Applications and status of gene drive in plants
Fig 1
Self-sustaining and self-limiting Cleave and Rescue mechanism (ClvR)-based gamete killers.
ClvR-based gamete killers produce, in the adult, loss-of-function (LOF) alleles of a gene whose expression in gametes is required for their survival. Following meiosis, these select against haploid gametes not inheriting the ClvR-bearing Rescue (gray, dying gametes), leading to almost exclusive transmission of ClvR-bearing versions of the chromosome (light green gametes) [2,3]. When Cas9 and the Cargo/Rescue are tightly linked (gamete combinations outlined in dashed red box) drive is self-sustaining because both sets of components continuously experience the drive benefits of LOF allele creation—an increase in frequency relative to that of the non-ClvR chromosome due to loss of the latter. In contrast, when Cas9 (or the gRNAs, not shown) are located at some distance from the Cargo/Rescue on the same chromosome they are initially inherited together at high frequency (thick upward arcing arrow; >50%), and drive is strong. However, over time recombination and chromosome segregation separate them into different gametes (gametes outlined in dashed black box; thin downward arcing arrow; <50%). Those carrying only Cas9 are lost, resulting in its progressive elimination from the population. Since it is the LOF alleles generated by Cas9/gRNAs that select for the presence of Cargo/Rescue by killing non-Cargo/Rescue-bearing gametes, loss of Cas9 leads to LOF allele loss and the end of Cargo/Rescue element spread. The closer these components are to each other on the same chromosome (<50% in adult plant), the longer drive is maintained—up to approximately a complete-self-sustaining element—while still being ultimately self-limited [3,6].