Characterization of an individual neural crest-like cell lineage in the invertebrate chordate Ciona intestinalis

During embryogenesis, all chordate embryos undergo neurulation to form a dorsal, hollow nerve cord. Neural crest cells (NCC), considered a vertebrate innovation, arise during neurulation and later differentiate into a multitude of tissues that account for much of the structural complexity that distinguishes craniates from invertebrate chordates [1, 2]. NCCs are induced and specified at the border of the neural and non-neural ectoderm by a complex network of inductive signals and transcriptional regulators then migrate throughout the embryo prior to differentiating [3, 4]. Invertebrate chordates, such as ascidians and amphioxus, possess cell types such as melanocytes, sensory neurons and even migratory cells [5] that resemble neural crest derivatives. However, it is not known whether these invertebrate cells share developmental specification mechanisms with vertebrate NCCs. Here we show that neural crest-like cells descending from the pigment cell lineage (PCL) of the ascidian Ciona intestinalis (Ci) originate at the neural plate border, express NCC transcriptional regulatory genes, migrate at the time of neural tube closure and differentiate into NCC derivatives including melanocytes and sensory neurons. Importantly, this careful analysis of the PCL has been carried out with single-cell resolution within a single ascidian species. Our results suggest that the specification and development of neural crest-like cells at the neural plate border is a symplesiomorphy of olfactores and provides insight into how this gene regulatory network was co-opted for NCC specification during vertebrate evolution. 1. Northcutt, R.G., The new head hypothesis revisited. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 2005. 304B(4): p. 274-297. 2. Northcutt, R.G. and C. Gans, The genesis of neural crest and epidermal placodes: a reinterpretation of vertebrate origins. Q Rev Biol, 1983. 58(1): p. 1-28. 3. LaBonne, C. and M. Bronner- Fraser, Induction and patterning of the neural crest, a stem cell-like precursor population. J Neurobiol, 1998. 36(2): p. 175-89. 4. Baker, C.V. and M. Bronner-Fraser, The origins of the neural crest. Part I: embryonic induction. Mech Dev, 1997. 69(1-2): p. 3-11. 5. Jeffery, W.R., A.G. Strickler, and Y. Yamamoto, Migratory neural crest-like cells form body pigmentation in a urochordate embryo. Nature, 2004. 431(7009): p. 696-9