Developmental cell-cell communication pathways in the cephalochordate amphioxus: actors and functions.

During embryonic development, cells of metazoan embryos need to communicate in order to construct the correct bodyplan. To do so, they use several signals that usually act through interactions between ligands and receptors. Interestingly, only a few pathways are known to be fundamental during animal development, and they are usually found in all the major metazoan clades, raising the following question: how have evolution of the actors and of the functions of these pathways participated in the appearance of the current diversity of animal morphologies? The chordate lineage comprises vertebrates, their sister group the urochordates, and the cephalochordates (i.e. amphioxus). Urochordates are quite derived relative to the chordate ancestor, whereas cephalochordates and vertebrates share many morphological traits. Thus, comparing embryonic development between vertebrates and cephalochordates should give us some insight into the ancestral characters present in chordates and into the morphological evolution in this clade. However, while much is known about the function of different signalling pathways in vertebrates, data are still scarce in the literature for cephalochordates. In this review, we summarize the current state of the field concerning the expression of actors and the function of the major cell-cell communication pathways, including Hedgehog (Hh), Notch, Nuclear Receptor (NR), Receptor Tyrosine Kinase (RTK), Transforming Growth Factor-β (TGF-β) and Wingless/Int (Wnt), in amphioxus.