OBSERVATIONS ON THE INDUCTION OF POSITION EFFECT VARIEGATION OF EUCHROMATIC GENES IN DROSOPHILA-MELANOGASTER

In the T(1;2)dor(var7) translocation, the IA-2B7-8 segment of the X ch romosome is brought to the vicinity of 2R-chromosome heterochromatin r esulting in position effect variegation of dor, BR-C and more distal g enes, as well as compaction of chromatin in this segment. By irradiati on of T(1;2)dor(var7), nine reversions (rev) to a normal phenotype wer e recovered. In two cases (rev27, rev226), the 1A-2B7-8 section is rel ocated to the 19A region of the X chromosome, forming free duplication s (1A-2B7--8/19A-20F-X-het). Modifiers of position effect do not chang e the normal expression of the BR-C and dor genes in these duplication s. In five reversions (rev3, rev40, rev60, rev167, rev175), free dupli cations have formed from the 1A-2B7-8 fragment and X chromosome hetero chromatin. In these rearrangements, modifiers of position effect (low temperature, removal of Y and 2R-chromosome heterochromatin and a gene tic enhancer (E-var(3)201) induce position-effect again. Two reversion s (rev45 and rev110) are associated with additional inversions in the original dor(var7) chromosomes. The inversions relocate part of the he terochromatin adjacent to the IA-2B7-8 section into new positions. In T(1;2)dor(rev45), position-effect is seen in the 2B7-8-7A element as c ompaction spreading from 2B7-8 proximally in some cases as far as the 5D region. Thus, in rev45 the pattern of euchromatin compaction is rec iprocal to that of the initial dor(var7) strain. Apparently, it is due to the same variegation-evoking center near the 2R centromere in both cases. In all nine revertants, weakening or complete disappearance of the position-effect is observed despite retention of the 20- kb heter ochromatic segment adjacent to the IA-2B7-8 region. Thus, a 20-kb hete rochromatic sequence does not inactivate euchromatin joined to it.