Sex determining protein required in the germline to promote the spermatogenesis to oogenesis switch during the late larval stages of development. Acts with the sex determining factor tra-1, and redundantly with wdr-5.1, to regulate fog-3 expression, which in turn determines germ cell fate. Not required for methylation of histone H3 'Lys-4'. No obvious phenotype at 20 degrees Celsius or 25 degrees Celsius. At 20 and 25 degrees Celsius, double knockdown mutants with wdr-5.1 have increased fog-3 expression. At 25 degrees Celsius, these mutants have increased fog-1 expression, reduced brood size accompanied by 42% embryonic lethality with 100% of the surviving progeny being sterile. Surviving progeny display defective spermatogenesis to oogenesis transition with 88% of the gonads only containing sperm. The remaining germ cells in the gonads switch to oogenesis, but the oocytes display either an endoreplication or endomitotic phenotype. Germ cells also have increased expression of the sex determining factor tra-1 in the cytoplasm and as a result there is reduced binding of tra-1 to the fog-3 promoter. No detectable defects in histone H3 'Lys-4' di- or tri-methylation in embryos or adults germ cells at 20 or 25 degrees Celsius.