Shortly after hatching of the Ll larva, the right and left Q neuroblasts undergo a series of divisions and migrations that produce a set of three neurons on each side of the animal. There is a left/right difference in the direction of migration for cells in the Q lineage: on the left side the Q cell and some of its descendants migrate towards the posterior of the animal while on the right side the Q cell and its descendants migrate in the anterior direction. Both genetic and molecular data indicate that the
mab-5 gene product is specifically required for posterior migration by descendants of the Q neuroblasts (Chalfie et al. Science 221:61, Hedgecock et al. Development 100:365, 5. Salser WBG 11:5). If
mab-5 activity is absent, the descendants of both QR and QL migrate towards the anterior of the animal while, in the presence of ectopically expressed
mah5, descendants of both QR and QL migrate towards the posterior of the animal. In contrast, little is known about what is required for the wild-type anterior migration of QR and its descendants. In a screen for mutations affecting the migration of cells in the Q lineage, I isolated two mutations (
mu26 and
mu28) that affect migration of the descendants of QR. In animals homozygous for either
mu26 or
mu28, QR.p and its descendants fail to migrate to their normal anterior positions.
mu26 is an allele of
lin-39, a gene originally identified on the basis of its role in vulval formation (Clark and Horvitz WBG 10:2). Iin-39 maps close to
mab-5 and, like
mab-5, seems likely to encodes an Antennapedia-class homeobox protein (Wang and Kenyon WBG 11:5). While
mab-5 is required for proper post-embryonic development of many cells in the posterior region of the worm, mutations in
lin-39 affect development of cells in the central body region. It is therefore interesting that whereas
mab-5 is required for migration of descendants of QL through the posterior body region, lin- 39 mutations affect anterior migration of descendants of QR through the central region of the animal. mig
(mu28) homozygotes show a defect in QR migrations similar to that seen for
lin-39(
mu26). In the case of mig
(mu28) this phenotype is variable, but this may be because mig
(mu28) is a hypomorph: the phenotype of mig
(mu28)/Df animals is stronger and more consistent than that seen in mig
(mu28) homozygotes.