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-poisoning”-and dissected their target-site requirements and repair dynamics. X-shredding resulted in sex distortion when Cas9 endonuclease activity occurred during the meiotic stages of spermatogenesis but not when Cas9 was expressed from the stem cell stages onwards. Our results suggest that X-shredding is counteracted by the NHEJ DNA repair pathway and can operate on a single repeat cluster of non-essential sequences, although the targeting of a number of such repeats had no effect on the sex ratio. X-poisoning by contrast, i.e. targeting putative haplolethal genes on the X chromosome, induced a high bias towards males (>92%) when we directed Cas9 cleavage to the X-linked ribosomal target gene RpS6. In the case of X-poisoning sex distortion was coupled to a loss in reproductive output, although a dominant-negative effect appeared to drive the mechanism of female lethality. These model systems will guide the study and the application of sex distorters to medically or agriculturally important insect target species.
Bibliographical noteFunding Information:
This study was funded by the BBSRC under the research grant BB/P000843/1 to NW. PAP was funded by the Italian Ministry Education, University and Research (MIUR-D.M. no. 79 04.02.2014), by the United States - Israel Binational Agricultural Research and Development Fund (Research Grant No. IS-5180-19) and by the Israel Science Foundation (Research Grant No. 2388/19). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
© 2020 Fasulo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.