Cell-specific gene regulation in early molluscan development

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Abstract
In order to understand the processes that regulate cell-specific gene expression in early molluscan development, we studied the expression of tubulin genes in embryos of Patella vulgata (Gastropoda, Mollusca). Tubulin genes are first expressed at the 32-cell stage, which is also the stage at which the embryonic transcription starts. At this stage expression of tubulin genes is observed exclusively in two cells in each quadrant, the trochoblasts. These cells, after one additional division, will differentiate into ciliated and cleavage arrested cells. Later they will form part of the prototroch, the locomotory organ of the free-swimming trochophore larva. A trochoblast-specific α-tubulin gene was cloned and its 5′ upstream region was fused to the lac-Z reporter gene. After micro-injection into 2-cell stage embryos, the expression of this fusion gene appears to be restricted to the trochoblasts at the right time. Mutagenic analysis showed that two elements located between −108 and −42 were essential for a correct spatiotemporal expression. The first element, which contains a putative recognition site for homeobox proteins, acts negatively, the second one positively. In south-western and gel retardation assays we could show that several nuclear proteins are able to bind to these elements. The molecular mechanisms that regulate trochoblast-specific gene expression appear to be evolutionarily highly conserved. The 5′ region of the Patella tubulin gene is not only cell type specifically activated in embryos of this gastropod, but also in embryos of spirally cleaving species from other taxonomic groups, e.g., in a polyplacophoran, a scaphopod, and an annelid.