A Nodal/Eph signalling relay drives the transition from apical constriction to apico-basal shortening in ascidian endoderm invagination

Gastrulation is the first major morphogenetic event during animal embryogenesis. Ascidian gastrulation starts with the invagination of 10 endodermal precursor cells between the 64- and late 112-cell stages. This process occurs in the absence of endodermal cell division and in two steps, driven by myosin-dependent contractions of the acto-myosin network. First, endoderm precursors constrict their apex. Second, they shorten apico-basally, while retaining small apical surfaces, thereby causing invagination. The mechanisms controlling the endoderm mitotic delay, the step 1 to step 2 transition, and apico-basal shortening have remained elusive. Here, we demonstrate the conserved role during invagination of Nodal and Eph signalling in two distantly related ascidian species (Phallusia mammillata and Ciona intestinalis). We show that the transition to step 2 is controlled by Nodal relayed by Eph signalling and that Eph signalling has a Nodal-independent role in mitotic delay. Interestingly, both Nodal and Eph signals are dispensable for endodermal germ layer fate specification.Summary statementIdentification of a regulatory developmental signalling sub-network driving endoderm cell shape changes during ascidian endoderm invagination, not involved in cell fate specification.