Fig. 7.

Dmbx reporter assays reveal conserved morphogenesis of the ddN and asymmetric unintelligibility of cis-regulatory sequences. a Dorsal view of a Molgula occidentalis embryo electroporated with a Molgula oculata (Moocul) Dmbx>Unc-76::eGFP reporter plasmid, specifically labeling (in green) a developing descending decussating neuron (ddN) on one side of the embryo. Inset at top right shows magnified view of boxed area, showing the conserved axon trajectory of the ddN, which is initially perpendicular to the anterior–posterior axis and then turns 90 degrees to continue posteriorly toward the tail. b Another embryo electroporated with Moocul.Dmbx>Unc-76::eGFP, this time labeling both right and left ddNs. Both axons cross the midline (dashed line) to continue down toward the tail on the other side. RS: right soma, RA: right axon, LS: left soma, LA: left axon. c M. occidentalis embryo electroporated with Ciona robusta Dmbx>Unc-76::Venus reporter, which is activated precisely and robustly in the ddN (green). d Ciona robusta embryo electroporated with Molgula oculata Dmbx>Unc-76::eGFP reporter, which is not active in this species’ ddN (approximate location indicated by dashed circle). Note that non-specific expression (green) can be seen in other unrelated tissues and cell types (palps, mesenchyme, etc.), suggesting that transcription/translation of GFP was not globally affected (see text for details)