AUTONOMY OF EXPRESSION OF EPIDERMIS-SPECIFIC GENES IN THE ASCIDIAN EMBRYO

An example of developmental autonomy, assessed at the level of express ion of tissue-specific genes, was characterized in this study. cDNA cl ones that corresponded to two different epidermis-specific genes (HrEp iA and HrEpiB) of the ascidian Halocynthia roretzi were isolated. Tran scripts of both genes first appeared around the time of gastrulation, suggesting zygotic expression. In the case of HrEpiA, transcripts beca me undetectable after metamorphosis had begun, while transcripts of Hr EpiB were still detectable in juveniles after metamorphosis. In situ h ybridization analysis using HrEpiA and B probes gave the following res ults. (1) All types of partial embryo that originated from blastomeres with epidermal fate from two-, four-, and eight-cell embryos expresse d these two epidermis-specific genes. (2) When cleavage was blocked by treatment with cytochalasin B at the one-cell stage, the cleavage-arr ested one-celled embryos expressed the two genes. (3) Even when blasto meres with epidermal fate were isolated from two-, four-, and eight-ce ll embryos and the division of the blastomeres was arrested immediatel y after isolation, they expressed the two specific genes. These result s suggest that the epidermal lineage in the early ascidian embryo is a developmental system with considerable self-sustainability. (C) 1994 Academic Press, Inc.