The significance of the germ line lies in its totipotency: it retains the ability to not only produce the next generation, but also to produce all somatic lineages present in the offspring. Transcriptional repression (silencing) of the embryonic germline appears to be essential for its development and maintenance in C. elegans. Mutations in the
pie-1 gene, for example, relieve transcriptional repression in the embryonic germ line and this results in transformation to soma. Repressive mechanisms are also important for maintaining the post-embryonic germline. Repetitive transgenes are strongly silenced in the adult germline, and mutations that disrupt this silencing cause defects in fertility. Examples include the Maternal Effect Sterile genes (mes) identified and studied by Susan Strome's group, and several of the rde and mut genes (
rde-2,
rde-3,
mut-7) studied by Craig Mello and Ronald Plasterk's groups. A screen for mutations that alleviate transgene desilencing recovered seven such mutations in loci termed Silencers In Germline (sig) genes. All sig mutants exhibit transgene silencing defects and fertility defects. Only one of the mutations,
sig-7, exhibits observable somatic defects, suggesting little overlap between germline silencing mechanisms and essential somatic function.
sig-7 is a fully recessive, zygotic sterile mutation whose phenotype is sterility, thin-body, and is often accompanied by the somatic defect of a protruding vulva.
sig-7 animals, like other sig mutants, also exhibit germ line desilencing.
sig-7 also exhibits a mog (masculinization of thegerm line) phenotype in which hermaphrodites produce only sperm: the functional status of these sperm is unknown. The ultimate goal is to understand why silencing of its germ line is needed for its proper development and to understand how its totipotency is retained.
sig-7's role in this process will be investigated by identifying its molecular identity through mapping its genetic locus, and characterizing the nature of its silencing defect. We have genetically mapped
sig-7 to an interval on LG I, between
unc-29 and
fog-3. Several evl mutants(
evl-9,-16,-17) have been mapped to this region, and we are testing them for complementation by
sig-7. Our progress in these studies will be presented.