Repression of P element-mediated hybrid dysgenesis in Drosophila melanogaster.

Inbred lines derived from a strain called Sexi were analyzed for their abilities to repress P element-mediated gonadal dysgenesis. One line had high repression ability, four had intermediate ability and two had very low ability. The four intermediate lines also exhibited considerable within-line variation for this trait; furthermore, in at least two cases, this variation could not be attributed to recurring P element movement. Repression of gonadal dysgenesis in the hybrid offspring of all seven lines was due primarily to a maternal effect; there was no evidence for repression arising de novo in the hybrids themselves. In one of the lines, repression ability was inherited maternally, indicating the involvement of cytoplasmic factors. In three other lines, repression ability appeared to be determined by partially dominant or additive chromosomal factors; however, there was also evidence for a maternal effect that reduced the expression of these factors in at least two of the lines. In another line, repression ability seemed to be due to recessive chromosomal factors. All seven lines possessed numerous copies of a particular P element, called KP, which has been hypothesized to produce a polypeptide repressor of gonadal dysgenesis. This hypothesis, however, does not explain why the inbred Sexi lines varied so much in their repression abilities. It is suggested that some of this variation may be due to differences in the chromosomal position of the KP elements, or that other nonautonomous P elements are involved in the repression of hybrid dysgenesis in these lines.