Patterning of brain precursors in ascidian embryos

Abstract

In terms of their embryonic origins, the anterior and posterior parts of the ascidian central nervous system (CNS) are associated with distinct germ layers. The anterior part of the sensory vesicle, or brain, originates from ectoderm lineages following a neuro-epidermal binary fate decision. In contrast, a large part of the remaining posterior CNS is generated following neuro-mesodermal binary fate decisions. Here, we address the mechanisms that pattern the anterior brain precursors along the medial-lateral axis (future dorsal-ventral) at neural plate stages. Our functional studies show that Nodal signals are required for induction of lateral genes including Delta-like, Snail, Msxb and Trp. Delta-like/Notch signalling induces intermediate (Gsx) over medial (Meis) gene expression in intermediate cells, while the combinatorial action of Snail and Msxb prevents the expression of Gsx in lateral cells. We conclude that despite the distinct embryonic lineage origins within the larval CNS, the mechanisms that pattern neural precursors are remarkably similar.