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Abstract

Commissural neurons have their cell body on one side of the body and project their axon across the midline to the other side of the body. The midline is a structure that is specific to Bilaterian animals and is formed when the blastopore closes. Whether the blastopore was closed in Urbilateria, the last common ancestor of Bilaterians, is an open question. Since commissural neurons require the blastopore to close, finding that homologous neurons are commissural across Bilateria would suggest that the blastopore was closed in Urbilateria. On the other hand, finding that commissural neurons have acquired the ability to cross the midline convergently would suggest that the blastopore closed independently in the Protosome and Deuterostome lineages. Transcription factors play a pivotal role in specifying neuronal cell fate. The analysis and comparison of transcription factor molecular signatures across Bilaterian animals thus represents a powerful approach to put forth and validate hypotheses about the homology of commissural neurons. One putatively-conserved commissural cell type is made up of ascending neurons that express the homeobox transcription factor even- skipped (eve/evx) in insects and vertebrates. To advance the molecular characterisation and comparison of the commissural neurons in Bilateria, I have studied the commissural neurons of the ventral nerve cord of the marine annelid Platynereis dumerilii. Several characteristics of this animal make it particularly well suited for evolutionary studies, including its ancient mode of development and life style. It is also a model for which powerful tools have been developed to study its morphology and gene expression, such as a serial block face scanning-electron microscopy dataset of the full 6-days-post-fertilization (dpf) larva and gene expression atlases at several developmental stages. Platynereis’ commissural neurons were identified following both a biased approach searching for molecular signatures known from insects or vertebrates, and an unbiased approach taking the axonal traces of an entire segmental complement of commissural neurons as a starting point. I thus determined that in the second segment of the 6 dpf larva, fewer than 15% of the neurons are commissural. Analysis of their specific transcription factor-expression pattern allowed for the classification of seven cell types, characterised by the expression of i) Phox2, Hox1, and Brn3, ii) Phox2, Hox1, and Isl, iii) Dbx1, Pax6, and Irx6, iv) Eve and Lhx1/5, v) Nk6, PitxB, and Maf, vi) Nk6, PitxB, and Tal, and vii) Nk6, PitxB, and Irx6. There are commissural neurons in the vertebrate neural tube with a similar transcription factor-expression pattern and developmental origin to the cell types i, ii and iv, consistent with these being conserved in Bilateria. This would support the hypothesis that the midline existed in Urbilateria. More comparative research will be needed to settle this question.