Early development of amphioxus links evolutionary events with vertebrates.

Comparison of early development is a powerful approach to understand how spherical embryos set up the basis for body patterning. Localization of the germ plasm likely couples with the site of gastrulation in many animals including cnidarians. A center of single or complex Wnt signaling pathway(s) is also co-localized with germ plasm and plays a role primarily in antero-posterior patterning in most animals. In addition, a Nodal signaling center appears in deuterostomes perpendicular to Wnt signaling and governs dorso-ventral patterning. Wnt and Nodal signaling pathways function as orthogonal coordinates to pattern embryos in three-dimensions. Amphioxus early embryos establish similar coordinates, but the location of Wnt signaling along the equator might modify the function of the Nodal signaling center to specify the chordate basic body pattern inverting dorso-ventral polarity, which is thought to have occurred during the evolution of deuterostomes. Surprising similarities in early developmental processes found in cnidarians and deuterostomes, and divergences of cnidarians-bilaterians, deuterostomes-protostomes, and ambulacrarians-chordates occurred one after another possibly within a geologically short period may link molecular mechanisms that gave rise to bilaterians, deuterostomes, and chordates.