Hedgehog crosses the snail's midline: Evolutionary biology

According to the dorsoventral axis-inversion theory 1 , protostomes (such as insects, snails and worms) are organized upside-down by comparison with deuterostomes (vertebrates) 2 , 3 , 4 , 5 , in which case their respective ventrally (belly-side) and dorsally (back-side) located nervous systems, as well as their midline regions, should all be derived from a common ancestor 5 . Here we provide experimental evidence for such homology by showing that an orthologue of hedgehog , an important gene in midline patterning in vertebrates, is expressed along the belly of the larva of the limpet Patella vulgata . This finding supports the existence of a similar mechanism for the development of the midline of the nervous system in protostomes and deuterostomes.