Neuronal Patterning Of The Tubular Collar Cord Is Highly Conserved Among Enteropneusts But Dissimilar To The Chordate Neural Tube

The dorsal neural tube of chordates and the ventral nerve cord of annelids exhibit a similar molecular mediolateral architecture. Accordingly, the presence of such a complex nervous system (CNS) has been proposed for their last common ancestor. Members of Enteropneusta, a group of non-chordate deuterostomes, possess a less complex CNS including a hollow neural tube, whereby homology to its chordate counterpart remains elusive. Since the majority of data on enteropneusts stem from Saccoglossus kowalevskii, a derived direct-developer, we investigated expression of key neuronal patterning genes in the indirect-developer Balanoglossus misakiensis.\n\nThe 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.\n\nComparative analyses reveal that CNS patterning is highly conserved among enteropneusts. BmiDlx and BmiPax6 have no corresponding expression domains in the chordate brain, which may be indicative of independent acquisition of a tubular CNS in Enteropneusta and Chordata. Moreover, mediolateral architecture varies considerably among chordates and enteropneusts, questioning the presence of a vertebrate-like patterned nervous system in the last common deuterostome ancestor.