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

Abstract

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 two 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 find 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. Article SummaryA series of foundational papers established that abnormal oocyte (ao), a euchromatic maternal-effect lethal gene, interacts with heterochromatin and the histone multigene cluster to dictate embryonic viability in D. melanogaster. An earlier report argued that ao encodes a protein that localizes to and represses histone gene expression, thereby connecting histone gene overexpression with ao mutant maternal-effect lethality. Using new reagents for genetics and cytology, we recapitulate findings that ao encodes a maternal-effect lethal gene, whose loss is ameliorated by excess heterochromatin or loss of histone genes. However, we find that ao does not affect histone gene expression. Thus, how ao loss causes maternal-effect lethality remains unknown.