Defense Abstracts

PhD thesis: Molecular patterning of the developing nervous system in the chaetognath Spadella cephaloptera

Date: 21.10.2026, 13:00

Location: SR 4.1, UBB

Supervisor: Tim Wollesen

Abstract

Understanding how nervous systems evolve requires comparative developmental data from lineages with distinctive body plans and unresolved phylogenetic affinities. Chaetognaths, or arrow worms, occupy an important position within Spiralia and have long attracted attention because of their unusual morphology, contentious phylogenetic placement, and highly organized nervous system. However, despite their relevance for discussions of bilaterian nervous system evolution, the molecular developmental basis of chaetognath neurogenesis and neural regionalization remains poorly characterized. 
This thesis investigates nervous system development and axial patterning in the benthic chaetognath Spadella cephaloptera using stage-resolved gene expression analyses spanning early neurogenesis, anterior–posterior regionalization, and mediolateral organization of the ventral nerve center. Together, these datasets provide a molecular developmental framework for interpreting how the distinctive chaetognath nervous system is established during embryonic and early post-embryonic development. Early neurogenesis in S. cephaloptera is associated with spatially restricted expression of conserved neurogenic and dorsoventral patterning markers. Expression of soxB and neuroD identifies candidate neurogenic territories during gastrulation and elongation, whereas bmp2/4, chordin, and pSmad1/5/8 reveal a transient dorsoventral patterning system during early embryogenesis. Additional expression of ventral neural markers, including nk6 and hb9, further indicates early molecular regionalization within the developing ventral nerve center. These findings suggest that chaetognath neurogenesis occurs within a patterned embryonic context, although the regulatory relationships among these markers remain to be tested functionally. Anterior–posterior regionalization of the nervous system is characterized by distinct expression domains of anterior patterning genes and Hox genes. Expression analyses of otx, six3/6, and nk2.1 reveal molecularly distinct anterior neural territories in hatchlings, whereas Hox gene analyses show that S. cephaloptera possesses an extensive Hox repertoire deployed in staggered domains along the ventral nerve center and posterior body regions. These patterns support conserved anterior–posterior regionalization while also highlighting lineage-specific deployment of axial patterning genes, particularly in the postanal tail. Mediolateral organization of the hatchling ventral nerve center is further supported by the expression of nk2.2, pax6, foxA, msx, and pax3/7, which define distinct molecular domains. Although these domains do not conform to a simple canonical columnar arrangement, their organization is compatible with mediolateral regionalization when interpreted in relation to the unusual geometry of the chaetognath nervous system. 
More broadly, these findings emphasize that comparative analyses of neural patterning across bilaterians must account for tissue architecture, developmental timing, and lineage-specific anatomical organization. Overall, this thesis shows that the morphologically distinctive nervous system of S. cephaloptera is molecularly regionalized along multiple developmental axes. The results support the view that chaetognath nervous system evolution involved both retention and modification of conserved bilaterian developmental patterning features within a lineage specific anatomical context. By providing a developmental framework for interpreting nervous system organization in an understudied spiralian lineage, this work establishes a foundation for future functional, cellular, and comparative studies aimed at clarifying how conserved developmental systems are deployed, modified, or reorganized during nervous system evolution across Spiralia and Bilateria.