The position of the nucleus in a polarized location within the cytoplasm is essential to many developmental events. Two related processes function to first migrate the nucleus to a specific location, and then to anchor the nucleus in place. Mutations in
unc-83 and
unc-84 disrupt nuclear migration in P-cells, resulting in cell death and loss of the P-cell lineage, which causes Egl and Unc phenotypes. Mutations in
anc-1 or
unc-84 disrupt the anchorage of nuclei. Our goal of this study is to identify additional loci involved in nuclear positioning. The P-cell nuclear migration defect is temperature sensitive in a null
unc-83 or
unc-84 background. Null mutations in
unc-83 or
unc-84 disrupt nuclear migration at 25 ! C, but at 16 ! C nearly all P-cell nuclei migrate normally, suggesting that additional pathways exist, which function redundantly to UNC-83 and UNC-84 1-3 . We therefore hypothesize that an enhancer pathway is sufficient for P-cell nuclear migration at 15 ! C, but that both the enhancer pathway and the UNC-83/UNC-84 pathway are required at 25 ! C. To test our hypothesis we carried out genetic screens for e nhancers of the nuclear m igration defect of u nc-83 or
unc-84 ( emu genes). Using EMS we mutagenized null
unc-83(
e1408) or
unc-84(
n369) worms and screened their F2 progeny for worms with P-cell nuclear migration defects at the normally permissive temperature of 16 ! C. In the primary screen we searched for Egl worms. Since this phenotype can result from a number of other defects besides failed P-cell nuclei migration a secondary screen was performed on Egl animals to directly count the number of GABA neurons in the ventral cord. 12 of the 20 GABA neurons normally present are derived from P-cell lineages. UNC-47::GFP, which is expressed in the GABA neurons 4 , was employed during this screen. We have isolated 11 eue alleles that disrupt P-cell nuclear migration at the normally permissive temperature of 16!C. We are in the process of crossing out
unc-83 or
unc-84 to determine if the emu mutants have a phenotype on their own. The
yc8 allele appears to have no phenotype on it's own. Since our EMS screen was so successful, we are continuing our search for emu genes using a genome-wide RNAi feeding screen. The original screen for anchorage defective (Anc) mutations isolated 5 alleles of
anc-1 , but no alleles of
unc-84 5 . This is not surprising given the huge target of
anc-1 , which encodes a 25 kb cDNA 6 . We therefore hypothesized that screens for Anc mutants are not saturated. We have undertaken both EMS and RNAi feeding screens for additional Anc mutant animals. EMS screens have isolated 5 new alleles of
anc-1 and one weak allele,
yc11 , that is not in the
anc-1 locus. In the RNAi feeding screen, we have completed chromosome I. The only gene on chromosome I found to disrupt nuclear anchorage was
anc-1 . We are continuing the RNAi feeding screen. 1. Sulston, J. E. & Horvitz, H. R. Dev Biol 82, 41-55 (1981). 2. Starr, D. A., Hermann, G. J., Malone, C. J., Fixsen, W., Priess, J. R., Horvitz, H. R. & Han, M. Development 128, 5039-50 (2001). 3. Malone, C. J., Fixsen, W. D., Horvitz, H. R. & Han, M. Development 126, 3171-81 (1999). 4. McIntire, S. L., Reimer, R. J., Schuske, K., Edwards, R. H. & Jorgensen, E. M. Nature 389, 870-6 (1997). 5. Hedgecock, E. M. & Thomson, J. N. Cell 30, 321-330 (1982). 6. Starr, D. A. & Han, M. Science 298, 406-409 (2002).