Characterization of Homeobox Genes Reveals Sophisticated Regionalization of the Central Nervous System in the European Cuttlefish Sepia officinalis

Cephalopod mollusks possess a number of anatomical traits that often parallel vertebrates in morphological complexity, including a centralized nervous system with sophisticated cognitive functionality. Very little is known about the genetic mechanisms underlying patterning of the cephalopod embryo to arrive at this anatomical structure. Homeodomain (HD) genes are transcription factors that regulate transcription of downstream genes through DNA binding, and as such are integral parts of gene regulatory networks controlling the specification and patterning of body parts across lineages. We have used a degenerate primer strategy to isolate homeobox genes active during late-organogenesis from the European cuttlefish Sepia officinalis. With this approach we have isolated fourteen HD gene fragments and examine the expression profiles of five of these genes during late stage (E24-28) embryonic development (Sof-Gbx, Sof-Hox3, Sof-Arx, Sof-Lhx3/4, Sof-Vsx). All five genes are expressed within the developing central nervous system in spatially restricted and largely non-overlapping domains. Our data provide a first glimpse into the diversity of HD genes in one of the largest, yet least studied, metazoan clades and illustrate how HD gene expression patterns reflect the functional partitioning of the cephalopod brain.