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

A tubular nervous systemis present in the deuterostomegroups Chordata ( cephalochordates, tunicates, vertebrates) and in the non- chordateEnteropneusta. However, the worm-shaped enteropneusts possess a less complex nervous systemfeaturing only a short hollow neural tube, whereby homology to its chordatecounterpart 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 +  Engrailedexpression, in a region corresponding to the chordatebrain. Neuronal Nk2.1/Nk2.2 expression is absent. Interestingly, we found median Dlx and lateral Pax6expression domains, i.e., a condition that is reversed compared to chordates. Comparative analyses reveal that adult nervous systempatterning is highly conserved among the enteropneust families Harrimaniidae, Spengelidae and Ptychoderidae. BmiDlx and BmiPax6 have no corresponding expression domains in the chordatebrain, which may be indicative of independent acquisition of a tubular nervous systemin Enteropneusta and Chordata.