Engrailed in cephalopods: a key gene related to the emergence of morphological novelties

The engrailed gene is a transcription factor required in numerous species for major developmental steps (neurogenesis, limb development, boundary establishment), and its evolution is known to be closely related to the evolution of the metazoan body plan. Cephalopods exhibit numerous morphological peculiarities among molluscs, such as a direct development, a complex sensory and nervous system (eyes, brain, giant axons), a reduced shell, a funnel, and a brachial crown. We assessed a potential recruitment of engrailed in the development of these derived traits and examined the expression pattern of engrailed during the organogenesis of the cuttlefish Sepia officinalis, by immunostaining. Engrailed was detected at the margin of the prospective internal shell, which is consistent with studies on molluscs having an external shell and confirms a conserved role of engrailed in delimitating the molluscan shell compartment. Interestingly, unexpected patterns were early detected in the emerging arms, funnel and optic vesicles and latter in tentacles and eye lids. We also identified an engrailed cognate in the squid Loligo, which provides new evidence that engrailed in molluscs is not restricted to a 'shell function' and has been recruited in the mollusc lineage for the emergence of morphological novelties in cephalopods.