New Sexing Strains for Mediterranean Fruit Fly Ceratitis capitata: Transforming Females into Males

Sex determination mechanisms, differing in their modality, are widely represented in all the various animal taxa, even at the intraspecific level. Within the highly diversified class Insecta, Drosophila has been used to unravel the molecular and genetic mechanistic interactions that are involved in sex determination. Indeed, the molecularly characterized genes of the Drosophila sex determination hierarchy X:A > Sxl > tra > dsx have been fruitful starting points in the cloning of homologous genes from other insect species. This genetic cascade seems to control sex determination in all Drosophila species. Sex determination in the tephritid Mediterranean fruit fly Ceratitis capitata (Wiedemann), which diverged from Drosophila 90-100 million years ago, contrasts to that found in Drosophila. A different primary signal, a Y-linked male-determining factor (M), still to be molecularly identified, dictates maleness whereas in Drosophila, the primary signal is the X:A (X chromosome:autosome) balance. However, the Drosophila sex-determining pathway, apart from the X:A > Sxl initial regulatory segment, is functionally conserved in C. capitata. The tra gene (Cctra) of C. capitata, as in Drosophila, is the master gene for femaleness through its regulation of the dsx gene and it is dispensable for maleness. In contrast to Drosophila however, where tra is a subordinate target of Sxl, Cctra seems to initiate an autoregulatory mechanism in XX embryos that provides continuous tra female-specific function and acts as a cellular memory maintaining the female pathway. Indeed, a transient interference with Cctra expression in XX embryos by RNA interference (RNAi) treatment can cause complete sexual transformation of both germ-line and soma in adult flies, resulting in fertile XX pseudomales. The development of new transgenic sexing strains of C. capitata able to produce male-only progeny following heat-shock treatments is now feasible and a concrete possibility. Evolutionary considerations strongly suggest that this biotechnological strategy to produce maleonly progeny could be developed for many other Tephritidae and other dipteran species where the sterile insect technique (SIT) is employed within the framework of area-wide integrated pest management programmes.

KEYWORDS Mediterranean fruit fly, Ceratitis capitata, Tephritidae, SIT, sexing, sex determination, RNA interference, transgenic, biotechnology, biological control