Gamete formation requires genetic programs that coordinate germ cell differentiation and meiosis with sexual fate choice and maintenance. At the posttranscriptional level these programs are integrated via a dense network of RNA-binding and RNA-modifying proteins. Especially translational regulators, such as PUF, Nanos and cytoplasmic polyadenylation element-binding (CPEB) family proteins are identified in many organisms as molecular nodes of the germ cell fate decision machinery. Caenorhabditis elegans germ cells undergo a programed sperm-to-oocyte switch in the hermaphrodite to produce -in essence- a self-fertile female animal. This is governed by the sequential action of the RNA-binding protein FOG-1CPEB, which initiates male fate specification, and the FBFPUF/NOS-3Nanos RNA regulatory complex that blocks several male fate-promoting (‘fem’ and ‘fog’) genes by preventing their efficient translation, thus limiting sperm production to the final larval stage prior to adulthood. Previously, we showed that in males the Bicaudal-C protein GLD-3 antagonizes FBF-mediated translational repression: while gld-3 mutant germ cells initiate spermatogenesis during larval stages, adults often fail to maintain their male fate and switch to oogenesis in an fbf-dependent manner, suggesting that GLD-3Bic-C is an important player but not the only one in males to keep the female fate-promoting machinery in check.
In search of additional regulators, we identified CPB-1CPEB as a molecular opponent of FBF in sexual fate maintenance. Although cpb-1 hermaphrodites produce -compared to wild type- a similar number of sperm, indicating no crucial role in the sperm-to-oocyte switch fate, we confirmed a previously suggested role in sperm differentiation; all animals are sterile. Importantly, we found that a large fraction of adult males produce oogenic cells after an initial period of spermatogenesis. Our genetic analysis places cpb-1 in the core sex determination pathway upstream of fbf and in parallel to gld-3. Intriguingly, all gld-3; cpb-1 double mutant male gonads produce exclusively oocytes suggesting that CPB-1 works redundantly with GLD-3 to antagonize FBF, which is further corroborated by their expression patterns. To our surprise, detailed structure-function analysis of transgenic rescuing experiments suggested that CPB-1’s RNA recognition motifs are essential for male fate maintenance, rather than a previously mapped FBF-binding region in its amino terminus. Therefore, CPB-1 likely functions in its capacity as an RNA-binding protein to target mRNAs required for maintenance of male germ cell fate. Our findings reveal a novel role for CPEBs and contrast to work in mammals where a transcriptional network is a key regulator of sexual fate maintenance.