Molluscan Shells, Spicules, and Gladii Are Evolutionarily Deeply Conserved.

Area of Science:

• Evolutionary developmental biology
• Comparative genomics
• Invertebrate zoology

Background:

• Shells, spicules, and chaetae are diverse hard structures in spiralians, with debated formation processes and evolutionary links.
• Understanding the genetic basis of these structures is crucial for deciphering their evolutionary history.

Purpose of the Study:

• To investigate the expression of conserved genes involved in shell and spicule formation across different spiralian groups.
• To explore the evolutionary relationship between spicules and molluscan shells.

Main Methods:

• Examined the expression of transcription factor genes (hox1, goosecoid, grainyhead, notch, delta, zic) and structural genes (chitin synthase, ferritin).
• Analyzed gene expression in shell-forming epithelia and spicule-bearing cells of aculiferans (Polyplacophora, Neomeniomorpha) and conchiferans (Cephalopoda, Scaphopoda).

Main Results:

• Conserved genes were expressed in both spicule-forming cells and shell fields, indicating a close relationship.
• Spicule-bearing cells in polyplacophorans and neomeniomorphs expressed specific genes, including hox1 in neomeniomorphs.
• Ferritin gene expression was absent in the studied spicule-bearing cells and conchiferan shell fields, suggesting it's a gastropod innovation.

Conclusions:

• Spicules and shell fields share a common genetic basis, suggesting homology.
• Spicules likely predated molluscan shells and may be homologous to brachiopod and annelid chaetae.
• Conchiferan mollusks may have secondarily lost spicules during evolution.