A male-biased sex-distorter gene drive for the human malaria vector Anopheles gambiae

In this study, a gene drive in malaria-transmitting mosquitoes (Anopheles gambiae) is presented in which an already described CRISPR/Cas-based gene drive (original publication: Kyrou K, Hammond AM, Galizi R, Kranjc N, Burt A, Beaghton AK, Nolan T, Crisanti A (2018) A CRISPR-Cas9 gene drive targeting doublesex causes complete population suppression in caged A. gambiae mosquitoes. Nat Biotechnol 36 (11): 1062-1066. doi: 10.1038 / nbt.4245) is combined with an already described sex-rato distorter (Original publication: 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. Nat Commun 5: 3977.doi: 10.1038 / ncomms4977).

The gene drive based on CRISPR/Cas integrates the gene drive construct defined target region within a gene called doublesex. Doublesex plays an important role during embryonic development: it is crucial for whether mosquitoes develop in females or in males. The integration of the gene drive construct at this region in the genome leads to the development of non-fertile pseudo-females and normal males. After about ten generations in the laboratory, the gene drive mosquito population collapsed.
The sex-ratio distorter (also called X-Shredder) uses a different mode of action: a specific nuclease is formed during the development of the sperm cells and recognizes a certain DNA sequence that exists in high copy numbers on the X chromosome of the mosquitoes. As a result, the sex chromosome X is cut into many small pieces in X-bearing sperm cells and only sperm cells that have a Y chromosome survive. Consequently, there are almost only male descendants of these genetically modified mosquitoes. So far, the scientists have not yet succeeded in creating a functioning gene drive from this sex-ratio distorter. This is due to various genetic and epigenetic obstacles.
In the study, genetically modified mosquitoes are presented that contain both the CRISPR/Cas-based gene drive and the sex-ratio distorter. To do this, the construct described as sex-distorter gene drive (SDGD) is integrated into the doublesex gene in the genome of the mosquitoes with the help of the CRISPR/Cas9 gene scissors. The SDGD construct contains the DNA encoding the CRISPR/Cas9 gene scissors, the guide for the target sequence of the doublesex gene and the DNA encoding the sex-ratio distorter. If females mate with males carrying the SDGD construct, the SDGD construct is integrated into the doublesex gene, which means that the development of fertile females is prevented. Also , the additionally integrated X-Shredder ensures that the X chromosome is cut during the formation of the germ line cells. Overall, the offspring are mostly all male. Depending on the amount of genetically modified mosquitoes used, the population collapses after 9-13 generations because there are mostly male offspring and hardly any fertilized eggs.
The two constructs of the SDGD mosquitoes have a redundant effect: if one system fails, the other will still work. According to the calculations of the scientists, the number of (biting) female mosquitoes is reduced much faster with the SDGD construct compared to the original gene drive. An indication for synergy of the two systems.