Embryonic expression of the period clock gene in the central nervous system of Drosophila melanogaster.

We have examined the temporal and spatial expression of the 4.5‐kb mRNA that is transcribed from the period locus of Drosophila melanogaster and is the best candidate for the per gene product. Both Northern blot analyses and hybridizations in situ to tissue sections reveal significant expression of the 4.5‐kb mRNA in embryos. This expression is limited to the central nervous system of the developing embryo and is localized within the brain and ventral ganglia. The 4.5‐kb mRNA is enriched in adult heads (by Northern blotting) although we were not able to detect specific localization (in situ). In addition to the physiological role the 4.5‐kb mRNA might have in maintaining biological rhythms, we now suggest that it has a developmental role for establishing mechanisms that are necessary for eventual expression of clock functions.