Hybrid Dysgenesis in Drosophila and the Stochastic Loss Hypothesis

Many Drosophila melanogaster genomes appear to carry multiple copies of an apparently mobile element known as the P factor. The positions and numbers of these elements vary widely from one strain to the next, and most long-established laboratory strains (usually 10 years or older) lack it completely. The P factor is defined by its ability to bring about, under certain well-specified conditions, a state called hybrid dysgenesis in which various germ-line abnormalities, including high mutability, the production of chromosomal rearrangements, and temperature-sensitive sterility, can be observed. In addition to the P factor, at least one other similar element, the I factor, is capable of causing hybrid dysgenesis. This element, which differs from the P factor in the details of its associated dysgenic state, is present in wild flies and also in many of the laboratory strains of intermediate age which lack the P factor. In this paper I review the...