Female germline expression of OVO transcription factor bridges <i>Drosophila</i> generations

OVO is required for female germ cell viability but has no known function in the male germline in Drosophila. ovo is autoregulated by 2 antagonistic isoforms, OVO-A and OVO-B. All ovo- alleles were created as partial revertants of the antimorphic ovoD1 allele. Creation of new targeted alleles in an ovo+ background indicated that disrupting the germline-specific exon extension of ovo-B leads to an arrested egg chamber phenotype, rather than germ cell death. RNA sequencing analysis, including >1 K full-length cDNAs, indicates that ovo has several unannotated splice variations in the extended exon and a minor population of ovo-B transcripts has an alternative splice. This indicates that classical ovo alleles, such as ovoD1rv23, are not truly null for ovo and are likely to be weak antimorphs. To generate bonafide nulls, we deleted the ovo-A and ovo-B promoters showing that only ovo-B is required for female germ cell viability, and there is an early and continual developmental requirement for ovo-B in the female germline. To visualize OVO expression and localization, we endogenously tagged ovo and found nuclear OVO in all differentiating female germ cells throughout oogenesis in adults. We also found that OVO is maternally deposited into the embryo, where it showed nuclear localization in newly formed pole cells. Maternal OVO persisted in embryonic germ cells until zygotic OVO expression was detectable, suggesting that there is continuous nuclear OVO expression in the female germline in the transition from one generation to the next.