A fly model establishes distinct mechanisms for synthetic CRISPR/Cas9 sex distorters

Synthetic sex distorters have recently been developed in the malaria mosquito, relying on endonucleases that target the X-chromosome during spermatogenesis. Although inspired by naturally-occurring traits, it has remained unclear how they function and, given their potential for genetic control, how portable this strategy is across species. We established Drosophila models for two distinct mechanisms for CRISPR/Cas9 sex-ratio distortion - "X-shredding" and "X-meddling" - and dissected their target-site requirements and repair dynamics. X-shredding relies on sufficient meiotic activity of the endonuclease to overpower DNA repair and can operate on a single repeat cluster of non-essential sequences. X-meddling by contrast, i.e. targeting putative haplolethal X-linked genes, induced a bias towards males that is coupled to a loss in reproductive output, although a dominant-negative effect may drive the mechanism of female lethality. Our model system will guide the study and the application of sex distorters to medically or agriculturally important insect target species.