Fig. 1. A simplified view of sex determination pathways in the somatic gonad and germline. In somatic cells, the ratio of X chromosomes to autosomes (X:A) influences the activity of Sex-lethal (Sxl), which, in turn, activates transformer (tra). tra, along with transformer 2 (tra2), controls the alternative RNA splicing of doublesex (dsx), which determines whether the somatic gonad will develop as male or as female. In the germ cells, the X:A ratio also influences sexual identity, and ovo, ovarian tumor (otu) and Sxl promote female germ cell development. Interactions between germ cells and somatic cells also influence germline sex determination and act through extracellular ligands, such as Unpaired (Upd), which promotes male development. Gap junctions (red) may also facilitate communication between the two cell types.