A chromosome inversion alters the pattern of specific DNA replication in drosophila follicle cells

The ocelliless (oc) mutation is associated with a small chromosome inversion (In(1)7F1,2-8A1,2) whose distal breakpoint is located 1-3 kb upstream from the s36-1 and s38-1 chorion protein structural genes. The breakpoint lies within the central portion of a 90 kb region, containing chorion genes and adjacent chromosomal sequences, which normally undergoes amplification in ovarian follicle cells (Spradling, 1981). The pattern of differential replication of these sequences is drastically altered in the ocelliless chromosome. The 40 kb of DNA lying distal to the 7F breakpoint completely fails to amplify. The remaining 50 kb, now located at cytogenetic locus 8A, still undergoes amplification, although of a somewhat reduced magnitude. Amplification "spreads" past the proximal breakpoint, however, because normally unamplified sequences adjacent to the breakpoint in 8A differentially replicate in ocelliless follicle cells. These observations suggest that a specific chromosomal region located proximal to the ocelliless breakpoint at 7F is required in cis for amplification, possibly as a specific origin of replication. The pattern of amplification in ocelliless is explained most simply by the repositioning of this control sequence due to the inversion. As a consequence, the dosage of large, continuous chromosome regions adjacent to the breakpoints is altered in follicle cells. The effects of the ocelliless inversion on chorion gene expression are analogous to the "position effects" associated with many other chromosome rearrangements in Drosophila. Disturbances of differential DNA replication may be reponsible for additional examples of position effects.