The QL and QR neuroblasts are born in bilaterally symmetric positions along the A-P body axis in C. elegans (QL on the left and QR on the right). Shortly after hatching, the two cells undergo characteristic, stereotypical L/R asymmetric migrations. QL extends a long process to the posterior and its nucleus then migrates into this projection. Following this migration, the Hox gene
mab-5 is expressed in QL and its descendants remain in the posterior.
mab-5 expression is both necessary and sufficient for Q descendants to remain in the posterior. Conversely, QR extends a process to the anterior and subsequently migrates anteriorly.
mab-5 is not expressed in QR, hence its descendants continue to migrate anteriorly. Each cell moves to a well-defined final position. In
unc-40 or
dpy-19 mutants, polarisation of both Q cells is randomised and the Q nuclei fail to undergo their migrations. This randomisation of Q polarisation is coupled with a randomisation of
mab-5 expression. Either cell, QL or QR, can express
mab-5, and this expression then determines the fate of that cell's descendants. In order to gain a more complete picture of how the initial L/R asymmetry of the Q cells is established, we are studying 11 new mutants from an extensive Q cell migration screen (Queelim Ch'ng and Mary Sym, unpublished results). These mutants fall into 4 complementation groups and have similar phenotypes to
unc-40 and
dpy-19, and are therefore good candidates for new genes in this pathway. Work is underway to characterise, map and clone these new genes. qid --1 (Q Is Defective, 8 alleles) maps to a 180kb region on the left arm of chromosome III, and is rescued by injection of a cosmid pool covering this region.
qid-2 (1 allele) maps to the right arm of chromosome X, and further mapping is underway. Mapping and further characterisation are also planned for
qid-3 (1 allele) and
qid-4 (1 allele), although these mutants have less penetrant Q phenotypes and appear to be more pleiotropic.