Pax37 gene function in Oikopleura dioica supports a neuroepithelial-like origin for its house-making Fol territory

Larvacean tunicates feature a spectacular innovation not seen in any other chordate or other animals - the oikoplastic epithelium of the trunk. This epithelium produces a house, a large and complex extracellular structure used for filtering and concentrating food particles. Previous studies have shown that several homeobox transcription factors may play a role in patterning the oikoplastic epithelium. Among these are two duplicates of the paired box transcription factor 3/7 gene (Pax3/7), that we named Pax37A and Pax37B. The vertebrate homologs, PAX3 and PAX7 genes, are involved in developmental processes related to neural crest, neural crest derived tissues and muscles. In the ascidian tunicate Ciona robusta, Pax3/7 has been given a role in the development of cells deriving from the neural plate border including epidermal sensory neurons of the trunk and tail nerve cord neurons as well as in neural tube closure. Here we have investigated the roles of the two O. dioica Pax3/7 genes in the development of the oikoplastic epithelium using CRISPR-Cas9 mutants, analyzing scRNA-seq data from wild-type animals that were finally compared with scRNA-seq data from C. robusta. We thus revealed that Pax37B but not Pax37A is essential for the differentiation of three cell fields that together produce the food concentrating filter of the house: the anterior Fol, the giant Fol and the Nasse cells. Lineage analysis supports that the expression of Pax37 is under the influence of Wnt signaling and that the Fol cell types have a neuroepithelial-like transcriptional signature. We propose that the highly specialized secretory epithelial cells of the Fol region either maintained or evolved neuroepithelial features as do "glue" secreting collocytes of ascidian tunicates. Their development seems to be controlled by a GRN that also operates in some ascidian neurons. ### Competing Interest Statement The authors have declared no competing interest.