Female-to-male sex conversion in Ceratitis capitata by CRISPR/Cas9 HDR-induced point mutations in the sex determination gene transformer-2

The Sterile Insect Technique (SIT) is based on the mass release of sterilized male insects to reduce the pest population size via infertile mating. Critical for all SIT programs is a conditional sexing strain to enable the cost-effective production of male-only populations. Compared to current female-elimination strategies based on killing or sex sorting, generating male-only offspring via sex conversion would be economically beneficial by doubling the male output. Temperature-sensitive mutations known from the D. melanogaster transformer-2 gene (tra2(ts)) induce sex conversion at restrictive temperatures, while regular breeding of mutant strains is possible at permissive temperatures. Since tra2 is a conserved sex determination gene in many Diptera, including the major agricultural pest Ceratitis capitata, it is a promising candidate for the creation of a conditional sex conversion strategy in this Tephritid. Here, CRISPR/Cas9 homology-directed repair was used to induce the D. melanogaster-specific tra2(ts) SNPs in Cctra2. 100% female to male conversion was successfully achieved in flies homozygous for the tra2(ts2) mutation. However, it was not possible, to identify a permissive temperature for the mutation allowing the rearing of a tra2(ts2) homozygous line, as lowering the temperature below 18.5 °C interferes with regular breeding of the flies.