P element-encoded regulatory products enhance Repeat-Induced Gene Silencing (RIGS) of P-lacZ-white clusters in Drosophila melanogaster.

In Drosophila melanogaster, some clusters of P transgenes ( P-lacZ-white) display a variegating phenotype for the white marker in the eye, a phenomenon termed "Repeat-Induced Gene Silencing" (RIGS). We have tested the influence of the P element repression state (P cytotype) on the eye phenotype of several P-lac-w clusters that differ in transgene copy number or genomic insertion site. P element-encoded regulatory products strongly enhance RIGS. The effect occurs in both sexes, is detectable with clusters having at least three copies and is observed at both genomic locations tested (cytogenetic regions 50C and 92E). Single variegating P-lac-w transgenes located in pericentromeric heterochromatin are not affected by P regulatory products. All P strain backgrounds tested enhance RIGS, including chromosomes bearing a single P element encoding a truncated P transposase or carrying a single internally deleted KP element. Therefore, clusters are highly sensitive to different types of P repressors. Finally, a chimeric gene in which the 5' portion of the P element is fused to the polyhomeotic coding sequence (ph(p1)) also strongly enhances silencing of P-lac-w clusters. These results have implications for the mechanism of action of the P repressors and show that P transgene clusters represent a new class of P-sensitive alleles, providing a simple assay for somatic P repression that can be completed in one generation.