Organisms have to maintain appropriate numbers of various stem cells. Too few can cause infertility or defective tissue regeneration; too many may increase the risk of cancer development. Stem cells are maintained mainly by preventing the expression of differentiation factors - sometimes this occurs through chromatin-mediated transcriptional repression. Maines and coworkers now report that epigenetic control mediated by the DNA-associated protein Stonewall (Stwl) maintains female germline stem cells (GSCs) in Drosophila (see ). The researchers show that clones of stwl-GSCs are lost by differentiation and that overexpression of stwlcauses an expansion of GSCs. Because stwl mutants act as suppressors of variegation (genes that prevent patchy gene silencing within tissues), they propose that Stwl is involved in chromatin-dependent gene repression. Finally,they show that Stwl represses the expression of many genes, some of which contain putative binding sites for Pumilio, a translation inhibitor that,together with Nanos, represses the translation of key differentiation factors in GSCs. Thus, the researchers conclude, two overlapping mechanisms block GSC differentiation.
