The Drosophila melanogaster shavenbaby (svb)-ovo gene region is a complex locus, containing two distinct but comutable genetic functions. ovo is required for survival and differentiation of female germ line cells and plays a role in germ line sex determination. In contrast, svb is required in both male and female embryos for the production of epidermal locomotor and sensory structures. Sequences required for the two genetic functions are partially overlapping. ovo corresponds to a previously described germ line-dependent 5.0-kb poly(A)+ mRNA that first appears in the germarium and accumulates in nurse cells during oogenesis. The 5.0-kb mRNA is stored in the egg, but it is rapidly lost in the embryos except for its continued presence in the germ line precursor pole cells. The ovo mRNA predicts a 1,028-amino-acid 110.6-kDa protein homologous with transcription factors. We have identified an embryonic mRNA, 7.1 kb in length, that contains exons partially overlapping those of the 5.0-kb poly(A)+ mRNA. The spatial distribution of this newly discovered transcript during midembryogenesis suggests that it corresponds to the svb function. The arrangement of exons common to the 5.0-and 7.1-kb mRNAs suggests that the Ovo and Svb proteins share DNA-binding specificity conferred by four Cys2-His2 zinc finger motifs but differ functionally in their capacity to interact with other components of the transcription machinery.The central role of Sex-lethal (Sxl) for sex determination in the somatic tissues of Drosophila melanogaster has been elegantly worked out (reviewed in reference 9). However, the mechanisms active in the germ line differ in three major ways from those operating in somatic sex determination. First, although Sxl is required in the germ line, it is not active in the germ line precursor cells of early embryos (4) when sex-specific germ line functions first appear. Second, the activation of Sxl in the germ line does not require the same X-linked genes as required for its activation in embryonic somatic cells (14, 37), nor are the downstream genes required in the germ line (reviewed in references 28 and 36). Third, germ line sex determination is not autonomous but depends upon proper somatic cell-germ line interactions (38). Thus, it is necessary to identify the genes required by germ line cells for the assessment of sex identity and to determine the biochemical pathways by which they operate.A key gene for sex-dimorphic germ line functions is ovo. Recessive, apparently null, alleles cause female-specific sterility due to the loss of germ cells in the embryo (27 ovo alleles to suppress partially P-M hybrid dysgenesisinduced female germ line cell death (39). Thus, ovo appears to play a prominent role in the early female-specific establishment of the germ line as well as in subsequent oocyte development and differentiation.Genomic DNA sequences required for ovo function are genetically overlapping and interdigitated with the genomic DNA sequences required for the shavenbaby (svb) function (13,22). svb is known to aff...