Developmental expression of the white locus of Drosophila melanogaster

We have isolated several cDNA clones of the white locus which are derived from embryonic and pupal transcripts of Drosophila melanogaster. The cDNA sequences map within 7.5 kb (coordinates -3.0 to +4.6) of the genomic DNA and correspond mainly to sequences within the distal region of the gene (coordinates -0.2 to -3.0). A major RNA species of 2.6 kb was detected on Northerns of poly(A) RNA isolated from all developmental stages. The total accumulation of this transcript peaks in the mature third instar larva to a level of 0.003% which is about ten times higher than that observed in embryos. The spatial distribution of white locus transcripts was determined by in situ hybridization to tissue sections. In embryos, hybridization signals are restricted to the cells of the developing Malpighian tubules and the signal strength corresponds with 50 transcripts per cell. Before the termination of the third instar stage, hybridization signals are also detected at a comparable level in the eye antennal disks. At the same stage, a third site of labeling is observed over a small cluster of cells which seems to be associated with the larval photoreceptor organs. Thus, white locus expression is largely restricted to tissues which are known to be involved in the biosynthesis of eye pigments and these different cell types act in a temporally autonomous manner with respect to the induction of the white gene during development.