Molluscan engrailed expression, serial organization, and shell evolution

Whether the serial features found in some molluscs are ancestral or derived is considered controversial. Here, in situ hybridization and antibody stud ies show iterated engrailed-gene expression in transverse rows of ectoderma l cells bounding plate field development and spicule formation in the chito n, Lepidochitona caverna, as well as in cells surrounding the valves and in the early development of the shell hinge in the clam, Transennella tantill a. Ectodermal expression of engrailed is associated with skeletogenesis acr oss a range of bilaterian phyla, suggesting a single evolutionary origin of invertebrate skeletons. The shared ancestry of bilaterian-invertebrate ske letons may help explain the sudden appearance of shelly fossils in the Camb rian. Our interpretation departs from the consideration of canonical metame res or segments as units of evolutionary analysis. In this interpretation, the shared ancestry of engrailed-gene function in the terminal/posterior ad dition of serially repeated elements during development explains the iterat ive expression of engrailed genes in a range of metazoan body plans.