Conditional Expression of Cas9 and dCas9 in Lucilia cuprina Reveals dCas9-Associated Lethality.

Conditional sex transformation systems could improve genetic control strategies against insect pests. Here, we developed and tested CRISPR-based, tetracycline-repressible sex transformation strains in the Australian sheep blowfly, Lucilia cuprina. Using Tet-Off-regulated expression of Cas9 and dCas9, we targeted the sex-determining gene transformer with the goal of converting females into males. Conditional Cas9 expression enabled knockout of a visual marker gene, confirming inducible genome editing. However, strains expressing transformer-targeting sgRNA arrays did not undergo sex transformation. Embryonic microinjection of transformer-targeting sgRNAs into Cas9-expressing embryos produced intersex individuals, indicating that sgRNA expression from the integrated arrays was insufficient to disrupt the sex determination pathway. In contrast, high-level dCas9 expression was associated with developmental delays, reduced body weight, and lethality. These findings establish the first conditional CRISPR expression system in L. cuprina and demonstrate that Cas9 is compatible with inducible gene editing, whereas dCas9 is poorly tolerated at high expression levels.