Brain regionalization genes are co-opted into shell field patterning in Mollusca

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Genetics

Background:

• Conserved gene expression patterns in brain regionalization exist across deuterostomes, ecdysozoans, and cephalopods.
• The expression of key brain regionalization genes (Six3/6, Otx, Pax2/5/8, Gbx, Hox1) in Mollusca remains largely unexplored.
• Investigating these genes in molluscan sister groups can elucidate conserved and novel developmental roles.

Purpose of the Study:

• To investigate the developmental expression of the Gbx gene in representatives of the molluscan sister groups, Aculifera and Conchifera.
• To determine if Gbx and other posterior brain regionalization genes are involved in central nervous system regionalization in Mollusca.
• To explore potential novel roles of these genes in molluscan shell and spicule formation.

Main Methods:

• Comparative analysis of Gbx gene expression during development.
• Examination of gene expression in aculiferan (polyplacophoran, solenogaster) and conchiferan (bivalve) mollusks.
• Utilizing molecular techniques to visualize gene expression patterns in the central nervous system and mantle tissues.

Main Results:

• Gbx is expressed in the posterior central nervous system of polyplacophorans and solenogasters, suggesting a role in CNS regionalization.
• Gbx exhibits anterior CNS expression in solenogasters, a pattern not observed in other bilaterians.
• Gbx, Pax2/5/8, and Hox1 are expressed in the mantle of mollusks, indicating a role in shell and spicule formation.
• Bivalves show a loss of ancestral Gbx and Pax2/5/8 expression in the CNS, with retained expression in mantle tissues.

Conclusions:

• Gbx, along with Six3/6, Otx, Pax2/5/8, and Hox1, likely participates in molluscan central nervous system regionalization.
• These genes possess an additional, novel function in the formation of molluscan shell fields and spicule-bearing cells.
• Evolutionary divergence in gene expression patterns, particularly in bivalves, highlights the plasticity of developmental pathways in Mollusca.