ENHANCER OF POLYCOMB IS A SUPPRESSOR OF POSITION-EFFECT VARIEGATION IN DROSOPHILA-MELANOGASTER

Polycomb group (PcG) genes of Drosophila are negative regulators of ho meotic gene expression required for maintenance of determination. Sequ ence similarity between Polycomb and Su(var)205 led to the suggestion that PcG genes and modifiers of position-effect variegation (PEV) migh t function analogously in the establishment of chromatin structure. If PcG proteins participate directly in the same process that leads to P EV, PcG mutations should suppress PEV. We show that mutations in E(Pc) , an unusual member of the PcG, suppress PEV of four variegating rearr angements: In(l)w(m4), B-SV, T(2;3)Sb-V; and In(2R)bw(YDe2). Using rev ersion of a P element insertion, deficiency mapping, and recombination mapping as criteria, homeotic effects and suppression of PEV associat ed with E(Pc) co-map. Asx is an enhancer of PEV whereas nine other PcG loci do not affect PEV. These results support the conclusion that the re are fewer similarities between PcC genes and modifiers of PEV than previously supposed. However, E(Pc) appears to be an important link be tween the two groups. We discuss why Asx might act as an enhancer of P EV.