The Sperm/Oocyte Decision, a C. elegans Perspective

The decision of germ cells to differentiate into spermatocytes or oocytes is one of the most complex choices in development, but it is made long before later changes in cell morphology, and its mechanisms remain elusive. Because of the advantages of hermaphrodite genetics, the nematode C. elegans has become a model for this process. In worms, the core sex-determination pathway controls sexual fates throughout the body, and acts through the transcription factor TRA-1 to regulate fog-1 and fog-3, which specify spermatogenesis and prevent oogenesis. Furthermore, the activities of key genes in this pathway are modulated in the germ line to allow hermaphroditic development, so that XX animals produce a small number of sperm. Many of these modulators work by controlling the translation of target messenger RNAs. Translational regulators play a major role in many aspects of germ cell fate, since some of them prevent maternal transcripts from influencing the sperm/oocyte decision, and some regulate essential transcripts in the germ line and early embryo. In fact, the expression patterns set up by interactions among these translational regulators, acting in concert with signals from the somatic gonad, can explain how germ cell fates are specified.