Chromosome-scale genome assembly of the European common cuttlefish <i>Sepia officinalis</i>

Coleoid cephalopods, a subclass of mollusks that includes octopuses, cuttlefish, and squid, exhibit sophisticated biological features, such as dynamic and neurally driven camouflage behavior, inter-individual communication, single-lens camera-like eyes, the largest brains among invertebrates, and a distinctive embryonic development. The common cuttlefish <i>Sepia officinalis</i> has served as a model organism in various research fields, spanning biophysics, neurobiology, behavior, evolution, ecology, and biomechanics. More recently, it has become a model to investigate the neural mechanisms underlying cephalopod camouflage, using quantitative behavioral approaches alongside molecular techniques to characterize the identity, evolution, and development of neuronal cell types. Despite significant interest in this animal, a high-quality, annotated genome of this species is still lacking. To address this, we sequenced and assembled a chromosome-scale genome for <i>S. officinalis</i>. Our assembly spans 5.68 billion base pairs and comprises 1n=47 repeat-rich chromosome scaffolds. This was unexpected because the haploid karyotypes of other decapods indicate 46 chromosomes. Detailed comparisons of our data to those from published decapod genome assemblies and to another recent genome assembly of <i>S. officinalis</i> (itself suggesting 1n=49 chromosomes) in fact revealed clear homologies between 46 scaffolds across all the datasets. In-depth comparison of datasets reveals highly repetitive regions at discordant scaffold boundaries and suggests that the true karyotype of <i>S. officinalis</i> is probably 1n=46 chromosomes, a likely ancestral and if true, conserved decapod karyotype. Our results include a comprehensive gene annotation and full-length transcript prediction, which we used to characterize orthologous gene families across mollusks. We identified several large-scale expansions specific to cephalopods, with many genes specific to neural or non-neural tissues of adult <i>S. officinalis</i>. In summary, this genome should provide a valuable resource for future research on the evolution, brain organization, information processing, development, and behavior in this important clade.