ABSTRACT
With their diverse body plans and different life history strategies, mollusks represent an ideal group to study body plan patterning, the evolution of morphological novelties, and the role of developmental regulators such as Hox genes. Data from representatives of both molluscan lineages, the Aculifera and the Conchifera, have shown that the last common molluscan ancestor probably exhibited staggered Hox expression, a condition that is still visible in the aculiferan polyplacophorans and to a certain degree in conchiferans such as scaphopods, gastropods, or cephalopods. Besides these remainders of staggered expression, Hox genes have been independently co-opted into the formation of morphological novelties such as the shell field, the foot, or the cephalopod arm crown and funnel. Hox gene expression studies are available for gastropods, a scaphopod, bivalves, a cephalopod, and polyplacophorans, but information is still missing on crucial taxa such as the enigmatic monoplacophorans and the worm-shaped aplacophorans. In particular, functional studies on the role of Hox genes using, e.g., genome editing tools are still in their infancy, yet provide exciting prospects for future research.
