Chorion gene amplification in Drosophila: A model for metazoan origins of DNA replication and S-phase control

The mechanisms controlling duplication of the metazoan genome are only begi nning to be understood. It is still unclear what organization of DNA sequen ces constitutes a chromosomal origin of DNA replication, and the regulation of origin activity during the cell cycle has not been fully revealed. We r eview recent results that indicate that chorion gene amplification in folli cle cells of the Drosophila ovary is a model for investigating metazoan rep lication. Evaluation of cis sequence organization and function suggests tha t chorion loci share attributes with other replicons and provides insights into metazoan origin structure. Moreover, recent results indicate that chor ion origins respond to S-phase control, but escape mechanisms that inhibit other origins from firing more than once in a cell cycle. Several identifie d genes that mediate amplification are critical for the cell cycle control of replication initiation. It is likely that further genetic screens for mu tations that disrupt amplification will identify the cadre of proteins asso ciated with origins and the regulatory pathways that control their activity . Furthermore, the recent development of methods to detect amplification in situ has uncovered new aspects of its developmental control. Examining thi s control will reveal links between developmental pathways and the cell cyc le machinery. Visualization of amplifying chorion genes with high resolutio n also represents an opportunity to evaluate the influence of nuclear and c hromosome structure on origin activity. The study of chorion amplification in Drosophila, therefore, provides great potential for the genetic and mole cular dissection of metazoan replication. (C) 1999 Academic Press.