Germ-line gene expression is restricted by many mechanisms which maintain the integrity of the germ line in C. elegans. We have discovered a novel form of silencing that can be prevented by maternally inherited transcripts, a rare example of a positive role for RNA in regulating gene expression. This phenomenon was discovered through its effect on the inheritance of
fem-1, a gene required for spermatogenesis in both sexes and for male somatic development. When females carrying any of three deficiency alleles,
fem-1(Df), are crossed to wild type males, their heterozygous progeny often exhibit germ line feminization (the Fog phenotype). In contrast, cross-progeny of females carrying other
fem-1 null alleles, such as the nonsense mutation
e2268, are phenotypically wild type. We found that injecting in vitro-transcribed
fem-1 RNA into the germ line of the
fem-1(Df) females rescued spermatogenesis in their progeny, suggesting that the Fog phenotype was caused by a failure of the deficiency chromosomes to provide
fem-1 RNA to the offspring. Non-overlapping fragments of
fem-1 RNA rescued, indicating that neither a specific region of
fem-1 RNA nor functional FEM-1 protein is required. We hypothesized that maternal
fem-1 RNA may be needed in order to promote the activity of the
fem-1 locus in the zygotic germ line. This idea is supported by the observations that the cross-progeny of
fem-1(Df) females have reduced
fem-1 transcript accumulation and genetic activity in the germ line. The reduction in
fem-1 activity is also heritable since the proportion of Fog progeny increases upon backcrossing to
fem-1(Df) females. Together, these data suggest that
fem-1 is heritably silenced in the germ line in the absence of maternal RNA, but provision of maternal
fem-1 RNA can promote expression of the gene. This mechanism could be a means of protecting the germ line by only licensing activity of those genes that were transcribed in the germ line of the previous generation. To investigate the mechanisms involved in silencing and licensing
fem-1, we performed an RNAi screen of candidate genes. Identification of the C. elegans piwi homologs
prg-1 and
prg-2 as strong suppressors of
fem-1 germ-line silencing implicates their products, perhaps in association with small RNAs whose accumulation depends on their activity, in the mechanism of germ-line silencing that is opposed by maternal
fem-1 RNA. Among the strongest enhancers were certain members of the C.elegans-specific branch of the Argonaute family. Thus the licensing of germ-line gene expression by maternal RNA adds to the expanding repertoire of regulatory roles served by Argonaute proteins.