An Otx/Nodal Regulatory Signature for Posterior Neural Development in Ascidians

Abstract

In chordates, neural induction is the first step of a complex developmental process through which ectodermal cells acquire a neural identity. In ascidians, FGF-mediated neural induction occurs at the 32-cell stage in two blastomere pairs, precursors respectively of anterior and posterior neural tissue. We combined molecular embryology and cis-regulatory analysis to unveil in the ascidian Ciona intestinalis the remarkably simple proximal genetic network that controls posterior neural fate acquisition downstream of FGF. We report that the combined action of two direct FGF targets, the TGFβ factor Nodal, acting via Smad- and Fox-binding sites, and the transcription factor Otx suffices to trigger ascidian posterior neural tissue formation. Moreover, we found that this strategy is conserved in the distantly related ascidian Phallusia mammillata, in spite of extreme sequence divergence in the cis-regulatory sequences involved. Our results thus highlight that the modes of gene regulatory network evolution differ with the evolutionary scale considered. Within ascidians, developmental regulatory networks are remarkably robust to genome sequence divergence. Between ascidians and vertebrates, major fate determinants, such as Otx and Nodal, can be co-opted into different networks. Comparative developmental studies in ascidians with divergent genomes will thus uncover shared ascidian strategies, and contribute to a better understanding of the diversity of developmental strategies within chordates.

Patterning of brain precursors in ascidian embryos

“…Msxb expression in a-lineage column 3 is also downstream of Nodal (Fig. S1), as has been shown previously for b-lineage Msxb expression (Roure et al, 2014). Using Msxb-MOs, we found that while knockdown of Msxb alone had no effect on Gsx expression, combined inhibition of both Msxb and Snail resulted in strong ectopic expression of Gsx in column 3 at the neurula stage ( Fig.…”

Tfap2 and Sox1/2/3 cooperatively specify ectodermal fates in ascidian embryos

“…At the 16-cell stage, Fgf9/16/20 is activated in the vegetal hemisphere and induces neural fate in the animal hemisphere (Bertrand et al, 2003;Hudson et al, 2003;Hudson and Lemaire, 2001;Khoueiry et al, 2010;Ohta and Satou, 2013;Ohta et al, 2015;Roure et al, 2014). Tfap2-r.b is first expressed in the entire animal hemisphere, and then the expression disappears in the neural lineage at the 64-cell stage (Fig.…”

“…S1), and give rise to the dorsal row of the nerve cord, epidermal sensory neurons and epidermal cells along the nerve cord (Imai et al, 2006;Pasini et al, 2006;Roure et al, 2014;Waki et al, 2015). Whereas Msx was expressed normally in embryos injected with the lacZ or Tfap2-r.b MOs, it was greatly reduced in Sox1/2/3 morphants ( Fig.…”

“…Because Msx is expressed under the control of Nodal and Otx (Imai et al, 2006;Roure et al, 2014), we examined Nodal and Otx expression in Sox1/2/3 morphants. Whereas Nodal was expressed normally at the 64-cell stage in embryos injected with the lacZ control MO, it was lost in Sox1/2/3 morphants (Fig.…”

The characterization of larval homology: transcriptomic insights into the origin and evolution of animal life cycles

“…TFs are known to play promiscuous roles in developmental systems, and can thus be co-opted to new functional roles (reviewed in Cheatle Jarvela and Hinman 2015). For instance, studies provide evidence that deeply conserved TFs across animal lineages have been co-opted to perform novel functional roles in specific lineages, such as Tbrain in echinoderms (Hinman et al 2007), Otx in ascidians (Roure et al 2014), and Hox genes in Haliotis asinina (Hinman et al 2003). Given these examples, it is possible that coexpressed TFs between sponges and sea urchins are not indicative of conserved regulatory roles involved in metamorphosis.…”