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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.
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