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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

developmental dynamics

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