In a recent paper in Scientific Reports, Galizi, Hammond, et al report on the use of CRISPR-Cas9 to develop a sex-distortion system in Anopheles gambiae, which they termed CRISPRSD. Earlier, the group had demonstrated that transgenic An. gambaie expressing the endonuclease I-PpoI could be used to generate “X-shredder” lines of An. gambiae. In this system, I-Ppol, which targets critical sequences on the X chromosome in An. gambiae, was expressed under control of a promoter that restricted the expression of the endonuclease to male meiosis, resulting in a favoring of Y-bearing spermatozoa, resulting in predominantly male offspring.
By expressing Cas9 using the same male meiotic promoter, and sgRNA guides under control of a ubiquitously-expressed U6 PolIII promoter, the researchers were able to replicate the same phenotype, establishing four lines of An. gambiae that exhibited male biases ranging from 86.1-94.8%, with three of these lines showing good male fertility, and a fourth with reduced fertility, which the authors attributed to possible position effects.
While the authors point out several current limitations of the system for pest management (development of cleavage-resistant alleles, full inheritance only if Y-linked, etc.), the demonstration of the utility of using CRISPR-Cas9 to generate sex-ratio distortions in insects with XY mating systems removes the limitation of the requirement that unique endonuclease targets must first be present on the X chromosome. Through careful bioinformatic analyses and validation testing in the whole animal, the direct targeting of X-linked essential genes, such as the 28S rDNA (which was the target for this work), represents an interesting approach to consider if interested in generating sex-distorted lines of insects with XY mating systems.
Galizi R, Hammond A, Kyrou K, Taxiarchi C, Bernardini F, O’Loughlin SM, Papathanos P-A, Nolan T, Windbichler N, Crisanti A. 2016. A CRISPR-Cas9 sex-ratio distortion system for genetic control. Scientific Reports 6:31139. DOI: 10.1038/srep31139.
Galizi R, Doyle LA, Menichelli M, Bernardini F, Deredec A, Burt A, Stoddard BL, Windbichler N, Crisanti A. 2014. A synthetic sex ratio distortion system for the control of the human malaria mosquito.Nature Communications 5, Article number: 3977. doi:10.1038/ncomms4977