Effect of DNA inhibitors upon DNA synthesis and development of Drosophila embryos

Abstract

Staged wildtype embryos Drosophila melanogaster were permeabilized and then subjected to a short pulse of either methyl‐³H‐thymidine, one of four different inhibitors of DNA synthesis (mitomycin C, 5‐fluorouracil, nalidixic acid, or 1‐β‐D‐arabino‐furanosylcytosine‐5′‐monophosphate), or a combination of both. The incorporation of methyl‐³H‐thymidine into acid insoluble material was at a maximum during the first half‐hour of embryogenesis, after which the incorporation dropped to half the initial value and remained constant throughout the remainder of development. There was no correlation between the rate of incorporation of methyl‐³H‐thymidine into DNA and the known periods of high mitotic activity. The time course of the estimated specific activity of the DNA newly synthesized in vivo closely paralleled the known changes in the DNA polymerase activity determined in vitro. The known periods of high mitotic activity in the embryo (0–3 hours, 5–12 hours) agree with the periods of maximal sensitivity of the embryo to the inhibitors of DNA synthesis. All four DNA inhibitors affected the incorporation of methyl‐³H‐thymidine into DNA, although they did not affect it in any simplistic manner. Inhibitor treatment during early cleavage resulted in arrested development, and treatment during late cleavage and blastoderm stages resulted in abnormal development, and treatment during late blastoderm and early gastrula resulted in normal development. The major phenotypic abnormality caused by the inhibitors is an abnormal distribution of blastoderm cells. As judged by the ID₅₀ values, the embryos remained very sensitive to the effects of the inhibitors until the stages of head and body segmentation, when they then very rapidly became insensitive.