Temporal expression patterns of 39 Brachyury-downstream genes associated with notochord formation in the Ciona intestinalis embryo

Expression of the Brachyury (Ci-Bra) gene of the ascidian Ciona intestinalis is initiated at the 64-cell stage. Gene expression is restricted to notochord precursor cells, and Ci-Bra plays a key role in notochord differentiation. In a previous study, nearly 50 cDNA clones for potential Ci-Bra-downstream genes that are expressed in notochord cells were isolated. The present determination, by whole-mount in situ hybridization, of the temporal expression patterns of 19 notochord-specific and 20 notochord-predominant genes demonstrated that the timings of initiation of the expression of various genes was not identical. The expression of several genes was initiated as early as the gastrula stage. However, the expression of most of the notochord- specific genes commenced at the neural plate stage. Partial nucleotide sequence data of these clones suggest that genes expressed earlier encode potential transcriptional factors and/or nuclear proteins, while those expressed later encode proteins implicated in cell adhesion, signal transduction, regulation of the cytoskeleton, and components of the extracellular matrix. These gene activities may be associated with changes in cell shape and adhesion during the intercalation and extension of the notochord cells.