Isolation of cDNA clones for genes that are expressed in the tail region of the ascidian tailbud embryo.

An ascidian tailbud embryo is comprised of the anterior trunk and posterior tail. We constructed cDNA libraries of the tail region and trunk region of the ascidian Halocynthia roretzi. The screening of the tail library by tail single-stranded cDNA minus the trunk library RNA as a probe, yielded cDNA clones for genes that are expressed in the tail epidermis, visceral ganglion, trunk lateral cells, muscle cells, and certain regions of the tail. Among them, a cDNA clone for a gene designated HrPost-1 is described in detail. HrPost-1 encodes a novel, possible secreted protein of 238 amino acids. The expression of the gene is zygotic. HrPost-1 transcript was first evident in the posterior B-line blastomeres including muscle cells and endodermal strand cells of the gastrula-stage embryo, and the expression in these regions disappeared by the early tailbud stage. Around neurulation, the HrPost-1 transcript appeared in epidermal cells of the posterior-most region of the embryo. As development proceeded, the gene expression spread anteriorly in the epidermal cells of the neurula and tailbud embryo, and thus at the early-to-mid tailbud stage, HrPost-1 expression appeared to define the boundary between the trunk and tail epidermis. These results suggest that, in addition to tissue-specific genes, the activities of a set of region-specific genes are associated with tail formation in the ascidian embryo.