Mutations in Su(var)205 and Su(var)3-7 Suppress P-Element-Dependent Silencing in Drosophila melanogaster

In Drosophila melanogaster, the w+ transgene in P{lacW}ciDplac is uniformly expressed throughout the adult eye. However, when other P elements are present, this w+ transgene is randomly silenced and this produces a variegated eye phenotype. This P-element-dependent silencing (PDS) is limited to w+ transgenes inserted in a specific region on chromosome 4. In a screen for genetic modifiers of PDS, we isolated mutations in Su(var)205, Su(var)3-7, and two unidentified genes that suppress this variegated phenotype. Therefore, only a few of the genes encoding heterochromatic modifiers act dose dependently in PDS. In addition, we recovered two spontaneous mutations of P{lacW}ciDplac that variegate in the absence of P elements. These P{lacW}ciDplac derivatives have a gypsy element inserted proximally to the P{lacW}ciDplac insert. The same mutations that suppress PDS also suppress w+ silencing from these P{lacW}ciDplac derivative alleles. This indicates that both cis-acting changes in sequence and trans-acting P elements cause a similar change in chromatin structure that silences w+ expression in P{lacW}ciDplac. Together, these results confirm that PDS occurs at P{lacW}ciDplac because of the chromatin structure at this chromosomal position. Studying w+ variegation from P{lacW}ciDplac provides a model for the interactions that can enhance heterochromatic silencing at single P-element inserts.