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The ‘brain regionalization genes’  Six3/6 ,  Otx ,  Pax2/5/8 ,  Gbx , and  Hox1  are expressed in a similar fashion in the deuterostome, ecdysozoan, and the cephalopod brain, questioning whether this holds also true for the remaining Mollusca. We investigated developmental  Gbx -expression in representatives of both molluscan sister groups, the Aculifera and Conchifera.  Gbx  is expressed in the posterior central nervous system of an aculiferan polyplacophoran and solenogaster but not in a conchiferan bivalve suggesting that  Gbx , together with  Six3/6 ,  Otx ,  Pax2/5/8 , and  Hox1 , is involved in central nervous system regionalization as reported for other bilaterians.  Gbx  is, however, also expressed in the anterior central nervous system, i.e. the anlagen of the cerebral ganglia, in the solenogaster, a condition not reported for any other bilaterian so far. Strikingly, all  Gbx- orthologs and the other ‘posterior brain regionalization genes’ such as  Pax2/5/8  and  Hox1  are expressed in the mantle that secretes shell(s) and spicules of mollusks (except cephalopods). In bivalves, the ancestral condition has even been lost, with  Gbx  and  Pax2/5/8  not being expressed in the developing central nervous system anymore. This suggests an additional role in the formation of the molluscan shell field(s) and spicule-bearing cells, key features of mollusks.

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