Larval body patterning and apical organs are conserved in animal evolution

Area of Science:

• Developmental biology
• Evolutionary biology
• Marine biology

Background:

• Planktonic ciliated larvae are crucial for marine invertebrate life cycles.
• The apical organ, a key feature of larvae, has sensory and neuronal roles that are not fully understood.
• Understanding larval and apical organ evolution is vital for deciphering animal life cycle evolution, but comparative molecular data are scarce.

Purpose of the Study:

• To investigate the regionalization of the episphere in Platynereis dumerilii trochophore larvae.
• To compare apical organs and larval body patterning using molecular markers.
• To elucidate the evolutionary relationships of apical organs across different animal phyla.

Main Methods:

• Studied the spatial distribution of transcription factors and Wnt signaling components.
• Used pharmacological activation of Wnt signaling with Gsk3β antagonists.
• Employed profiling by image registration for parallel cell expression profiling.

Main Results:

• Identified a Wnt-sensitive 'apical plate' expressing six3 and foxq2, crucial for apical tissue specification.
• Unraveled a conserved molecular signature of the apical organ (foxj, irx, nkx3, hox) shared with cnidarians.
• Characterized apical organ cell types, including hox-expressing apical tuft cells, putative sensory cells, and peptidergic cells.

Conclusions:

• The conserved apical plate and apical tuft suggest evolutionary conservation in the last common ancestors of cnidarians and bilaterians.
• Propose that the apical organ, with an apical tuft and basal plexus, was anciently involved in controlling metamorphosis.
• Hypothesize that diverse apical plate cells were added to the apical organ in later bilaterian lineages, supporting a common origin for ciliated larvae.