Polycomb-group genes devoid of effect on Telomeric Position Effect (TPE) may however act synergistically. A. Boivin , D. Anxolabehere , S. Ronsseray. Dynamique genome evolution, Institut Jacques Monod, Paris, Paris, France.

   In Drosophila, Telomeric Position Effect (TPE) occurs when a transgene containing a reporter gene is inserted near telomeres in Telomeric Associated Sequences (TAS). The TPE resembles other epigenetic phenomena (PEV and Pc-G repression), but the dominant modifiers of the classical PEV (including Y dosage) and most of the mutants of Pc-G genes have no detectable effect on it (Wallrath et Elgin 1995; Crydermann et al. 1999). Recently, Bob Levis and colleagues have shown that strong alleles of the polyhomeotic gene have a strong Suppressor effect on TPE. On the basis of these previous data, we hypothesized that Pc-G genes may act synergistically on TPE. Here, we show that specific combinations of Pc-G alleles (including ph, Asx, Psc, Pcl, Sce) have a synergistic suppressor effect whereas others have not. Moreover, the effect of Pc-G mutants can strongly depend on the location of the telomeric insert. Consequently, a given mutant may have no effect alone on TPE, but may however show a suppressor effect in combination with another Pc-G gene mutant. These results indicate that a number of Pc-G genes could be involved in TPE but only a strong disturbance of the repressor multiproteic complexes is phenotypically detectable. This disturbance cannot be assayed in the absence of one partner since in most cases, homozygous state for a null allele is lethal. Conversely, a reduced dosage of two partners, tested in trans-heterozygous flies, may be viable and allows therefore TPE modification to be detected. This contrasts with the effect of the Pc-G mutants on their euchromatic target genes which are sensitive to a single null mutation. This suggests therefore that Pc-G-mediated TPE would be basically more strongly stabilized than the euchromatic Pc-G-mediated repression.