Initiation of Drosophila chorion gene amplification requires Claspin and mus101, whereas Claspin, but not mus101, plays a major role during elongation

Background: Claspin and TopBP1 are checkpoint mediators that are required for the phosphorylation of Chk1 by ATR to maintain genomic stability. Here, we investigated the functions of Drosophila Claspin and mus101 (TopBP1 ortholog) during chorion (eggshell component) gene amplification, which occurs in follicle cells in the absence of global genomic DNA replication. Results: Unlike Drosophila mei‐41 (ATR ortholog) mutant embryos, Claspin and mus101 mutant embryos showed severe eggshell defects resulting from defects in chorion gene amplification. EdU (5‐ethynyl‐2′‐deoxyuridine) incorporation assay during initiation and elongation stages revealed that Claspin and mus101 were required for initiation, while only Claspin had a major role in the efficient progression of the replication forks. Claspin proteins were enriched in the amplification foci both in the initiation and elongation stage‐follicle cell nuclei in a mei‐41‐independent manner. The focal localization of ORC2, a component of the origin recognition complex, was not significantly affected in the Claspin mutant, whereas it was reduced in the mus101 mutant. Conclusions: Drosophila Claspin plays a major role in the initiation and elongation stages of chorion gene amplification by localizing to the amplification foci in a mei‐41‐independent manner. Drosophila mus101 is also involved in chorion gene amplification, mostly functioning in initiation, rather than elongation. Developmental Dynamics 246:466–474, 2016. © 2017 The Authors Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists