Figure 3.

Proteins Determining Sex in Somatic Tissues of D. melanogaster and A. gambiae

In fruit fly females, a double dose of X linked elements activates productive transcription of Sxl, leading to establishment of a positive feedback loop of Sxl production. Sxl controls splicing of tra into a productive form, which, together with Tra2, is necessary for splicing of dsx into a productive form and fru into a non-productive form. Sxl also prevents assembly of the dosage compensation (DC) complex by blocking translation of msl2, a critical component of the complex. In fruit fly males, a single dose of X linked proteins is insufficient to initiate a productive transcription of Sxl. As a result, dosage compensation is activated, tra is spliced into a non-productive form, and the default splicing of dsx and fru productive male forms occurs. In Anopheles females, Fle is necessary for the splicing of dsx and fru into productive and non-productive forms, respectively, as well as for repression of dosage compensation. The upstream sex-determining molecules remain to be identified; it is unclear whether Fle requires a cofactor to promote splicing of dsx and fru. In males, the primary sex determiner Yob triggers the sex determination pathway and inactivates Fle by a yet unknown mechanism, allowing activation of dosage compensation. Fle does not take part in the splicing of dsx or fru into male forms.