DNA representation of variegating heterochromatic P-element inserts in diploid and polytene tissues of Drosophila melanogaster.

Position-effect variegation (PEV) is the mosaic expression of a euchromatic gene brought into juxtaposition with heterochromatin. Fourteen different transformed Drosophila melanogaster lines with variegating P-element inserts were used to examine the DNA levels of these transgenes. Insert sites include pericentric, telomeric and fourth chromosome regions. Southern blot analyses showed that the heterochromatic hsp26 transgenes are underrepresented 1.3- to 33-fold in polytene tissue relative to the endogenous euchromatic hsp26 gene. In contrast, the heterochromatic hsp26 transgenes are present in approximately the same copy number as the endogenous euchromatic hsp26 gene in diploid tissue. It appears unlikely that DNA loss could account for the lack of gene expression in diploid tissues seen with these examples of PEV.