Evolutionary biology: hedgehog crosses the snail's midline.

According to the dorsoventral axis-inversion theory, protostomes (such as insects, snails and worms) are organized upside-down by comparison with deuterostomes (vertebrates), 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. 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.