Autonomy of expression of epidermis-specific genes in the ascidian embryo.

An example of developmental autonomy, assessed at the level of expression of tissue-specific genes, was characterized in this study. cDNA clones that corresponded to two different epidermis-specific genes (HrEpiA and HrEpiB) of the ascidian Halocynthia roretzi were isolated. Transcripts of both genes first appeared around the time of gastrulation, suggesting zygotic expression. In the case of HrEpiA, transcripts became undetectable after metamorphosis had begun, while transcripts of HrEpiB were still detectable in juveniles after metamorphosis. In situ hybridization analysis using HrEpiA and B probes gave the following results. (1) All types of partial embryo that originated from blastomeres with epidermal fate from two-, four-, and eight-cell embryos expressed these two epidermis-specific genes. (2) When cleavage was blocked by treatment with cytochalasin B at the one-cell stage, the cleavage-arrested one-celled embryos expressed the two genes. (3) Even when blastomeres with epidermal fate were isolated from two-, four-, and eight-cell embryos and the division of the blastomeres was arrested immediately after isolation, they expressed the two specific genes. These results suggest that the epidermal lineage in the early ascidian embryo is a developmental system with considerable self-sustainability.