the species. This remarkable scenario (reviewed by ENGEU 1992) was followed by a Pelement invasion of another kind. Within the last 10 years these elements have become ubiquitous tools for Drosophila geneticists of all stripes and have changed the way Drosophila research is conducted. The spread of P elements through natural populations of D. melanogaster went largely unnoticed while it was happening, with the possible exception of some early observations of unstable mutations in the Soviet Union that might have been due to P mobility (BERG 1974). It was not until the 1970s that P elements were recognized as mobile genetic sequences, and by then the invasion was essentially complete. At that time, Drosophila research itself seemed to be on the wane. Most of the exciting and fundamental work on genetic mechanisms was being done with Escherichia coli, and flies were increasingly associated with old-fashioned classical genetics. Only the field of population genetics clung to Drosophila as the experimental organism of choice. Many population genetic experiments involved capturing flies from nature and crossing them to laboratory stocks in order to “extract” chromosomes for fitness measurements or to study variability in other traits such as recombination frequency. It is now known that crosses of this kind provided precisely the conditions needed to mobilize P elements and bring them to the attention of experimenters.