Developmental expression of the Drosophila zeste gene and localization of zeste protein on polytene chromosomes.

The expression of the zeste gene varies through the life cycle of the fly. Its transcription is most abundant in maternal RNA, declines to very low levels during larval growth, but rises again in late third instar larvae and pupae. Using transposons containing a zeste-lacZ gene, we found a corresponding variation in the tissue distribution of zeste from stage to stage. Nearly ubiquitous expression of the zeste-lacZ gene is found in late embryos and first instar larvae, but disappears almost completely except in brain and gonads by third instar larva. Shortly before pupation expression rises again in imaginal discs, Malpighian tubules, and salivary glands and again becomes nearly ubiquitous in pupae. zeste continues to be expressed in adult brain and gonads. We constructed flies carrying a zeste gene controlled by the heat shock promoter and studied the distribution of zeste protein in their polytene chromosomes as well as those of wild-type flies. Using affinity-purified anti-zeste antibodies, we find that wild-type salivary gland chromosomes contain about 60 strong bands of zeste immunofluorescence at specific cytological locations. After heat induction of larvae containing the hs-zeste gene, many hundreds of bands appear. These results suggest the involvement of zeste in the expression of a wide variety of genes at different developmental stages.