Program in Cellular and Molecular Biology, Department of Biochemistry, and Howard Hughes Medical Institute, University of Wisconsin, Madison, WI 53706, USA.
PUF proteins control both growth and differentiation in the C. elegans germ line. These conserved RNA-binding proteins inhibit expression of target mRNAs, either by repressing translation or promoting degradation. Previous studies showed that PUF-8, a PUF protein with striking similarity to human Pumilio, prevents return of primary spermatocytes to the mitotic cell cycle [Subramaniam, K. & Seydoux, G. (2003) Curr. Biol. 13, 134-139]. We now report that PUF-8 is also critical for the hermaphrodite sperm/oocyte switch. Most puf-8 mutant hermaphrodites make both sperm and oocytes and are self-fertile, but some make a vast excess of sperm and fail to switch into oogenesis. This puf-8 defect is dramatically enhanced by removal of another puf gene called fbf-1: all fbf-1 puf-8 double mutants fail in the hermaphrodite sperm/oocyte switch. Therefore, puf-8 and fbf-1 act redundantly to control this decision. Epistasis analyses place puf-8 and fbf-1 upstream of fog-2, a gene near the top of the germ-line sex determination pathway. Furthermore, the abundance of FOG-2 increases dramatically in the distal region of fbf-1 puf-8 double mutants. We suggest that PUF-8 and FBF-1 may control fog-2 expression, and that the sperm/oocyte decision occurs in the distal germ line.