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

OVO is required for karyotypically female germ cell viability but has no known function in the male germline in Drosophila. <i>ovo</i> is autoregulated by two antagonistic isoforms, OVO-A and OVO-B. All <i>ovo</i>^- alleles were created as partial revertants of the antimorphic <i>ovo</i>^<i>D1</i> allele. Creation of new targeted alleles in an <i>ovo</i>⁺ background indicated that disrupting the germline-specific exon extension of <i>ovo-B</i> leads to an arrested egg chamber phenotype, rather than germ cell death. RNA-seq analysis, including >1K full length cDNAs, indicates that <i>ovo</i> utilizes a number of unannotated splice variations in the extended exon and a minor population of <i>ovo-B</i> transcripts utilizes an alternative splice. This indicates that classical <i>ovo</i> alleles such as <i>ovo</i>^{<i>D1rv23</i>}, are not truly null for <i>ovo</i>, and are likely to be weak antimorphs. To generate bonafide nulls, we deleted the <i>ovo-A</i> and <i>ovo-B</i> promoters showing that only <i>ovo-B</i> is required for female germ cell viability and there is an early and polyphasic developmental requirement for <i>ovo-B</i> in the female germline. To visualize OVO expression and localization, we endogenously tagged <i>ovo</i> 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.