Nicky Faber

In this thesis, I have investigated the evolutionary prerequisites for safe and effective use of gene drive technology in invasive pest management. My work can be classified into three themes that fall under this broader goal: evolutionary stability, efficiency, and safety. For gene drives to achieve population suppression or modification, they must remain evolutionarily stable. In Chapter 2, I tested whether standing genetic variation in several European Drosophila melanogaster populations could affect gene drive efficiency, but found no evidence for such polygenic resistance. However, even with a drive targeting a highly conserved gene, mutations in the gene drive target site still arose, potentially indicating simple resistance. This result suggests that resistance forming from drive activity itself is the most significant evolutionary instability for this technology. Because gene drives also need to be robust to environmental variation, in Chapter 3 I examined how a single ecological factor, yeast availability, affects drive performance in D. melanogaster. Even this single change in environmental conditions caused considerable variability in drive efficiency. Furthermore, although the gene drive appeared effective in single crosses, it failed to spread in most cage experiments, implying hidden fitness costs. These results indicate that laboratory fitness estimates are likely overly optimistic, and that gene drives are not yet ready for open field trials without intermediate, contained, small-scale testing. Achieving the necessary gene drive efficiency to enable spread through a population is still a major challenge in many species. In Chapter 4, I explored how species-specific developmental biology may explain why many gene drives achieve only low or moderate efficiency. We found numerous biological factors that have been overlooked in design strategies, which are often based on trial-and-error. Taking developmental and genomic context into account during gene drive design is likely the most promising strategy to improve gene drive efficiency going forward. When such improvements are not feasible, alternative gene drive designs may help. In Chapter 5, I developed and modelled a two-part gene drive that separates the spreading and sterilising functions of a gene drive for improved suppressive power. This design shows stronger suppressive potential under intermediate gene drive inheritance rates as well as a greater robustness to resistance. However, laboratory testing in D. melanogaster revealed unexpectedly high fitness costs that prevented spread, potentially indicating a limitation of multi-part gene drive designs. Finally, in Chapter 6 I assessed the feasibility of a safety mechanism that relies on locally fixed alleles. Population genetic modelling showed that such alleles are highly unlikely to form within the short time frames typical of invasive species on islands, making this strategy unlikely to be a realistic mechanism for spatial containment. In general, I conclude that long-term evolutionary processes are unlikely to be highly relevant for gene drive technology: pre-existing genetic diversity is unlikely to provide a broadly useful basis for gene drive localisation mechanisms, nor will it be a major barrier in terms of efficiency through selection for resistance based on standing genetic variation. Rather, it is the mechanistic aspects of gene drive technology at smaller scales (molecular, cellular, and tissue-specific) that could benefit from further optimisation, as gene drives can suffer from low homing rates and subtle, yet impactful, fitness costs to their carriers. I believe that it is time to engage more deeply with the biology of each target species, rather than continuing to rely on optimising technical aspects via trial-and-error, to be able to design gene drives with the efficiency, stability, and safety that would make them a viable management tool for invasive pests. Equally important is the development of rigorous ecological risk assessments, robust governance structures, and transparent science communication to ensure that any potential deployment proceeds responsibly and with public trust.