Anterior-posterior patterning in the chaetognath Spadella cephaloptera informs bilaterian nervous system and tail evolution

The formation of the bilaterian anterior-posterior axis relies on deeply conserved patterning systems, yet how these evolved remains incompletely understood. As sister to or within Gnathifera, chaetognaths provide an informative phylogenetic context for investigating anterior-posterior patterning, yet their developmental genetics remain poorly studied. Here, we examine anterior patterning (otx, nk2.1, six3/6) and Hox gene expression in the chaetognath Spadella cephaloptera. Anterior patterning genes are expressed in cerebral neural regions as in other bilaterians. In contrast, Hox genes, including the previously undescribed med6, postC, and postD, show staggered expression along the nervous system, resembling the proposed ancestral bilaterian condition. Posterior Hox genes are also expressed in the postanal tail, a trait only present in chaetognaths and chordates, suggesting a link between Hox expansion and the independent emergence of this structure. Our results indicate that chaetognaths retain ancestral bilaterian features and provide insights into Hox-driven lineage-specific innovations within Spiralia.