Cell-lineage analysis of the prototroch of the gastropod molluscPatella vulgata shows conditional specification of some trochoblasts

Embryos of many spirally cleaving species possess a characteristic cell type, the trochoblasts. These cells differentiate early in development into ciliated cells and give rise to the prototroch, the locomotory organ of the trochophore larva. As a necessary prelude to the investigation of the mechanisms that are responsible for specification of trochoblasts in the equally cleaving gastropod molluscPatella vulgata, the cell-lineage of the prototroch was studied. This was done by microinjection of the cell-lineage tracer lucifer yellow-dextran in trochoblasts and by scanning electron microscopical analysis of formation of the prototroch. The results show that trochoblasts that form the prototroch are of different clonal origin and that the four quadrants of the embryo have an unequal contribution to the prototroch. Since the four quadrants of the equally cleaving embryo are initially equipotent, some trochoblasts must become conditionally specified. Other trochoblasts seem to become autonomously specified. After initial ciliation some trochoblasts become deciliated and for some cells the choice between a larval and an adult cell fate is conditionally specified. Cell-lineage analysis demonstrates that the various autonomously and conditionally specified trochoblasts are organised according to the dorsoventral axis of the embryo. Possible mechanisms that can account for the conditional specification of trochoblasts - including a role for the 3D macromere, which forms the primary mesoderm and is responsible for the formation of the dorsoventral axis of the embryo - are discussed.