In C. elegans , the gonad primordium forms during embryogenesis when two somatic precursors, Z1 and Z4, migrate posteriorly to flank two central germline precursors. Despite the different polarity of the adult male and hermaphrodite gonads, the four-celled gonadal primordium is morphologically indistinguishable in the two sexes. We have identified four genes that regulate assembly of the gonadal primordium: gnd-1, gnd-2, gnd-3 and tra-1 . The gnd genes are critical for generation of the primordium and also for patterning within the primordium. Thus, newly hatched animals lacking gnd function can be missing Z1 or Z4 entirely, or Z1 and Z4 can be misplaced within the primordium. Therefore, the gnd genes either control generation of Z1/Z4 or their migration to the primordium in addition to establishing the symmetry of the primordium. The gnd-1 gene encodes a bHLH putative transcription factor that is expressed in the MS lineage and becomes restricted to Z1 and Z4 after the primordium forms. Cloning of gnd-2 and gnd-3 is in progress. The role of tra-1 in gonadogenesis overlaps that of the gnd genes: tra-1 is essential for patterning of Z1 and Z4 within the gonadal primordium of both XX and XO animals. Previous studies showed that tra-1 promotes female development in somatic tissues and is required for gonadogenesis in both hermaphrodites and males (Hodgkin, 1987). We find that tra-1 is essential for gonadal primordium formation: Z1 and Z4 are always present in tra-1 null mutants, but they are often misplaced. Furthermore, in tra-1 null mutants, the polarities of Z1 and Z4 divisions are the same with respect to the A/P axis, whereas in wild-type animals, they have opposite polarities. Thus, the tra-1 gonadal primordium appears to have lost the two-fold rotational symmetry typical of the wild-type primordium in both hermaphrodites and males. Instead, the tra-1 primordium has an asymmetry that is typical of developing male gonads at a later stage. We conclude that tra-1 is essential for primordium symmetry in both sexes. Genetic interactions between gnd mutants and tra-1 suggest that they might be part of a common pathway. We are exploring the possibility that the gnd genes control patterning of the primordium by regulating tra-1 . We are also currently examining other sex determination pathway members for a role in this process. Hodgkin, J. (1987) Genes & Development 1 , 731-745.