[Effects of two inhibitors of RNA synthesis (Actinomycin D and α-amanitin) on embryonic development of an insect: Gene activity from the beginning of embryogenesis inLeptinotarsa decemlineata].

Actinomycin D and α-amanitin were injected into the embryos ofLeptinotarsa from intravitelline cleavage stages to gastrula stages. The effect of injection was controlled. Following the treatment, embryogenesis was blocked at different stages: blastula (pseudo-blastula), abnormal gastrula and atypical germ band. The stages reached depend on the age of treatment. Treatment of intravitelline cleavage stages strongly inhibits morphogenesis: embryos are blocked mainly at the blastula stage (79 to 97%). Treatment of blastoderm formation stages shows different features: gastrulation is at first prevented in 80% of cases, but the capacity to gastrulate is acquired rapidly during the first blastodermic cell cycle; embryos treated at late blastoderm stages reach the germ band stage.From these results, two periods have been characterized in the early embryogenesis ofLeptinotarsa: 1. the intravitelline cleavage period is unaffected by the inhibitor of RNA synthesis. All information needed for cleavage and early blastoderm formation is already present in the fertilized egg, probably in the form of materialmRNA. 2. The blastoderm formation period is involved in further morphogenesis. New information, for the development of late blastula, gastrula and germ band is produced during succesive blastoderm cell cycles. The actinomycin D and α-amanitin treatments show that the acquisition of new morphogenetic capacities is contemporaneous with the arrival of nuclei in the periplasm.