Origins of anteroposterior patterning andHoxgene regulation during chordate evolution

Abstract

All chordates share a basic body plan and many common features of early development. Anteroposterior (AP) regions of the vertebrate neural tube are specified by a combinatorial pattern ofHoxgene expression that is conserved in urochordates and cephalochordates. Another primitive feature ofHoxgene regulation in all chordates is a sensitivity to retinoic acid during embryogenesis, and recent developmental genetic studies have demonstrated the essential role for retinoid signalling in vertebrates. Two AP regions develop within the chordate neural tube during gastrulation: an anterior ‘forebrain–midbrain’ region specified byOtxgenes and a posterior ‘hindbrain–spinal cord’ region specified byHoxgenes. A third, intermediate region corresponding to the midbrain or midbrain–hindbrain boundary develops at around the same time in vertebrates, and comparative data suggest that this was also present in the chordate ancestor. Within the anterior part of theHox–expressing domain, however, vertebrates appear to have evolved unique roles for segmentation genes, such asKrox–20, in patterning the hindbrain. Genetic approaches in mammals and zebrafish, coupled with molecular phylogenetic studies in ascidians, amphioxus and lampreys, promise to reveal how the complex mechanisms that specify the vertebrate body plan may have arisen from a relatively simple set of ancestral developmental components.