The germline is a polar tissue with cells in the distal end proliferating in a mitotic cell cycle, while more proximal cells enter meiosis eventually differentiating as sperm or oocytes. The distal mitotic stem cell population is maintained by a signal from the somatic distal tip cell (DTC) that activates the GLP-1/Notch signaling pathway. When this signal is removed, germs cells enter meiosis prematurely and the stem cell population is lost. DTC/GLP-1 signaling serves to down regulate two genetic pathways, one containing GLD-1, and the other containing GLD-2 (Kadyk and Kimble, 1998). Each of these pathways is able to promote meiotic entry as single mutants in
gld-1 or
gld-2 are able to enter meiosis normally (Francis et al., 1995, Kadyk and Kimble, 1998). It is only when both pathways are not functioning that germ cells fail to enter meiosis. The result is that
gld-2 gld-1 double mutants form a germline tumor of mitotic cells, similar the tumor caused by a constitutive activate form of GLP-1 (Kadyk and Kimble, 1998, Berry et al., 1997). We have undertaken two screens to identify genes that are involved in regulating entry into meiotic prophase. In the first we screened for mutations that enhance a weak gain-of-function allele of
glp-1. These mutations correspond to three genes,
teg-1,
teg-2 and
teg-4. We are currently attempting to clone
teg-1 and have narrowed down its location to a ~30 kbp region to the left of
dpy-18 on chromosome III. The second screen was designed to identify genes that specifically function in the GLD-1 pathway for entry into meiosis. Since the GLD-1 and GLD-2 pathways are each sufficient for entry into meiosis, we screened for mutations that are synthetic tumorous with a loss-of-function allele of
gld-2. To date, we have identified mutations corresponding to at least five genes. As expected, one of these is
gld-1. We also isolated another allele of
teg-1. The remaining genes are termed syt as they are synthetic tumorous with
gld-2. Since this second screen was designed to identify genes that function in the GLD-1 pathway, we performed genetic tests to confirm their position. Our criteria for determining if these genes function in the GLD-1 pathway are the following: 1) The mutation must be synthetic tumorous with
gld-2(lf). 2) The mutation is not tumorous on its own. 3) The mutation is not synthetic tumorous with
gld-1(lf). 4) The synthetic tumorous phenotype with
gld-2(lf) must be epistatic to
glp-1(lf). While
teg-1 meets the first three criteria,
gld-2;
glp-1 teg-1 animals do not have tumorous germlines but rather show a Glp phenotype. This suggests that
teg-1 is not in the GLD-1 pathway, but rather may function further upstream. Another gene that was identified in the second screen,
syt-1, does meet the four criteria for functioning in the GLD-1 pathway for entry into meiosis. In particular,
gld-2;
syt-1;
glp-1 animals have tumorous germlines. We have mapped
syt-1 to linkage group II near
unc-4. We are also in the process of mapping the other mutations that were identified in the second screen.