Fig. 5. DNA damage triggers a Chk2-dependent block in the expression of a subset of zygotic genes. Enhanced fluorescence in situ hybridization for runt (A-F) and slam (G-L) during early interphase 14 is shown. Embryo stage was judged by blastoderm nuclear density (insets) and DIC images (not shown). (A,C) In w¹¹¹⁸ and mnk mutant embryos, runt mRNA is distributed broadly over the central region of the embryo (Klingler and Gergen, 1993). (G,I) slam mRNA localizes to the invaginating membrane front, similar to the Slam protein localization reported previously (Lecuit et al., 2002), resulting in a hexagonal network structure around each nucleus. (B) In w¹¹¹⁸ embryos treated with bleomycin, runt transcript is not detectable. (D) By contrast, similarly treated mnk mutant embryos express runt at the same levels as the embryos without DNA damage. Following bleomycin treatment, slam is expressed in both w¹¹¹⁸ and mnk mutant embryos, although slam transcript localization is disrupted. This transcript is dispersed in w¹¹¹⁸ embryos (H) and localizes around the large aneuploid nuclei in mnk embryos (J). runt expression is also blocked in grp mutant embryos, and expression is restored in mnk grp double mutants (E,F). By contrast, slam transcripts are detected in both grp single-mutant and mnk grp double-mutant embryos (K,L). DNA damage during the syncytial blastoderm stage and the grp mutation thus produce similar gene-specific expression defects. Scale bar: 100 µm.