During embryogenesis DD, DA, and DB neuroblasts arise from left and right lineages, move towards the midline and intercalate into a single tract to form the ventral nerve cord (VNC). VNC assembly involves rosette-mediated convergent extension and planar cell polarity (PCP) and Robo pathways [1]. Disruption of this process results in the mispositioning, usually manifested as an anterior displacement, of embryonically-derived motor neurons in the VNC. A more recent analysis of canonical Wnt pathway components also revealed motor neuron position defects. The six DD neurons are evenly spaced along the anterior-posterior (AP) axis and therefore a convenient marker of VNC assembly. We found that mutations in
bar-1/beta-catenin and
pop-1/TCF display DD spacing defects. For example, in
bar-1 and
pop-1 mutants, DD2 is displaced anteriorly and therefore in closer proximity to DD1. In contrast, disruption of the negative Wnt signaling regulator PRY-1/Axin results in the posterior displacement of DD1 such that it is in closer proximity to DD2. To begin to understand the roles of BAR-1 and PRY-1, we examined where endogenously-tagged proteins were expressed during VNC assembly. Using CRISPR/Cas9, we generated
bar-1(
zy97[GFP::
bar-1]) and found that BAR-1 is expressed in right but not left side-derived VNC neuroblasts. In contrast, PRY-1::mNG (
cp383) is expressed in left but not right side-derived neuroblasts. In
pry-1 mutants, BAR-1 is expressed symmetrically in right and left neuroblasts indicating a left-sided repression of BAR-1 signaling. We are examining how neuroblasts intercalate into a single VNC tract in
bar-1 and
pry-1 mutants to understand the role of this asymmetric expression. Preliminary observations suggest that motor neuron positioning defects begin to manifest around the 1.5-fold stage and later. We hypothesize that BAR-1 asymmetry in right side neuroblasts may help contribute to the proper ordering of DD, DA and DB neurons along the VNC. [1] Shah et al. Dev Cell 2017.