We have been investigating the cellular remodeling events that enable correct ventral movement of lateral epidermal cells during the first larval stage (L1) in order to complete the ventral hypodermis of the developing worm. Unique to the L1 seam (Vn) cells, we have observed apical junction projections that are always associated with the hypodermally-fated anterior daughters of asymmetric seam divisions (Vn.a cells). The projection (first appearing at the anterior of undivided seam cells in embryos following dorsal intercalation) provides a migratory route for Vn.a cells towards the ventral midline during the hitherto overlooked ventral intercalation of these cells with the underlying P cells. Vn.p seam daughters, with no projection, remain in the seam line and maintain the stem-like fate in order to contribute further hypodermal daughters during asymmetric cell divisions in subsequent larval stages. We describe a crucial role for the homeobox gene
pal-1 in regulating the A-P polarity of these projections and consequently correct seam vs. hypodermal cell positioning.In order to probe the molecular basis of
pal-1 function we performed an RNAi-based modifier screen. Several members of the Wnt and Netrin signaling pathways were isolated as either suppressors or enhancers of the
pal-1 Vn daughter migration defect. Strikingly, a ts mutation in the beta-catenin
wrm-1 phenocopies the
pal-1 epidermal cell positioning defect, and the Wnt effector POP-1 is a putative direct regulator of a
pal-1 seam-specific regulatory element. This suggests a direct interaction between Wnt signaling and
pal-1 function. A null
unc-6 (netrin) allele synergizes strongly with
pal-1, with double mutants displaying markedly increased severity and penetrance of epidermal positioning defects, even though the single netrin allele has no epidermal aberrations.Thus, we describe a novel mechanism for the generation of ventral hypodermis during early larval development. This involves polarized apical junction remodeling allowing ventral movement of correct seam daughters to complete the ventral hypodermis. This newly described mechanism is controlled by
pal-1, working through Wnt and Netrin.