Deuterostomes in a twist: the origins of a radical new body plan.

Echinoderms have a unique ontogeny and adult structure, and, among Bilateria, are the phylum that has diverged most radically in appearance from the ancestral body plan. Embryology and gene expression studies suggest how this transformation may have occurred while paleontological data provide direct evidence for the order in which these events took place. Comparing echinoderm ontogeny and genetic developmental signalling patterns with those of their sister group, the hemichordates, suggests that an evolutionary switch from posterior facultative to anterior obligate larval attachment proved the critical trigger. This necessitated introduction of a phase of torsion in development to bring the mouth into a more appropriate orientation for filter feeding, which in turn rotated the axis of the developing adult 90 degrees out of alignment with Hox and other body patterning genes. As a result the developing echinoderm rudiment came to receive a complex mosaic of anterior-posterior signalling, and extensive co-option of signalling pathways was able to take place. The fossil record shows that early (pre-radiate) echinoderms were much more hemichordate-like, with a muscular post-anal stalk and facultative attachment, and probably developed maintaining continuity with larval axes, as in hemichordates, although left-right asymmetry was more highly developed. Anterior attachment and torsion, however, were clearly part of the developmental pattern of helicoplacoids and (to a much greater extent) in subsequent pentaradiate forms.