Sex determination in Ceratitis relies on a conserved binary genetic ON/OFF switch, splicing-based and epigenetic.

The transformer gene in the Medfly Ceratitis capitata (Cctra ep ) is the founding member of a family of conserved Serine Argi- nine-rich (RS) splicing factor encoding genes that act as master epigenetic ON/OFF switches in sex determination in insects as distantly related as dipterans and hymenopterans. A functional TRA protein is produced only in individuals with a female karyotype where it is required to establish and maintain the female fate during development through a positive feedback loop (Cctra ep ON). When zygotic activation of this loop is prevented (Cctra ep OFF), male development follows. This epigenetic self regulatory mechanism is based on alternative splicing analogous to the one operating in Drosophila melanogaster at the level of the gene, Sex-lethal (Sxl). In Drosophila XX individuals the splicing regulator Sxl is able to positively autoregulate (ON) and to direct and maintain female-specific splicing of transformer (ON). TRA together with its co-regulator transformer-2 in turn promotes female-specific splicing of the target genes doublesex and fruitless . Likewise, CcTRA and CcTRA-2 are required for female-specific splicing of dsx and fru in Ceratitis. Both target genes contain putative tra/tra-2 binding sites (13 nt long repeats) which were first identified in the corresponding targets of Drosophila. tra ep orthologs isolated in other insects seem to act as binary switches that govern all aspects of sexual development. Like Cctra, the feminizing activity of these orthologs rely on a self-regulatory function which maintains the productive female splicing mode. In Musca it was shown that maternal provision of TRA is required to activate the female promoting tra feedback loop in the zygote. tra/tra-2 regulatory elements are present and highly conserved in sequence and distribution in Ceratitis, Musca and all of the dipteran tra orthologs. Surprisingly, tra ortho- logs appear to be absent in the lower dipterans Aedes aegypti and Anopheles gambiae, eventhough the tra targets dsx and fru are present and sex-specifically expressed. We will discuss how understanding the molecular basis of these regulatory mechanisms will help to design new genetic strategies for pest control management of the Medfly and other insects of economical and medical relevance.