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Background  It has been hypothesized that the  ParaHox  gene  Gsx  patterned the foregut of the last common bilaterian ancestor. This notion was corroborated by  Gsx  expression in three out of four lophotrochozoan species, several ecdysozoans, and some deuterostomes. Remarkably,  Gsx  is also expressed in the bilaterian anterior-most central nervous system (CNS) and the gastropod and annelid apical organ. To infer whether these findings are consistent with other mollusks or even lophotrochozoans, we investigated  Gsx  expression in developmental stages of representatives of two other molluscan classes, the scaphopod  Antalis entalis  and the cephalopod  Idiosepius notoides .    Results   Gsx  is not expressed in the developing digestive tract of  Antalis entalis  and  Idiosepius notoides . Instead, it is expressed in cells of the apical organ in the scaphopod trochophore and in two cells adjacent to this organ. Late-stage trochophores express  Aen - Gsx  in cells of the developing cerebral and pedal ganglia and in cells close to the pavilion, mantle, and foot. In postmetamorphic specimens,  Aen - Gsx  is expressed in the cerebral and pedal ganglia, the foot, and the nascent captacula. In early squid embryos,  Ino - Gsx  is expressed in the cerebral, palliovisceral, and optic ganglia. In late-stage embryos,  Ino - Gsx  is additionally expressed close to the eyes and in the supraesophageal and posterior subesophageal masses and optic lobes. Developmental stages close to hatching express  Ino - Gsx  only close to the eyes.    Conclusions  Our results suggest that  Gsx  expression in the foregut might not be a plesiomorphic trait of the Lophotrochozoa as insinuated previously. Since neither ecdysozoans nor deuterostomes express  Gsx  in their gut, a role in gut formation in the last common bilaterian ancestor appears unlikely.  Gsx  is consistently expressed in the bilaterian anterior-most CNS and the apical organ of lophotrochozoan larvae, suggesting a recruitment of  Gsx  into the formation of this organ in the Lophotrochozoa. The cephalopod posterior subesophageal mass and optic ganglia and the scaphopod pedal ganglia also express  Gsx . In summary,  Gsx  expression only appears to be conserved in the anterior-most brain region during evolution. Accordingly,  Gsx  appears to have been recruited into the formation of other expression domains, e.g., the apical organ or the foregut, in some lophotrochozoans.

The ParaHox gene Gsx patterns the apical organ and central nervous system but not the foregut in scaphopod and cephalopod mollusks