Evidence of P-element-induced sister-chromatid exchange in a ring-X chromosome in Drosophila, with implication for a high rate of formation of hybrid elements.

Activation of a single incomplete P element induces recombination at a rate of approximately 0.5-1% in the male germline of Drosophila. Male recombination rises by an order of magnitude to approximately 20% if homologous P elements are involved. The high rate of recombination suggests the possibility that sister-chromatid exchange (SCE) might be elevated to a similar extent, since homologous P elements must always be present in sister chromatids. This possibility was tested by recombining a single P element onto a ring-X chromosome and using sex-ratio distortion to measure the loss of the ring-X due to SCE in the male germline. The results confirmed a rate of loss comparable to that expected with homologous elements, although the rate of loss was variable. Both SCE and recombination results are consistent with the "hybrid element insertion" model, in which the left and right ends from different elements associate, providing that insertion occurs preferentially in the vicinity of a P-element end. For autosomes, hybrid element formation may thus occur at a much higher rate than the 0.5-1% implied by single element recombination, with only a small minority of hybrid element excision events being resolved by recombination.