Regulated expression of genes injected into early Drosophila embryos.

DNA of cloned genes injected into 15-30 min Drosophila embryos reaches nearly all the cells of the later embryo and its expression can be detected by Northern blot hybridisation. The injected DNA continues to be expressed at least up to the first instar larval stage and some developmental control of expression is retained. Genes normally expressed in the embryo are strongly transcribed upon injection while genes characteristic of later stages are not, or only very weakly expressed. Some transcription was detected with a larval salivary gland secretion gene (sgs-4) and with the adult promoter of the alcohol dehydrogenase gene (Adh) but not from the Adh larval promoter or from larval cuticle genes. A foreign gene (Semliki Forest virus capsid protein) was used to monitor the activity of promoters attached to it. In situ immunofluorescence of thin sections showed that the copia promoter is active mostly in yolk cells. In contrast, the hsp-70 heat-shock promoter is active, upon heat-shock, in nearly all cells in the embryo but most of the injected DNA is gradually lost so that in first instar larvae only 15% of the cells still express it.