Novel amplification and transcriptional activity of chorion genes in Drosophila melanogaster follicle cells.

Single-copy chorion genes coding for egg shell proteins are amplified in the follicle cells of Drosophila melanogaster egg chambers. Chromatin spreads of appropriately staged follicle cells reveal complex, multi-forked chromosomal structures in which one chromatin strand branches into two, which themselves branch out, and so on. In one micrograph, 13 strands originating from a single strand were observed. These structures can account for the maximal amplification occurring in the center of the domain, where the major chorion genes are located, and the decreasing amplification of flanking sequences to both sides. The amplification, high transcriptional rate, stage-specific expression, and correlation with known molecular sizes have allowed the putative identification of the single-copy, major chorion genes on the X chromosome and on chromosome III.