The Drosophila maternal-effect gene abnormal oocyte ( ao ) does not repress histone gene expression

The abnormal oocyte (ao) gene of Drosophila melanogaster is a maternal-effect lethal gene previously identified as encoding a transcriptional regulator of core histones. However, background genetic mutations in existing ao mutant strains could compromise their utility in manipulating histone levels. To distinguish the true ao phenotype from background effects, we created 2 new ao reagents: a CRISPR/Cas9-mediated knockout of the ao allele for genetic and molecular analyses and an epitope-tagged ao allele for cytological experiments. Using these reagents, we confirm previous findings that loss of ao causes maternal-effect lethality, which can be rescued by either a decrease in the histone gene copy number or by Y chromosome heterochromatin. Our data indicate that ao genetically interacts with the heterochromatin, as previously suggested. However, contrary to a prior study, we detected neither Ao localization to histone genes nor ao repression of core histone transcript levels. Thus, the molecular basis for ao-associated maternal-effect lethality remains unknown.