Figure 7.

Hierarchy of sexual differentiation genes in C. elegans and Drosophila. In both C. elegans and Drosophila, the primary sex determining signal, the X chromosome to autosome ratio (X : A), sets the activity of the globally acting sex determination pathway. DM domain genes (blue) in C. elegans and Drosophila differ with respect to their relationship to this pathway. In C. elegans, the primary sexually regulated transcription factor is a Gli-related Zn finger protein, TRA-1A, encoded by the terminal gene of the sex determination pathway, tra-1. tra-1 activity affects the development of most sexually dimorphic tissues, acting through non-sex-specific developmental gene targets (e.g., cell death gene egl-1), as well as dedicated sexual regulators (mab-3). mab-3 (in tissues other than the gut) and mab-23 are not directly targeted by TRA-1A. The sex specificity of their activity derives from the cellular context in which they are expressed. In Drosophila, the primary sexually regulated transcription factor is the DM domain gene dsx. In contrast to C. elegans DM domain genes, dsx is a direct target of the sex-determination pathway in most somatic tissues. Like tra-1, dsx is broadly acting and functions early in sexual differentiation to sex-specifically regulate multiple developmental genes. In the CNS, fru appears to be the primary sexually regulated transcription factor.