P elements are transposable elements found in drosophila, with high transposition rates. Lab strains, which are P-, have much higher transposition rates. However, the wild-type P+ strains have a much lower transposition rate due to a silencing mechanism found in the oocyte cytoplasm.
Therefore, crossing a P- female with any male drosophila will result in unsuccessful offspring, due to the absence of this silencing mechanism. This silencing mechanism is not fully understood, but evolved in wild populations as a method of increasing the likelihood of successful offspring, via natural selection.