Targeting mosquito X-chromosomes reveals complex transmission dynamics of sex ratio distorting gene drives

Abstract

Engineered sex ratio distorters (SRDs) have been proposed as a powerful component of both self-limiting and self-sustaining genetic control strategies designed to suppress harmful insect pest or disease vector populations such as malaria mosquitoes. Two different CRISPR-based SRD mechanisms, X-shredding which eliminates X-bearing sperm in males and X-poisoning which causes postzygotic mortality of daughters inheriting disrupted X-linked genes, have been proposed. The nature of the sex distortion mechanism is predicted to affect the population dynamics of released Y-chromosome SRDs: A Y-linked X-shredder is invasive constituting a classical meiotic Y-drive, whereas Y-linked X-poisoning is self-limiting, unable to invade but also insulated from selection and disappearance. Here, we established the first X-poisoning SRD system in the malaria vectorAnopheles gambiaetargeting three X-linked ribosomal protein (RP) genes during spermatogenesis. Surprisingly, when we characterized transgenic strains showing a significant male bias, we found that sex distortion was primarily driven by a loss of X-bearing sperm with limited evidence for postzygotic lethality of daughters. By leveraging theDrosophilamodel, we show unambiguously that engineered SRD traits can operate differently in these two insects. Unlike X-shredding, X-poisoning could theoretically operate at early stages of spermatogenesis. We therefore explored premeiotic Cas9 expression to target the mosquito X-chromosome. We find that, by pre-empting the onset of meiotic sex chromosome inactivation, this approach may enable the development of Y-linked SRDs, once trade-offs between sex chromosome transmission distortion and male fertility are functionally balanced.

The haplolethal gene wupA of Drosophila exhibits potential as a target for an X-poisoning gene drive

“…Taken together, the above results suggest that wupA could be a good target for an X-poisoning gene drive ( Burt and Deredec 2018 ; Fasulo et al . 2020 ; Haber et al . 2023 ).…”

“…A related approach, dubbed “X-poisoning,” also targets loci on the X chromosome, not so that the chromosome is physically destroyed but so that mutations are introduced such that progeny receiving the X chromosome will not be viable ( Burt and Deredec 2018 ; Fasulo et al . 2020 ; Haber et al . 2023 ).…”

The haplolethal genewupAof Drosophila exhibits potential as a target for an X-poisoning gene drive

“…If the X-shredding transgenes could be placed on the Y chromosome, then an inheritance can be established such that fathers only have sons. Originally, the X-shredder was attempted in lab populations of Anopheles gambiae mosquito using the endonuclease PpoI , but subsequent implementations in A. gambiae mosquitos, Drosophila melanogaster and Ceratitis capitata fruit flies have deployed CRISPR/Cas9 machinery (Galizi et al 2014; Fasulo et al 2020; Meccariello et al 2021; Haber et al 2023).…”

“…A related approach, dubbed ‘X-poisoning’, also targets loci on the X chromosome; not so the chromosome is physically destroyed but so that mutations are introduced such that progeny receiving the X chromosome will not be viable (Burt and Deredec 2018; Fasulo et al 2020; Haber et al 2023). This strategy uses CRISPR/Cas9 to target X-linked haplolethal genes.…”