Organizing chordates with an organizer.

Understanding how the chordate body plan originated and evolved is still controversial. The discovery by Spemann and Mangold in 1924 of the vertebrate organizer and its inductive properties in patterning the AP and DV axis was followed by a long gap until the 1960s when scientists started characterizing the molecular events responsible for such inductions. However, the evolutionary origin of the organizer itself remained obscure until very recently; did it appear together with the origin and radiation of vertebrates, or was it a chordate affair? A recent study by Yu and collaborators,1 which analyses the expression of several organizer-specific genes in amphioxus together with recent phylogenetic data that reversed the position of invertebrate extant chordates (e.g. urochordates and cephalochordates), indicates that the organizer probably appeared in early chordates. It likely had separate signalling centres generating BMP and Wnt signalling gradients along the DV and AP axis. The organizer was then lost in the urochordate lineage, most probably as an adaptation to a rapid and determinate development. (c) 2007 Wiley Periodicals, Inc.