Genetic control of macromolecular synthesis during development of an ascidian: Ascidia nigra

During early tunicate development, there are two periods of rapid protein synthesis. The first begins shortly after fertilization and continues until about the four cell stage, the second begins at gastrulation. Only the second increase in the rate of protein synthesis is inhibited by actinomycin‐D. In addition to the initial high rate of synthesis there is some protein synthesis throughout early development which is insensitive to treatment with actinomycin‐D. The tunicate embryo thus shows two kinds of genomic control during development. It is suggested that one of them operates at the level of the synthesis of new messenger RNA, and the other at the level of translation of already existing, stable, messenger RNA. This dichotomy of regulatory mechanisms is similar to that occurring during sea urchin development. It is also shown that RNA synthesis first begins around the four cell stage in the Ascidian embryo and that this synthesis is sensitive to treatment with actinomycin‐D. It is postulated that this early synthesis of RNA accounts for the difference between mosaic and regulative development.