TheDrosophilamaternal-effect geneabnormal oocyte(ao) does not repress histone gene expression

Theabnormal oocyte(ao) gene ofDrosophila melanogasteris a maternal-effect lethal gene previously identified as encoding a transcriptional regulator of core histones. However, background genetic mutations in existingaomutant strains could compromise their utility in manipulating histone levels. To distinguish the trueaophenotype from background effects, we created two newaoreagents: a CRISPR/Cas9-mediated knockout of theaoallele for genetic and molecular analyses and an epitope-taggedaoallele for cytological experiments. Using these reagents, we confirm previous findings thataoexhibits maternal-effect lethality, which can be rescued by either a decrease in the histone gene copy number or by Y chromosome heterochromatin. We also confirm that the Ao protein localizes to the histone locus bodies in ovaries. Our data also suggest thataogenetically interacts with the histone genes and heterochromatin, as previously suggested. However, contrary to prior findings, we find thataodoes not repress core histone transcript levels. Thus, the molecular basis forao-associated maternal-effect lethality remains unknown.