| In C. elegans, the QL and QR neuroblasts are bilateral homologs that polarize and migrate in opposite directions along the anterior-posterior body axis. QL and its descendants migrate towards the posterior, whereas QR and its descendants migrate towards the anterior. The netrin receptor UNC-40 and the novel transmembrane protein DPY-19 are required for the early asymmetry of the Q cells. The Hox gene mab-5 is expressed in QL, but not QR, and is necessary and sufficient for the posterior migrations of cells in the QL lineage.; This work describes a genetic screen to identify new genes involved in the migrations of the Q neuroblasts. A set of new mutants is described ( qid-5, qid-6, qid-7, qid-8 and qid-9) that primarily disrupt the migrations of the QL descendants. Most of these mutants were defective in mab-5 expression in the QL lineage, and might identify genes that interact directly or indirectly with the EGL-20/Wnt signaling pathway. In one of these mutants, the migrations in the QL lineage were defective even though mab-5 was still expressed in these cells; this mutation could identify a candidate mab-5 target.; The screen also identified mutants with defects in the migrations of both the QL and QR lineages. Two of these genes, mig-21 and ptp-3, were characterized and were found to be required for the asymmetric polarizations and migrations of the Q cells. mig-21 was cloned and found to encode a novel transmembrane protein that is specifically expressed in the Q cells during their migrations. Thus, MIG-21 may act as a receptor within the Q cells to guide their migrations. Finally, mig-15, a STE20/NIK ortholog, was characterized and found to be required for the migrations, but not the polarizations, of the Q cells. mig-15 also appears to be required for mab-5 expression specifically in the QL lineage.; This thesis presents several new components of a genetic pathway for the guidance of Q cell migration, their interactions with previously known components, and suggests a model for how the genes may function in the migrations of the Q cells. |
