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A tubular nervous system is present in the deuterostome groups Chordata (cephalochordates, tunicates, vertebrates) and in the non-chordate Enteropneusta. However, the worm-shaped enteropneusts possess a less complex nervous system featuring only a short hollow neural tube, whereby homology to its chordate counterpart remains elusive. Since the majority of data on enteropneusts stem from the harrimaniid  Saccoglossus kowalevskii , putative interspecific variations remain undetected resulting in an unreliable ground pattern that impedes homology assessments. In order to complement the missing data from another enteropneust family, we investigated expression of key neuronal patterning genes in the ptychoderid  Balanoglossus misakiensis . The collar cord of  B. misakiensis  shows anterior  Six3/6  and posterior  Otx  +  Engrailed  expression, in a region corresponding to the chordate brain. Neuronal  Nk2.1/Nk2.2  expression is absent. Interestingly, we found median  Dlx  and lateral  Pax6  expression domains, i.e., a condition that is reversed compared to chordates. Comparative analyses reveal that adult nervous system patterning is highly conserved among the enteropneust families Harrimaniidae, Spengelidae and Ptychoderidae.  BmiDlx  and  BmiPax6  have no corresponding expression domains in the chordate brain, which may be indicative of independent acquisition of a tubular nervous system in Enteropneusta and Chordata.

Neuronal patterning of the tubular collar cord is highly conserved among enteropneusts but dissimilar to the chordate neural tube