Little is known regarding the molecular basis for early proliferation of the primordial germ cells, Z2 and Z3. The first several rounds of germ cell division are independent of GLP-1, 1 yet cell-cell signaling appears to be required. 2 The gonad primordium contains four cells: Z1, Z2, Z3 and Z4 3,4 . Ablation of Z1 and Z4, the somatic gonad precursors, prevents Z2 and Z3 from dividing or entering meiosis. 2 Therefore, cell-cell communication is necessary for both the proliferation and meiotic competence of Z2 and Z3. The result of ablation of Z1 and Z4 differs from the
glp-1 null mutant phenotype in which Z2 and Z3 divide several times, enter meiosis early, and form mature sperm. 1 The effect of this ablation also differs from the phenotype of other germline proliferation mutants in which Z2 and Z3 divide several times but do not enter meiosis (e.g.,
glp-4 , 5 and
glp-3 6 ). The molecular components of the Z1/Z4-to-Z2/Z3 signaling event are not known, and we are using a genetic approach to identify these components. We have found several mutants in which the adult hermaphrodite somatic gonad appears normal but apparently houses no germ cells (Nog phenotype). A host of developmental defects could result in a Nog phenotype. For example, Z2 and Z3 may not be generated, may die, or their descendents may die. Alternatively, Z2 and Z3 may be generated but may not reach the gonad primordium or they may reach the primordium but fail to proliferate. Interference with cell-cell communication between the somatic and germ lineages suggested by the Z1/Z4 ablation experiment could account for the last scenario. One of our Nog mutants,
ar228 , hatches with a normal-looking L1 gonad primordium containing four cells. During the late L1, however, at a time when the germ line of heterozygous siblings has begun to proliferate, the two central cells take on a slightly abnormal morphology under Nomarski optics. To assess the identity and fate of the two central cells in the primordium, we are performing a time-course analysis with a germ line-specific antibody. The
ar228 mutants display no other obvious behavioral or morphological defects. Therefore, characterization of the gene defined by the
ar228 allele could provide a molecular handle on signaling that initiates the proliferation of Z2 and Z3. Alternatively, this mutation could produce a germ line-specific cell-cycle defect or otherwise affect the maintenance of the germ cell fate. Genetic analysis indicates that
ar228 is recessive, completely penetrant and maps to the left arm of LGV.
ar228/sDf50 displays the same phenotype as an
ar228 homozygote, suggesting that
ar228 is a strong loss-of-function allele. Preliminary data also suggests that
ar228 fails to complement
e2173 , a small deletion that originally defined the
lin-40 locus. This allele deletes at least three complementation groups. 7,8 Mutants representing two of the genes within the
e2173 deletion (
s1669 and
s1611 ) both complement
ar228 . Therefore,
ar228 identifies one of the other genes in this region. Experiments are underway to locate the
ar228 locus from among the ORFs in the region. We gratefully acknowledge Bob Johnsen and David Baillie for providing stains and information pertaining to the
e2173 deletion. Thanks also to the Kimble lab for providing a GFP-tagged version of the nT1 balancer that has proven very helpful in our experiments. 1. Austin and Kimble (1987) 2. Kimble and White (1981) 3. Hirsh et al., (1976) 4. Klass et al. (1976) 5. Beanan and Strome (1992) 6. Kadyk et al. (1997) 7. Solari and Ahringer (2000) 8. R. Johnsen and D. Baillie (personal communication)