CELL-SPECIFIC GENE-REGULATION IN EARLY MOLLUSCAN DEVELOPMENT

In order to understand the processes that regulate cell-specific gene expression in early molluscan development, we studied the expression o f 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 e xpression of tubulin genes is observed exclusively in two cells in eac h quadrant, the trochoblasts. These cells, after one additional divisi on, will differentiate into ciliated and cleavage arrested cells. Late r they will form part of the prototroch, the locomotory organ of the f ree-swimming trochophore larva. A trochoblast-specific alpha-tubulin g ene was cloned and its 5' upstream region was fused to the lac-Z repor ter gene. After micro-injection into 2-cell stage embryos, the express ion of this fusion gene appears to be restricted to the trochoblasts a t the right time. Mutagenic analysis showed that two elements located between -108 and -42 were essential for a correct spatiotemporal expre ssion. The first element, which contains a putative recognition site f or homeobox proteins, acts negatively, the second one positively. In s outh-western and gel retardation assays we could show that several nuc lear proteins are able to bind to these elements. The molecular mechan isms that regulate trochoblast-specific gene expression appear to be e volutionarily highly conserved. The 5' region of the Patella tubulin g ene is not only cell type specifically activated in embryos of this ga stropod, but also in embryos of spirally cleaving species from other t axonomic groups, e.g., in a polyplacophoran, a scaphopod, and an annel id.