Isolation of cDNA clones for mRNAs transcribed zygotically during cleavage in the ascidian, Halocynthia roretzi.

The ascidian larva consists of a relatively small number of different cell types, and the cell lineages during embryogenesis have been well described. The clonal restriction of developmental fate takes place considerably early in development. The fates of most of the blastomeres become tissue-restricted by the 110-cell stage, just before the onset of gastrulation. To elucidate the molecular basis of the early events of fate determination in the ascidian Halocynthia roretzi, we isolated the genes for which zygotic expression is initiated during the early cleavage stages. Here we report 18 genes isolated by subtractive hybridization screening between 110-cell embryos and fertilized eggs. The expression of most (13) of the genes was initiated at the 32-cell stage. The genes were subdivided into three groups according to their spatial expression patterns. The first group included clones expressed throughout almost the entire embryo. The second and third groups represented clones expressed mainly in the animal hemisphere and in a subset of vegetal blastomeres, respectively. One of the genes, HrHesl1, encoded a polypeptide containing the bHLH domain that is similar to those of the Hairy/Enhancer of split/Deadpan family of transcriptional repressors. HrHesl1 was expressed exclusively in epidermal precursor cells during cleavage. Another gene named HrWnt-5 beta was expressed in muscle precursors.