Lineage-specific regulation of the Ciona snail gene in the embryonic mesoderm and neuroectoderm.

The snail gene encodes a highly conserved, zinc-finger transcription factor that has been implicated in the specification of mesodermal and neuronal tissues in a variety of organisms. In the ascidian, Ciona intestinalis, snail (Ci-sna) is expressed at the 32-cell stage in derivatives of the B4.1 blastomere, including B6.2, B6.4, and B7.5, which give rise to the primary-lineage tail muscles of the tadpole. At later stages, Ci-sna is expressed in the lineages that will form the secondary tail muscle, the lateral ependymal cells of the spinal cord, and the dorsal cells of the cerebral vesicle. A minimal, 504-bp cis-regulatory sequence from the Ci-sna promoter region, the B4.1 enhancer, is shown to direct the expression of heterologous promoters in primary-lineage muscles. Furthermore, evidence is presented that cis-regulatory elements necessary for expression in both the secondary muscle and neuronal lineages are separate from the B4.1 enhancer. We discuss the possibility that the classical muscle determinant present in ascidian eggs may correspond to bHLH activators, which bind to specific E-box sequences contained in the B4.1 enhancer.