The evolution of the hedgehog gene family in chordates: insights from amphioxus hedgehog

The hedgehog family of intercellular signalling molecules have essential fu nctions in patterning both Drosophila and vertebrate embryos. Drosophila ha s a single hedgehog gene, while vertebrates have evolved at least three typ es of hedgehog genes (the Sonic, Desert and Indian types) by duplication an d divergence of a single ancestral gene. Vertebrate Sonic-type genes typica lly show conserved expression in the notochord and floor plate, while Deser t- and Indian-type genes have different patterns of expression in vertebrat es from different classes. To determine the ancestral role of hedgehog in v ertebrates, I have characterised the hedgehog gene family in amphioxus. Amp hioxus is the closest living relative of the vertebrates and develops a sim ilar body plan, including a dorsal neural tube and notochord. A single amph ioxus hedgehog gene, AmphiHh, was identified and is probably the only hedge hog family member in amphioxus, showing the duplication of hedgehog genes t o be specific to the vertebrate lineage. AmphiHh expression was detected in the notochord and ventral neural tube, tissues that express Sonic-type gen es in vertebrates. This shows that amphioxus probably patterns its ventral neural tube using a molecular pathway conserved with vertebrates. AmphiHh w as also expressed on the left side of the pharyngeal endoderm, reminiscent of the left-sided expression of Sonic hedgehog in chick embryos which forms part of a pathway controlling left/right asymmetric development. These dat a show that notochord, floor plate and possibly left/right asymmetric expre ssion are ancestral sites of hedgehog expression in vertebrates and amphiox us. In vertebrates, all these features have been retained by Sonic-type gen es. This may have freed Desert-type and Indian-type hedgehog genes from sel ective constraint, allowing them to diverge and take on new roles in differ ent vertebrate taxa.