Building divergent body plans with similar genetic pathways.

Deuterostome animals exhibit widely divergent body plans. Echinoderms have either radial or bilateral symmetry, hemichordates include bilateral enteropneust worms and colonial pterobranchs, and chordates possess a defined dorsal-ventral axis imposed on their anterior-posterior axis. Tunicates are chordates only as larvae, following metamorphosis the adults acquire a body plan unique for the deuterostomes. This paper examines larval and adult body plans in the deuterostomes and discusses two distinct ways of evolving divergent body plans. First, echinoderms and hemichordates have similar feeding larvae, but build a new adult body within or around their larvae. In hemichordates and many direct-developing echinoderms, the adult is built onto the larva, with the larval axes becoming the adult axes and the larval mouth becoming the adult mouth. In contrast, indirect-developing echinoderms undergo radical metamorphosis where adult axes are not the same as larval axes. A second way of evolving a divergent body plan is to become colonial, as seen in hemichordates and tunicates. Early embryonic development and gastrulation are similar in all deuterostomes, but, in chordates, the anterior-posterior axis is established at right angles to the animal-vegetal axis, in contrast to hemichordates and indirect-developing echinoderms. Hox gene sequences and anterior-posterior expression patterns illuminate deuterostome phylogenetic relationships and the evolution of unique adult body plans within monophyletic groups. Many genes that are considered vertebrate 'mesodermal' genes, such as nodal and brachyury T, are likely to ancestrally have been involved in the formation of the mouth and anus, and later were evolutionarily co-opted into mesoderm during vertebrate development.