Characterization of shell field populations in gastropods and their autonomous specification mechanism independent of inter-quartet interactions.

The embryonic shell field of mollusks appears during gastrulation on the dorsal ectoderm and later develops into the adult shell-secreting mantle. Although several lines of evidence have revealed that the shell field is exclusively derived from the second quartet (2q) of 16-cell embryos, it is generally believed that its fate is established only after receiving inductive signals from cells derived from other quartets, such as the invaginated endoderm. However, the induction hypothesis remains questionable due to limited experimental evidence and contradictory results. Here, we re-investigated the induction hypothesis for shell field specification in the limpet. We identified three cell populations within the developing shell field using two-color in situ hybridization and single-cell transcriptome analysis, each characterized by distinct effector and transcription factor genes. The specification of each population was examined in 2q blastomeres isolated from 16-cell embryos. Even without inter-quartet interactions, marker gene expression for each shell field population was detected in the 2q-derived partial embryos. We conclude that the early specification of shell field in 2q-derived cells occurs largely independently of interactions with other quartets.