Modifiers of Terminal Deficiency-Associated Position Effect Variegation in Drosophila

Corresponding author: Gary H. Karpen, The Salk Institute for Biological Studies, 10010 N. Torrey Pines Rd., La Jolla, CA 92037., karpen{at}salk.edu (E-mail)

Communicating editor: R. S. HAWLEY

Terminal deletions of a Drosophila minichromosome (Dp(1;f)1187) dramatically increase the position effect variegation (PEV) of a yellow⁺ body-color gene located in cis. Such terminal deficiency-associated PEV (TDA-PEV) can be suppressed by the presence of a second minichromosome, a phenomenon termed "trans-suppression." We performed a screen for mutations that modify TDA-PEV and trans-suppression. Seventy suppressors and enhancers of TDA-PEV were identified, but no modifiers of trans-suppression were recovered. Secondary analyses of the effects of these mutations on different PEV types identified 10 mutations that modify only TDA-PEV and 6 mutations that modify TDA-PEV and only one other type of PEV. One mutation, a new allele of Su(var)3-9, affects all forms of PEV, including silencing associated with the insertion of a transgene into telomeric regions (TPE). This Su(var)3-9 allele is the first modifier of PEV to affect TPE and provides a unique link between different types of gene silencing in Drosophila. The remaining mutations affected multiple PEV types, indicating that general PEV modifiers impact TDA-PEV. Modifiers of TDA-PEV may identify proteins that play important roles in general heterochromatin biology, including proteins involved in telomere structure and function and the organization of chromosomes in the interphase nucleus.

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