HOX Genes in the Sepiolid Squid Euprymna scolopes: Implications for the Evolution of Complex Body Plans

Molluscs display a rich diversity of body plans ranging from the wormlike appearance of aplacophorans to the complex body plan of the cephalopods with highly developed sensory organs, a complex central nervous system, and cognitive abilities unrivaled among the invertebrates. The aim of the current study is to define molecular parameters relevant to the developmental evolution of cephalopods by using the sepiolid squid Euprymna scolopes as a model system. Using PCR-based approaches, we identified one anterior, one paralog group 3, five central, and two posterior group Hox genes. The deduced homeodomain sequences of the E. scolopes Hox cluster genes are most similar to known annelid, brachiopod, and nemertean Hox gene homeodomain sequences. Our results are consistent with the presence of a single Hox gene cluster in cephalopods. Our data also corroborate the proposed existence of a differentiated Hox gene cluster in the last common ancestor of Bilaterians. Furthermore, our phylogenetic analysis and in particular the identification of Post-1 and Post-2 homologs support the Lophotrochozoan clade.