Live Analysis of Position Effect Variegation Reveals Different Modes of Action for HP1 and Su(var)3-9

Abstract

AbstractPosition Effect Variegation (PEV) results from the juxtaposition of euchromatic and heterochromatic components of eukaryotic genomes, silencing genes near the new euchromatin/heterochromatin junctions. The degree of silencing is itself heritable through S-phase, giving rise to distinctive random patterns of cell clones expressing the genes intermixed with clones in which the genes are silenced. Much of what we know about epigenetic inheritance in the soma stems from work on PEV aimed at identifying the components of the silencing machinery and its mechanism of inheritance. Despite identifying two central gene activities – the Su(var)3-9 histone H3-Lysine-9 methyltransferase and the Su(var)205/HP1 methyl-H3-Lysine-9 binding protein – their role in PEV has been inferred from terminal phenotypes, leaving considerable gaps in understanding how PEV behaves through development. Here, we investigate the phenotypes of Su(var)3-9 and Su(var)205/HP1 mutations in live developing tissues. We discovered that mutations in Su(var)205/HP1 compromise the initial establishment of PEV in early embryogenesis. Later gains of heterochromatin-induced gene silencing are possible, but are unstable and lost rapidly. In contrast, mutations in Su(var)3-9 exhibit robust silencing early in development, but fail to maintain it through subsequent cell divisions. Our analyses show that while the terminal phenotypes of these mutations may appear identical, they have arrived at them through different developmental trajectories. We discuss how our findings further challenge existing models for epigenetic inheritance of heterochromatin-induced gene silencing.