GLP-1 is a member of the Notch family of transmembrane receptors and functions to promote germ cell proliferation in C. elegans. Activation of the GLP-1 receptor, expressed in the germline, is spatially regulated by the transmembrane ligand, LAG-2, expressed in the distal tip cell. Binding of LAG-2 to GLP-1 induces receptor cleavage generating GLP-1(INTRA), which is then presumed to translocate to the nucleus, bind LAG-1, and alter the pattern of transcription. Disruption of this pathway causes all proliferating germ cells to prematurely enter meiosis. Conversely, constitutive activation by the
glp-1(
oz112) gain-of-function (gf) mutation results in the formation of a germline tumor where proliferative cells are found throughout the gonad. Two genes,
gld-1 and
gld-2, function redundantly downstream of GLP-1 to inhibit proliferation and/or promote meiotic development.
glp-1 signaling, in effect, inhibits both genes to promote germ cell proliferation. We have taken a forward genetic approach to identify genes that either function downstream of GLP-1 to promote entry of germ cells into meiosis, or that function to negatively regulate GLP-1 signaling. A number of screens have been conducted in sensitized backgrounds in order to identify mutations that confer a synthetic tumorous (Syt) phenotype. One of the first screens utilized a weak
glp-1(
oz112oz120 gf) background and identified mutations in the genes
teg-1 and
teg-4.
gld-2;
teg-1 and
gld-2teg-4 are also tumorous. Interestingly, in contrast to the
gld-2gld-1;
glp-1(null) triple mutant, which is tumorous,
gld-2;
glp-1teg-1 and
gld-2teg-4;
glp-1 triple mutants display the Glp premature meiotic entry phenotype. The
gld-1 pathway gene,
nos-3, was used in another screen to identify genes that function in the
gld-2 pathway. 13 Syt mutants were identified: 8
gld-2 alleles, 4
gld-3 alleles, and one weak
glp-1(gf) allele,
oz272. Another screen using a
teg-1 mutant background identified two weak
glp-1(gf) mutants,
oz264 and
oz274; molecular lesions are located in the second and third LNG repeats, regions where other
glp-1/lin-12(gf) mutations have been localized. Finally, another screen utilized the
glp-1(
oz264) mutant background, which identified one Syt mutant,
oz273. It appears to be associated with a Mog (masculinization of the germline) phenotype in the
glp-1(+) background. The mutation maps to LG1, suggesting that it may be a
gld-1 allele, but complementation tests with
gld-1(null) suggest otherwise. Further mapping and sequencing analysis need to be conducted to identify this gene.