Scholarly Literature
This is an aggregation of scholarly literature from peer-reviewed journals, bioRxiv , medRxiv and arXiv preprint servers focused on natural and engineered selfish genetic elements and genetic biocontrol. New citations are added as they appear. This content is intended for anyone interested in the scholarly literature on gene drive and genetic biocontrol.
Alternative Techniques and Options for Risk Reduction of Gene Drives
11265Bernd Giese, Arnim von Gleich and Johannes L. Frieß, Gene Drives at Tipping Points, 2020-04-28 18:59:49.
In this chapter, we analyse and compare different types of gene drives as well as promising alternative approaches that may provide a reduced risk.
Limits of Knowledge and Tipping Points in the Risk Assessment of Gene Drive Organisms
11263Arnim von Gleich and Winfried Schröder, Gene Drives at Tipping Points, 2020-04-28 18:50:47.
New challengesTipping point in the risk assessment of genetically engineered (GE) organisms are expected to emerge in the context of so-called ‘gene drives’. Based on a review of findings from current knowledge of GE organisms, it is concluded that the risk assessment of gene drive organisms intended for release into the environment will inevitably suffer from major uncertainties, ‘unknowns’ and methodological problems: subsequent generations of GE organisms might show effects that were not observed or intended in the first generation. Unintended effects can, for example, emerge from interaction with genetic backgrounds within natural populations or be triggered by changing environmental conditions. Due to the increasing spatio-temporal complexity associated with the long-term persistence and propagation of GE organisms, risk assessment can no longer be expected to produce sufficiently reliable results. It has to be assumed that at a certain point in the dissolution of spatio-temporal boundariesa tipping point will be reached, that will make reliable risk assessment impossible. Moreover, methodological problems need to be overcome: the comparative approach that is the starting point for current European Food Safety Authority (EFSA)European Food Safety Authority (EFSA)’s environmental risk assessment might not be applicable due to the lack of suitable ‘comparators’. Despite increasing uncertainties, riskassessors and risk managers need to solve the problems of how to come to robust conclusions and make reliable decisions that take the precautionary principle(PP) into sufficient consideration. The introduction of a new step in the risk assessment of genetically engineered organisms has been suggested to solve these problems—which is the ‘spatio-temporal controllability’ and takes three criteria into account:(1)the biology of the target organisms,(2)their naturally occurring interactions with the environment (biotic and abiotic),(3)the intended biological characteristics (traitsTraits) of the GE organismsGenetically engineered organism.
Steps Towards a Precautionary Risk Governance of SPAGE Technologies Including Gene-Drives
11261Arnim von Gleich, Gene Drives at Tipping Points, 2020-04-28 18:44:32.
In view of the rapid dynamics of genetic engineering development (in particular regarding the ‘new gene-technologies’ gene editing, self-propagating artificial genetic elements (SPAGESelf-Propagating Artificial Genetic Elements (SPAGE)) and synthetic biology), the question is being intensively discussed whether the currently practiced risk governance is sufficient to guarantee the desired high level of health, consumer and environmental safety. Especially the extreme spatial and temporal environmental exposure due to released SPAGEs is identified as a problem because it leads to an enormous expansion of ignorance about possible situations and interactions. Therefore it requires measures based on the precautionary principle. Workable ways to integrate the precautionary principle into environmental risk assessment and risk regulation are being developed.
Model Concepts for Gene Drive Dynamics
11259Johnannes L. Frieß, Merle Preu and Broder Breckling, Gene Drives at Tipping Points, 2020-04-28 18:40:56.
The GeneTip project works on the conception and design modeling of population dynamics influenced by gene drives. In this pursuit, multiple different approaches and concepts have been developed to on one hand, be able to cover a broad perspective on the topic but on the other hand to also focus on different key aspects. In the following we present seven concepts based on different modeling approaches. In these model concepts our model organism is the olive fruit fly (Bactrocera oleae), which is a major pest species in agricultural olive production.
Case Study 2: Oilseed Rape (Brassica napus L.)
11257Johnannes L. Frieß, Broder Breckling, Kathrin Pascher and Windfried Schröder, Gene Drives at Tipping Points, 2020-04-28 18:36:22.
SPAGESelf-Propagating Artificial Genetic Elements (SPAGE) (Self-Propagating Artificial Genetic Element) technologies allow for a proliferation of genetic information on the populationPopulation level at a higher rate than usual Mendelian inheritanceMendelian inheritance. Currently projected developments of SPAGESelf-Propagating Artificial Genetic Elements (SPAGE) mainly aim at a reduction or suppressionSuppression of animal populationsPopulation which are considered to be harmful or undesirable (Oye et al. 2014). However, the application of SPAGESelf-Propagating Artificial Genetic Elements (SPAGE) is not limited to animals only. In principle, also plant populationsPopulation can be targeted (National Academies of Sciences 2016). The GeneTip case study on oilseed rape (Brassica napus) is intended to assess, which interactions play a role in a plant-specific context to address relevant ecological interactions that need to be fully explored in order to estimate potential risksRisk.
Case Study 1: Olive Fruit Fly (Bactrocera oleae)
11255Merle Preu, Johannes L. Frieß, Broder Breckling and Winfried Schröder, Gene Drives at Tipping Points, 2020-04-28 18:31:15.
The olive fruit fly Bactrocera oleae is a phytophagous insect associated to olive trees (Olea europaea, Oleaceae). Its larvae monophagously feed on olive fruits, the fly is therefore considered the most severe pest of olive cultivation causing tremendous economic losses. The olive fly therefore poses a good example of a potential target organism in a European context. This case study revealed that uncertainties exist with regard to the dispersal capacity of gene drive-bearing olive flies, as well as concerning the high gene flow between different populations and most importantly with regard to the population bottlenecks that regularly occur in winter. These would significantly increase or decrease genetic variability between subpopulations and thereby severely jeopardize the intended outcome of any SPAGE-application.
Vulnerability Analysis of Ecological Systems
11253Carina R. Lalyer, Arnim von gleich, Bernd Giese, Gene Drives at Tipping Points, 2020-04-28 18:28:04.
Vulnerability analysis can be seen as the counterpart to technology characterization. Technology characterisation scrutinises the intervening technology. Vulnerability analyses potentially affected systems. That may be socio-ecological, socio-technical, socio-economic or other systems. In this chapter ecological systems are in focus.
Gene Drives Touching Tipping Points
11251Brodee Breckling, Arnim von Gleich,, Gene Drives at Tipping Points, 2020-04-28 18:24:58.
Tipping points and tipping elements, phase transitions and similar critical phenomena are widely discussed in scientific as well as socio-economic contexts as components to understand unforeseen far reaching changes and critical transitions from one stage into another in complex systems caused by small perturbations or gradual changes. For the risk assessment of self-propagating artificial genetic elements in self-sustaining population wild populationsof animals or plants, it is crucial to understand, where tipping elements could become relevant, how they could be anticipated and to what extent surprises and unexpected effects might occur.
Technology Characterisation
11249Johannes L. Frieß, Bernd Giese, Arnim von Gleich, Gene Drives at Tipping Point, 2020-04-28 18:21:27.
In recent years, innovation in genetic engineering brought forth a number of technologies to manipulate the fate of entire wild typeWild type populations. These technologies rely on the dissemination of synthetic genetic elements within a population of sexually reproducing species via the germline and are identified as Self-Propagating Artificial Genetic Elements (SPAGESelf-Propagating Artificial Genetic Elements (SPAGE)). Some secure their dissemination passively so that only offspring carrying the SPAGESelf-Propagating Artificial Genetic Elements (SPAGE) will survive or be fertile. Others overcome the limitations of the Mendelian inheritance pattern by a distortion of allelic segregation or a fragmentation of chromosomes, resulting in e.g. an altered sex ratio. Genetic elements may also promote their preferred inheritance by a molecular mechanism. If a SPAGESelf-Propagating Artificial Genetic Elements (SPAGE) overcomes the Mendelian pattern of inheritance and is thereby enabled to spread and distribute a novel traits throughout a population – even defying natural selection – it is called a gene drive. If organisms have a comparably short generation time, as e.g. insects, then already after a few months, a large part of the population could express a new property transmitted by the gene drive. In particular, very invasivegene drives may be able to impose properties on entire populations that otherwise could not spread.
Gene Drives at Tipping Points
11247Amin von Gleich and Winfried Schroder, Gene Drives at Tipping Points, 2020-04-28 18:14:49.
This open access book reports on a pilot project aiming at collecting information on the socio-ecological risks that could arise in the event of an uncontrolled spread of genetically engineered organisms into the environment. The researchers will, for instance, be taking a closer look at genetically engineered oilseed rape, genetically engineered olive flies as well as plants and animals with so-called gene drives. The book mainly adresses researchers.
The Enterprise: A massive transposon carrying Spokt meiotic drive genes
15127A. A. Vogan, S. L. Ament-Velásquez, E. Bastiaans, O. Wallerman, S. J. Saupe, A. Suh and H. Johannesson, bioRxiv, 2020.03.25.007153. 2020-04-28 16:03:56.
Previously, we described a large genomic feature called the Spok block which is notable due to the presence of meiotic drive genes of the Spok gene family. The Spok block ranges from 110 kb to 247 kb and can be present in at least four different genomic locations within P. anserina, despite what is an otherwise highly conserved genome structure. We have determined that the reason for its varying positions is that the Spok block is not only capable of meiotic drive, but is also capable of transposition. More precisely, the Spok block represents a unique case where the Enterprise has captured the Spoks, thereby parasitizing a resident genomic parasite to become a genomic hyperparasite.
The yeast mating-type switching endonuclease HO is a domesticated member of an unorthodox homing genetic element family
11244A. Y. Coughlan, L. Lombardi, S. Braun-Galleani, A. A. R. Martos, V. Galeote, F. Bigey, S. Dequin, K. P. Byrne and K. H. Wolfe, eLife, 9:e55336. 2020-04-27 17:30:45.
The mating-type switching endonuclease HO plays a central role in the natural life cycle of Saccharomyces cerevisiae, but its evolutionary origin is unknown. HO is a recent addition to yeast genomes, present in only a few genera close to Saccharomyces. Here we show that HO is structurally and phylogenetically related to a family of unorthodox homing genetic elements found in Torulaspora and Lachancea yeasts. These WHO elements home into the aldolase gene FBA1, replacing its 3' end each time they integrate. They resemble inteins but they operate by a different mechanism that does not require protein splicing. We show that a WHO protein cleaves Torulaspora delbrueckii FBA1 efficiently and in an allele-specific manner, leading to DNA repair by gene conversion or NHEJ. The DNA rearrangement steps during WHO element homing are very similar to those during mating-type switching, and indicate that HO is a domesticated WHO-like element.
Mosquito-Borne Diseases Emergence/Resurgence and How to Effectively Control It Biologically
12383H. Dahmana and O. Mediannikov, Pathogens, 9:26. 2020-04-23 17:49:22.
Deadly pathogens and parasites are transmitted by vectors and the mosquito is considered the most threatening vector in public health, transmitting these pathogens to humans and animals. We are currently witnessing the emergence/resurgence in new regions/populations of the most important mosquito-borne diseases, such as arboviruses and malaria. This resurgence may be the consequence of numerous complex parameters, but the major cause remains the mismanagement of insecticide use and the emergence of resistance. Biological control programmes have rendered promising results but several highly effective techniques, such as genetic manipulation, remain insufficiently considered as a control mechanism. Currently, new strategies based on attractive toxic sugar baits and new agents, such as Wolbachia and Asaia, are being intensively studied for potential use as alternatives to chemicals. Research into new insecticides, Insect Growth Regulators, and repellent compounds is pressing, and the improvement of biological strategies may provide key solutions to prevent outbreaks, decrease the danger to at-risk populations, and mitigate resistance.
Opinions of key stakeholders on alternative interventions for malaria control and elimination in Tanzania
11461M. F. Finda, N. Christofides, J. Lezaun, B. Tarimo, P. Chaki, A. H. Kelly, N. Kapologwe, P. Kazyoba, B. Emidi and F. O. Okumu, Malaria Journal, 19:164. 2020-04-23 15:13:29.
Malaria control in Tanzania currently relies primarily on long-lasting insecticidal nets and indoor residual spraying, alongside effective case management and behaviour change communication. This study explored opinions of key stakeholders on the national progress towards malaria elimination, the potential of currently available vector control interventions in helping achieve elimination by 2030, and the need for alternative interventions that could be used to supplement malaria elimination efforts in Tanzania.
Development and testing of a novel killer–rescue self-limiting gene drive system in Drosophila melanogaster
11214S. H. Webster, M. R. Vella and M. J. Scott, Proceedings of the Royal Society B: Biological Sciences, 287:20192994. 2020-04-15 15:04:49.
Here we report the development and testing of a novel self-limiting gene drive system, Killer–Rescue (K–R), in Drosophila melanogaster. This system is composed of an autoregulated Gal4 Killer (K) and a Gal4-activated Gal80 Rescue (R). Overexpression of Gal4 is lethal, but in the presence of R activation of Gal80 leads to much lower levels of Gal4 and rescue of lethality. We demonstrate that with a single 2 : 1 engineered to wild-type release, K drives R through the population and after nine generations, more than 98% of the population carry R and less than 2% of the population are wild-type flies. We discuss how this simple K–R gene drive system may be readily adapted for population replacement in a human health pest, Aedes aegypti, or for population suppression in an agricultural pest, Drosophila suzukii.
Selfish genes and sexual selection: the impact of genomic parasites on host reproduction
11970N. Wedell, Journal of Zoology, 311:1-12. 2020-04-08 18:18:42.
Selfish genetic elements (SGEs) such as replicating mobile elements, segregation distorters and maternally inherited endosymbionts, bias their transmission success relative to the rest of the genome to increase in representation in subsequent generations. As such, they generate conflict with the rest of the genome. Such intragenomic conflict is also a hallmark of sexually antagonistic (SA) alleles, which are shared genes between the sexes but that have opposing fitness effects when expressed in males and females. However, whilst both SGEs and SA alleles are recognized as common and potent sources of genomic conflict, the realization that SGEs can also generate sexually antagonistic selection and contribute to sexual conflict in addition to generate sexual selection is largely overlooked. Here, I show that SGEs frequently generate sex-specific selection and outline how SGEs that are associated with compromised male fertility can shape female mating patterns, play a key role in the dynamics of sex-determination systems and likely be an important source of sexually antagonistic genetic variation. Given the prevalence of SGEs, their contribution to sexual conflict is likely to be greatly overlooked.
Spatio-temporal controllability and environmental risk assessment of genetically engineered gene drive organisms from the perspective of EU GMO Regulation
11208C. Then, K. Kawall and N. Valenzuela, Integrated Environmental Assessment and Management, 2020-04-06 14:38:52.
Gene drive organisms are a recent development created by using methods of genetic engineering; they inherit genetic constructs that are passed on to future generations with a higher probability than with Mendelian inheritance. There are some specific challenges inherent to the environmental risk assessment (ERA) of genetically engineered (GE) gene drive organisms, since subsequent generations of these GE organisms might show effects that were not observed or intended in the former generations. Unintended effects can emerge from interaction of the gene drive construct with the heterogeneous genetic background of natural populations and/or be triggered by changing environmental conditions. This is especially relevant in case of gene drives with invasive characteristics and typically takes dozens of generations to render the desired effect. Under these circumstances, ‘next generation effects’ can substantially increase the spatial and temporal complexity associated with a high level of uncertainty in ERA. To deal with these problems, we suggest the introduction of a new additional step in the ERA of GE gene drive organisms that takes three criteria into account: the biology of the target organisms, their naturally occurring interactions with the environment (biotic and abiotic) and their intended biological characteristics introduced by genetic engineering. These three criteria are merged to form an additional step in ERA, combining specific ‘knowns’ and integrating areas of 'known unknowns' and uncertainties, with the aim of assessing the spatio‐temporal controllability of GE gene drive organisms. The establishment of assessing spatio‐temporal controllability can be used to define so‐called ‘cut‐off’ criteria in the risk analysis of GE gene drive organisms: if it is likely that GE gene drive organisms escape spatio‐temporal controllability, the risk assessment cannot be sufficiently reliable because it is not conclusive. Under such circumstances, the environmental release of the GE gene drive organisms would not be compatible with the precautionary principle (PP).
Natural gene drives offer potential pathogen control strategies in plants
11206D. M. Gardiner, A. Rusu, L. Barrett, G. C. Hunter and K. Kazan, bioRxiv, 2020-04-06 14:21:48.
Globally, fungal pathogens cause enormous crop losses and current control practices are not always effective, economical or environmentally sustainable. Tools enabling genetic management of wild pathogen populations could potentially solve many problems associated with plant diseases. A natural gene drive from a heterologous species can be used in the globally important cereal pathogen, Fusarium graminearum, to remove pathogenic traits from contained populations of the fungus. The gene drive element became fixed in a freely crossing populations in only three generations. Repeat induce point mutation, a natural genome defence mechanism in fungi, may be useful to recall the gene drive following release, should a failsafe mechanism be required. We propose that gene drive technology is a potential tool to control plant pathogens.
Efficient production of male Wolbachia-infected Aedes aegypti mosquitoes enables large-scale suppression of wild populations
22639J. E. Crawford, D. W. Clarke, V. Criswell, M. Desnoyer, D. Cornel, B. Deegan, K. Gong, K. C. Hopkins, P. Howell, et al., Nature Biotechnology, 38:482-492. 2020-04-06 06:19:43.
The range of the mosquito Aedes aegypti continues to expand, putting more than two billion people at risk of arboviral infection. The sterile insect technique (SIT) has been used to successfully combat agricultural pests at large scale, but not mosquitoes, mainly because of challenges with consistent production and distribution of high-quality male mosquitoes. We describe automated processes to rear and release millions of competitive, sterile male Wolbachia-infected mosquitoes, and use of these males in a large-scale suppression trial in Fresno County, California. In 2018, we released 14.4 million males across three replicate neighborhoods encompassing 293 hectares. At peak mosquito season, the number of female mosquitoes was 95.5% lower (95% CI, 93.6–96.9) in release areas compared to non-release areas, with the most geographically isolated neighborhood reaching a 99% reduction. This work demonstrates the high efficacy of mosquito SIT in an area ninefold larger than in previous similar trials, supporting the potential of this approach in public health and nuisance-mosquito eradication programs.
Engineering multiple species-like genetic incompatibilities in insects
12543M. Maselko, N. Feltman, A. Upadhyay, A. Hayward, S. Das, N. Myslicki, A. J. Peterson, M. B. O’Connor and M. J. Smanski, bioRxiv, 2020-04-05 17:32:23.
Speciation constrains the flow of genetic information between populations of sexually reproducing organisms. Gaining control over mechanisms of speciation would enable new strategies to manage wild populations of disease vectors, agricultural pests, and invasive species. Additionally, such control would provide safe biocontainment of transgenes and gene drives. Natural speciation can be driven by pre-zygotic barriers that prevent fertilization or by post-zygotic genetic incompatibilities that render the hybrid progeny inviable or sterile. Here we demonstrate a general approach to create engineered genetic incompatibilities (EGIs) in the model insect Drosophila melanogaster. Our system couples a dominant lethal transgene with a recessive resistance allele. EGI strains that are homozygous for both elements are fertile and fecund when they mate with similarly engineered strains, but incompatible with wild-type strains that lack resistant alleles. We show that EGI genotypes can be tuned to cause hybrid lethality at different developmental life-stages. Further, we demonstrate that multiple orthogonal EGI strains of D. melanogaster can be engineered to be mutually incompatible with wild-type and with each other. Our approach to create EGI organisms is simple, robust, and functional in multiple sexually reproducing organisms.
Bioethical issues in genome editing by CRISPR-Cas9 technology
11216F. B. Ayanoglu, A. E. Elcin and Y. M. Elcin, Turkish Journal of Biology, 44:110-120. 2020-04-02 15:13:15.
Genome editing technologies have led to fundamental changes in genetic science. Among them, CRISPR-Cas9 technology particularly stands out due to its advantages such as easy handling, high accuracy, and low cost. It has made a quick introduction in fields related to humans, animals, and the environment, while raising difficult questions, applications, concerns, and bioethical issues to be discussed. Most concerns stem from the use of CRISPR-Cas9 to genetically alter human germline cells and embryos (called germline genome editing). Germline genome editing leads to serial bioethical issues, such as the occurrence of undesirable changes in the genome, from whom and how informed consent is obtained, and the breeding of the human species (eugenics). However, the bioethical issues that CRISPR-Cas9 technology could cause in the environment, agriculture and livestock should also not be forgotten. In order for CRISPR-Cas9 to be used safely in all areas and to solve potential issues, worldwide legislation should be prepared, taking into account the opinions of both life and social scientists, policy makers, and all other stakeholders of the sectors, and CRISPR-Cas9 applications should be implemented according to such legislations. However, these controls should not restrict scientific freedom. Here, various applications of CRISPR-Cas9 technology, especially in medicine and agriculture, are described and ethical issues related to genome editing using CRISPR-Cas9 technology are discussed. The social and bioethical concerns in relation to human beings, other organisms, and the environment are addressed.
Auditing preparedness for vector control field studies
11210C. M. Collins and M. M. Quinlan, American Journal of Tropical Medicine and Hygiene, 102:707-710. 2020-04-01 14:52:00.
The value of baseline entomological data to any future area-wide release campaign relies on the application of consistent methods to produce results comparable across different times and places in a stepwise progression to larger releases. Traditionally, standard operating procedures (SOPs) and operational plans support this consistency and, thus, the validity of emergent data. When release plans include transgenic mosquitoes for vector control or other novel beneficial insects, additional factors come into play such as biosafety permits, stakeholder acceptance, and ethics approval, which require even greater coordination and thoroughness. An audit approach was developed to verify the correct use of SOPs and appropriate performance of tasks during mosquito mark, release, recapture (MRR) studies. Audit questions matched SOPs, permit terms and conditions, and other key criteria, and can be used to support subsequent “spot check” verification by field teams. An external team of auditors, however, was found to be effective for initial checks in this example before the use of a transgenic strain of laboratory mosquitoes. We recommend similar approaches for field studies using release of novel beneficial insects, to ensure useful and valid data as an outcome and to support confidence in the rigor of the step-wise process.
Engineering a minimal gene drive system for integral replacement in Drosophila melanogaster
25648A. Nash, Imperial College London, 2020-04-01 07:48:50.
Gene drives represent a powerful tool for the control of vector-borne diseases. By suppressing or replacing vector populations, laboratory studies have highlighted the potential for this group of tools to make a powerful impact on the burden of zoonotic disease. Current genetic drive systems have a number of limitations, namely their complexity, susceptibility to genetic resistance, and a high regulatory threshold. Here we suggest a novel design paradigm for the creation of replacement gene drives, which we have termed ‘Integral Replacement’. By splitting drive constructs, and integrating components into endogenous loci, we have aimed to engineer a minimal drive system, with low fitness cost, higher resilience to resistance alleles, and with greater flexibility for field testing. In so doing we have generated a model that illustrates increases in efficacy versus existing drive systems, and expanded on work performed on intronic gRNA cassettes. We subsequently were able to build prototype Integral Gene Drive (IGD) components, and demonstrate their efficacy using the model organism Drosophila melanogaster, providing evidence for an initial proof-of-principle for this novel design paradigm.
Development of control and sterilization technology for bluegill by genome editing
20100M. A. Madsen, Nippon Suisan Gakkaishi, 86:100-100. 2020-03-31 09:55:36.
This article is in Japanese
Male-biased adult production of the striped fruit fly, Zeugodacus scutellata, by feeding dsRNA fpecific to Transformer-2
11271M. A. Al Baki, M. Vatanparast and Y. Kim, Insects, 11:211. 2020-03-28 19:17:02.
Sterile insect release technique (SIT) is effective for eradicating quarantine insects including various tephritid fruit flies. When SIT is used for fruit flies, it is challenging to remove females from sterile males due to oviposition-associated piercing damage. This study developed a sex transition technique by feeding double-stranded RNA (dsRNA) specific to a sex-determining gene, Transformer-2 (Zs-Tra2) of the striped fruit fly, Zeugodacus scutellata. Zs-Tra2 is homologous to other fruit fly orthologs. It is highly expressed in female adults. RNA interference (RNAi) of Zs-Tra2 by injecting or feeding its specific dsRNA to larvae significantly increased male ratio. Recombinant Escherichia coli cells expressing dsRNA specific to Zs-Tra2 were prepared and used to feed larvae to suppress Zs-Tra2 gene expression levels. When these recombinant bacteria were fed to larvae during the entire feeding stage, the test population was significantly male-biased. Some females treated with such recombinant E. coli exhibited mosaic morphological characters such as the presence of male-specific abdominal setae in females. This study proposes a novel technique by feeding dsRNA specific to Transformer-2 to reduce female production during mass-rearing of tephritid males for SIT.
The value of existing regulatory frameworks for the environmental risk assessment of agricultural pest control using gene drive
8194J. Romeis, J. Collatz, D. C. M. Glandorf and M. B. Bonsall, Environmental Science & Policy, 108:19-36. 2020-03-27 20:20:04.
The application of (synthetic) gene drives is a powerful tool to control populations of insects that are agricultural pests, vectors of diseases, or a threat to biodiversity potentially leading to the local or global eradication of a species. The potential use of gene drive organisms has triggered a heated discussion regarding their environmental impacts and regulatory oversight. However, experience exists in assessing the environmental impacts of a number of established agricultural pest control methods that require the release of living organisms, that provide high levels of area-wide control and that might be irreversible. This includes classical biological control, the sterile insect technique, the incompatible insect technique that is based on the cytoplasmic incompatibility caused by Wolbachia endosymbionts, and genetically modified insects containing self-limiting traits. The different technologies are described, the regulatory practice and experience is summarized and pathways through which these control technologies could harm valued ecosystem services are presented. With a focus on the application of gene drives in agriculture, using the invasive Drosophila suzukii (Diptera: Drosophilidae) as a case study we then discuss to what extent the existing frameworks could assist the risk assessment of insects carrying gene drives. We suggest that drawing on existing practices, experiences and legislative frameworks will provide a pragmatic and proportionate approach to evaluate the environmental risks of novel solutions based on gene drive technologies.
Editorial Expression of Concern: Transgenic Aedes aegypti Mosquitoes Transfer Genes into a Natural Population
16199B. R. Evans, P. Kotsakiozi, A. L. Costa-Da-Silva, R. S. Ioshino, L. Garziera, M. C. Pedrosa, A. Malavasi, J. F. Virginio, M. L. Capurro and J. R. Powell, Scientific Reports, 10:2. 2020-03-24 15:17:41.
Shortly after publication of this Article in September 2019, the Editors were alerted to concerns regarding the interpretation of the data and some of the conclusions.
Experimental manipulation of selfish genetic elements links genes to microbial community function
11218S. D. Quistad, G. Doulcier and P. B. Rainey, Philosophical Transactions of the Royal Society B-Biological Sciences, 375:12. 2020-03-23 15:17:13.
Microbial communities underpin the Earth's biological and geochemical processes, but their complexity hampers understanding. Motivated by the challenge of diversity and the need to forge ways of capturing dynamical behaviour connecting genes to function, biologically independent experimental communities comprising hundreds of microbial genera were established from garden compost and propagated on nitrogen-limited minimal medium with cellulose (paper) as sole carbon source. After 1 year of bi-weekly transfer, communities retained hundreds of genera. To connect genes to function, we used a simple experimental manipulation that involved the periodic collection of selfish genetic elements (SGEs) from separate communities, followed by pooling and redistribution across communities. The treatment was predicted to promote amplification and dissemination of SGEs and thus horizontal gene transfer. Confirmation came from comparative metagenomics, which showed the substantive movement of ecologically significant genes whose dynamic across space and time could be followed. Enrichment of genes implicated in nitrogen metabolism, and particularly ammonification, prompted biochemical assays that revealed a measurable impact on community function. Our simple experimental strategy offers a conceptually new approach for unravelling dynamical processes affecting microbial community function. This article is part of the theme issue 'Conceptual challenges in microbial community ecology'.
Can a population targeted by a CRISPR-based homing gene drive be rescued?
11269N. O. Rode, V. Courtier-Orgogozo and F. Débarre, bioRxiv, 2020.03.17.995829. 2020-03-20 19:12:23.
CRISPR-based homing gene drive is a genetic control technique aiming to modify or eradicate natural populations through the release of individuals carrying an engineered piece of DNA that can be inherited by all their progeny. Developing countermeasures is important to control the spread of gene drives, should they result in unanticipated damages. One proposed countermeasure is the introduction of individuals carrying a brake construct that targets and inactivates the drive allele but leaves the wild-type allele unaffected. Here we develop models to investigate the efficiency of such brakes. We consider a variable population size and use a combination of analytical and numerical methods to determine the conditions where a brake can prevent the extinction of a population targeted by an eradication drive. We find that a brake is not guaranteed to prevent eradication and that characteristics of both the brake and the drive affect the likelihood of recovering the wild-type population. In particular, brakes that restore fitness are more efficient than brakes that do not. Our model also suggests that threshold-dependent drives (drives that can spread only when introduced above a threshold) are more amenable to control with a brake than drives that can spread from an arbitrary low introduction frequency (threshold-independent drives). Based on our results, we provide practical recommendations and discuss safety issues.Article summary for Issue Highlights Homing gene drive is a new genetic control technology that aims to spread a genetically engineered DNA construct within natural populations even when it impairs fitness. In case of unanticipated damages, it has been proposed to stop homing gene drives by releasing individuals carrying a gene-drive brake; however, the efficiency of such brakes has been little studied. The authors develop a model to investigate the dynamics of a population targeted by a homing drive in absence or in presence of brake. The model provides insights for the design of more efficient brakes and safer gene drives.CRISPRClustered Regularly Interspaced Short Palindromic Repeats
Simulating effects of fitness and dispersal on the use of Trojan sex chromosomes for the management of invasive species
11488C. C. Day, E. L. Landguth, R. K. Simmons, W. P. Baker, A. R. Whiteley, P. M. Lukacs and A. Bearlin, Journal of Applied Ecology, 2020-03-19 20:15:27.
The use of Trojan Y chromosomes (TYC) for controlling invasive species involves manipulating the sex chromosomes of captive-raised individuals. Following release, the offspring of these individuals consist of only one sex, thereby skewing the sex ratio of the invasive population and potentially leading to eradication. Simulation models are needed that can inform managers about how to maximize the likelihood of invasive species eradication, since implementation of this novel management approach in the field is still rare. Here, we present the first spatially explicit, mechanistic simulation model of a real-world TYC program for invasive species eradication. Using a brook trout Salvelinus fontinalis system model, we investigated the effects of competitive and reproductive fitness of the captive-raised YY males, dispersal behaviour upon their release and landscape heterogeneity on eradication success. Likelihood of eradication was dependent on both the competitive and reproductive fitness of the Trojan individuals. Competitive fitness (i.e. survival) had a higher threshold for eradication, below which the invasive populations were not eradicated. Movement ecology of both the wild and YY male populations was important for eradication. Under a restricted dispersal scenario for YY males following their release, the wild population was not extirpated but maintained a stable, yet reduced, population size. Analysis of landscape configuration indicated that time to eradication of local patches increased with greater connectivity within the stream network. In addition to sex ratio distortion, density-dependent mortality resulting from outplantings made an important contribution to population decline and therefore may also affect native competitors. Synthesis and applications. The use of Trojan sex chromosomes to skew population sex ratios is a novel method for the suppression and eradication of aquatic invasive species. Results from our modelling work indicate that while eradication is possible, maximizing its likelihood requires an understanding of the fitness and movement ecology of both the wild and YY male populations of the invasive species. Both our model and the principles derived from this study related to fitness and behavioural landscape ecology can be broadly applied to other invaded species and systems.
Computational and experimental performance of CRISPR homing gene drive strategies with multiplexed gRNAs
7962S. E. Champer, S. Y. Oh, C. Liu, Z. Wen, A. G. Clark, P. W. Messer and J. Champer, Science Advances, 6:eaaz0525. 2020-03-16 14:22:56.
The rapid evolution of resistance alleles poses a major obstacle for genetic manipulation of populations with CRISPR homing gene drives. One proposed solution is using multiple guide RNAs (gRNAs), allowing a drive to function even if some resistant target sites are present. Here, we develop a model of homing mechanisms parameterized by experimental studies. Our model incorporates several factors affecting drives with multiple gRNAs, including timing of cleavage, reduction in homology-directed repair efficiency due to imperfect homology, Cas9 activity saturation, gRNA activity level variance, and incomplete homology-directed repair. We find that homing drives have an optimal number of gRNAs, usually between two and eight, depending on the specific drive type and performance parameters. These results contradict the notion that resistance rates can be reduced to arbitrarily low levels by gRNA multiplexing and highlight the need for combined approaches to counter resistance evolution in CRISPR homing drives.
Anti-CRISPR protein applications: natural brakes for CRISPR-Cas technologies
11201Marino, N. D., Pinilla-Redondo, R. , Csorgo, B., Bondy-Denomy, J., Nature Methods, 2020-03-16 13:52:42.
Clustered, regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) genes, a diverse family of prokaryotic adaptive immune systems, have emerged as a biotechnological tool and therapeutic. The discovery of protein inhibitors of CRISPR-Cas systems, called anti-CRISPR (Acr) proteins, enables the development of more controllable and precise CRISPR-Cas tools. Here we discuss applications of Acr proteins for post-translational control of CRISPR-Cas systems in prokaryotic and mammalian cells, organisms and ecosystems. This Review highlights recent discoveries and applications of anti-CRISPR (Acr) proteins that enable the regulation of CRISPR-Cas technology.
Abundance of conserved CRISPR-Cas9 target sites within the highly polymorphic genomes of Anopheles and Aedes mosquitoes
14309H. Schmidt, T. C. Collier, M. J. Hanemaaijer, P. D. Houston, Y. Lee and G. C. Lanzaro, Nature Communications, 11. 2020-03-16 12:39:04.
ere we report the results of a survey of 1280 genomes of the mosquitoes Anopheles gambiae, An. coluzzii, and Aedes aegypti in which we determine that similar to 90% of all protein-encoding CGD target genes in natural populations include at least one target site with no DRAs at a frequency of >= 1.0%.
A fly model establishes distinct mechanisms for synthetic CRISPR/Cas9 sex distorters
7973B. Fasulo, A. Meccariello, M. Morgan, C. Borufka, P. A. Papathanos and N. Windbichler, PLOS Genetics, 16:e1008647. 2020-03-13 15:02:19.
Author summary Harmful insect populations can be eliminated for a lack of females if they are made to produce mostly male offspring. There are genes that occur naturally that make males produce mostly sons and, although we don’t know exactly how they work, this appears to coincide with damage to the X-chromosome during the production of sperm. Recently, we showed in a mosquito species that such sex-biasing genes could also be constructed artificially from first principles. To better understand if this works in other species too, we designed and built male-biasing genes of two types in the fruit fly and determined what is needed to for a shift towards males. We show how different ways of cutting the X-chromosome DNA at different times with CRISPR, results in distinct outcomes and started to ask what cellular processes are involved in this. These models will help us to design such genes for the control of insect species that transmit disease or threaten crops.
Performance analysis of novel toxin-antidote CRISPR gene drive systems
7969J. Champer, I. K. Kim, S. E. Champer, A. G. Clark and P. W. Messer, BMC Biology, 18:27. 2020-03-12 14:48:01.
CRISPR gene drive systems allow the rapid spread of a genetic construct throughout a population. Such systems promise novel strategies for the management of vector-borne diseases and invasive species by suppressing a target population or modifying it with a desired trait. However, current homing-type drives have two potential shortcomings. First, they can be thwarted by the rapid evolution of resistance. Second, they lack any mechanism for confinement to a specific target population. In this study, we conduct a comprehensive performance assessment of several new types of CRISPR-based gene drive systems employing toxin-antidote (TA) principles, which should be less prone to resistance and allow for the confinement of drives to a target population due to invasion frequency thresholds.
Stakeholder workshop “Problem formulation for the environmental risk assessment of gene drive modified insects” (15 May 2019, Brussels)
8192European Food Safety, A., Devos, Y., Gallani, B. & Firbank, L. G, A. European Food Safety, Y. Devos, B. Gallani and L. G. Firbank, 17:1819E. 2020-03-09 20:12:26.
Recent advances in molecular and synthetic biology are enabling the engineering of gene drives that spread genes of interest through interbreeding populations at a frequency greater than the rate expected by simple Mendelian inheritance. At present, insects represent the most likely cases of gene drive modified organisms for deliberate release into the environment. Through an open workshop, the European Food Safety Authority (EFSA) aimed to engage with stakeholders to discuss potential environmental risks associated with the deliberate release into the environment of gene drive modified insects. Workshop participants were invited to contribute to an example problem formulation to: (1) identify relevant broad protection goals and make them operational for use in environmental risk assessment; (2) formally devise examples of plausible pathways to harm that describe how the deployment of gene drive modified insects could be harmful; (3) formulate example risk hypotheses about the likelihood and severity of such events; (4) identify possible information that would be useful to test these risk hypotheses; and (5) identify how to acquire new data for hypothesis testing when existing information is deemed insufficient for regulatory decision-making. The problem formulation exercise was run for two hypothetical case studies (i.e. self-sustaining low threshold gene drives to control disease-spreading mosquitoes (Aedes albopictus, the Asian tiger mosquito) and agricultural pests (Drosophila suzukii, the spotted-wing Drosophila)). Points raised by the workshop participants reveal different often contrasting opinions/perspectives toward gene drive and their risk assessment. Overall, there was agreement that the problem formulation process is fit-for-purpose for the environmental risk assessment of gene drive modified insects, but it was acknowledged that practical challenges may be encountered. Points raised by the workshop participants, on defining protection goals, formulating specific pathways to harm and on structuring risks, have been considered by EFSA's Panel on genetically modified organisms during its deliberations.
Toward the definition of efficacy and safety criteria for advancing gene drive-modified mosquitoes to field testing
7971S. L. James, J. M. Marshall, G. K. Christophides, F. O. Okumu and T. Nolan, Vector Borne and Zoonotic Diseases, 20:237. 2020-03-09 14:54:00.
Mosquitoes containing gene drive systems are being developed as complementary tools to prevent transmission of malaria and other mosquito-borne diseases. As with any new tool, decision makers and other stakeholders will need to balance risks (safety) and benefits (efficacy) when considering the rationale for testing and deploying gene drive-modified mosquito products. Developers will benefit from standards for judging whether an investigational gene drive product meets acceptability criteria for advancing to field trials. Such standards may be formalized as preferred product characteristics and target product profiles, which describe the desired attributes of the product category and of a particular product, respectively. This report summarizes discussions from two scientific workshops aimed at identifying efficacy and safety characteristics that must be minimally met for an investigational gene drive-modified mosquito product to be deemed viable to move from contained testing to field release and the data that will be needed to support an application for first field release.
Population-level multiplexing: A promising strategy to manage the evolution of resistance against gene drives targeting a neutral locus
11212M. P. Edgington, T. Harvey-Samuel and L. Alphey, Evolutionary Applications, 10. 2020-03-06 14:52:40.
CRISPR-based gene drives bias inheritance in their favour by inducing double-stranded breaks (DSBs) at wild-type homologous loci and using the drive transgene as a repair template-converting drive heterozygotes into homozygotes. Recent studies have shown that alternate end-joining repair mechanisms produce cut-resistant alleles that rapidly induce drive failure. Multiplexing-simultaneously targeting multiple sites at the wild-type locus-is commonly assumed to overcome this issue since resistance would need to develop at all target sites for the system to fail. This may work for some population suppression drives targeting essential (e.g. viability or fertility) genes if careful design can ensure cut-resistant alleles themselves have low fitness. However, here, models are used to demonstrate that this approach will be ineffective when targeting neutral loci. We then go on to compare the performance of four alternative population-level multiplexing approaches with standard individual-level multiplexing. Two of these approaches have mechanisms preventing them from becoming linked, thus avoiding multiple simultaneous DSBs and giving a large improvement. Releasing multiple unlinked drives gives a modest improvement, while releasing multiple drives that may become linked over time produces a decrease in performance under the conditions tested here. Based on performance and technical feasibility, we then take one approach forward for further investigation, demonstrating its robustness to different performance parameters and its potential for controlling very large target populations.
A unifying approach to gene drive
7957P. Verma, R. Reeves and C. S. Gokhale, bioRxiv, 2020.02.28.970103. 2020-03-03 13:53:12.
Synthetic gene drive technologies aim to spread transgenic constructs into wild populations even when they impose organismal fitness disadvantages. The properties of gene drive constructs are diverse and depend on their molecular construction, and differential selection pressure they impose in the varied ecological situations they encounter. The extraordinary diversity of conceivable drive mechanisms and the range of selective parameters they may encounter makes it very difficult to convey their relative predicted properties. The sheer number of published manuscripts in this field, experimental and theoretical, is a testament to the possibilities presented by this technology. We evaluate and condense the essential synthetic drive mechanisms from a variety of studies and present a unified mathematical paradigm (and a user-friendly tool DrMxR - Drive Mixer) describing the properties of a wide variety of single construct gene drives (non-suppression). Within this common framework, we have been able to recapitulate key published results derived using bespoke modelling frameworks. Because a unified framework is employed, it is also possible to seamlessly explore the consequences of combining multiple drive approaches within a single construct. We provide a method for analytically assessing the measure of invasiveness of a drive construct. As opposed to typical studies of synthetic drives, we explore the resilience of such drives in a spatially explicit manner advancing the connection between realistic spatial dynamics and typical well-mixed populations. Besides a scientific advance, our results and the tools provided an intuitive and objective way for regulators, scientists and NGOs to evaluate the properties and robustness of proposed and future gene drive approaches.
Public health concerns over gene-drive mosquitoes: will future use of gene-drive snails for schistosomiasis control gain increased level of community acceptance?
7950D. O. Famakinde, Pathogens and Global Health, 2020-02-26 20:49:43.
With the advent of CRISPR (clustered regularly interspaced short palindromic repeat)-based gene drive, present genetic research in schistosomiasis vector control envisages the breeding and release of transgenic schistosome-resistant (TSR) snail vectors to curb the spread of the disease. Although this approach is still in its infancy, studies focussing on production of genetically modified (GM) mosquitoes (including gene-drive mosquitoes) are well advanced and set the pace for other transgenic vector research. Unfortunately, as with other GM mosquitoes, open field release of gene-drive mosquitoes is currently challenged in part by some concerns such as gene drive failure and increased transmission potential for other mosquito-borne diseases among others, which might have adverse effects on human wellbeing. Therefore, not only should we learn from the GM mosquito protocols, frameworks and guidelines but also appraise the applicability of its current hurdles to other transgenic vector systems, such as the TSR snail approach. Placing these issues in a coherent comparative perspective, I argue that although the use of TSR snails may face similar technical, democratic and diplomatic challenges, some of the concerns over gene-drive mosquitoes may not apply to gene-drive snails, proposing a theory that community consent will be no harder and possibly easier to obtain for TSR snails than the experience with GM mosquitoes. In the future, these observations may help public health practitioners and policy makers in effective communication with communities on issues regarding the use of TSR snails to interrupt schistosomiasis transmission, especially in sub-Saharan Africa.
Gene Drives: New and Improved
7945R. M. Friedman, J. M. Marshall and O. S. Akbari, Issues in Science and Technology, 36:1-7. 2020-02-18 20:39:12.
Our goal here is to describe the various options under development in nontechnical terms for a policy-making audience, review how far along each is, and examine the broader context of how this new suite of technologies compares with other available alternatives. Early engagement by the policy and regulatory communities will be of great help to product developers, and thus ultimately to society through better and more appropriate products. Our hope is that this improved understanding of what is in the development pipeline will enhance the dialogue between the policy community and developers.
Genome engineering in insects: focus on the CRISPR/Cas9 system
19405Hillary, V. Edwin Ceasar, Stanislaus Antony Ignacimuthu, S., Genome Engineering via CRISPR-Cas9 System, 2020-02-18 18:07:54.
Genome engineering is a precise tool used to alter the genome of desired organism. Zinc finger nuclease (ZFN), transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats (CRISPR), and the CRISPR-associated RNA guided endonuclease Cas9 (CRISPR/Cas9) are the major genome engineering tools used in these days. CRISPR/Cas9 system has redeemed the precise genome engineering in different species including insects. This chapter covers the details on genome engineering studies reported in various insects including mosquitoes, butterflies, silkworm and fruit fly with a focus on CRISPR/Cas9 system. Many studies have been reported on application of ZFN, TALEN and CRISPR/Cas9 in insects. In recent years, many scientists have adopted CRISPR/Cas9 system for insect genome modification due to its affordability and quick designing of the constructs. We also discuss the details and applications of gene drive. Further studies with CRISPR/Cas9in insects will help researchers to find an effective strategy to combat the vector borne diseases spread by insects like mosquitoes.
Modeling the impacts of a simple meiotic gene drive on small, homeostatic populations
7956K. R. Pilkiewicz and M. L. Mayo, Physical Review E, 101:11. 2020-02-18 13:43:10.
Gene drives offer unprecedented control over the fate of natural ecosystems by leveraging non-Mendelian inheritance mechanisms to proliferate synthetic genes across wild populations. However, these benefits are offset by a need to avoid the potentially disastrous consequences of unintended ecological interactions. The efficacy of many gene-editing drives has been brought into question due to predictions that they will inevitably be thwarted by the emergence of drive-resistant mutations, but these predictions derive largely from models of large or infinite populations that cannot be driven to extinction faster than mutations can fixate. To address this issue, we characterize the impact of a simple, meiotic gene drive on a small, homeostatic population whose genotypic composition may vary due to the stochasticity inherent in natural mating events (e.g., partner choice, number of offspring) or the genetic inheritance process (e.g., mutation rate, gene drive fitness). To determine whether the ultimate genotypic fate of such a population is sensitive to such stochastic fluctuations, we compare the results of two dynamical models: a deterministic model that attempts to predict how the genetics of an average population evolve over successive generations, and an agent-based model that examines how stable these predictions are to fluctuations. We find that, even on average, our stochastic model makes qualitatively distinct predictions from those of the deterministic model, and we identify the source of these discrepancies as a dynamic instability that arises at short times, when genetic diversity is maximized as a consequence of the gene drive's rapid proliferation. While we ultimately conclude that extinction can only beat out the fixation of drive-resistant mutations over a limited region of parameter space, the reason for this is more complex than previously understood, which could open new avenues for engineered gene drives to circumvent this weakness.
Dramatically diverse Schizosaccharomyces pombe wtf meiotic drivers all display high gamete-killing efficiency
7943M. A. Bravo Núñez, I. M. Sabbarini, M. T. Eickbush, Y. Liang, J. J. Lange, A. M. Kent and S. E. Zanders, PLOS Genetics, 16:e1008350. 2020-02-07 20:35:38.
During gametogenesis, the two gene copies at a given locus, known as alleles, are each transmitted to 50% of the gametes (e.g. sperm). However, some alleles cheat so that they are found in more than the expected 50% of gametes, often at the expense of fertility. This selfish behavior is known as meiotic drive. Some members of the wtf gene family in the fission yeast Schizosaccharomyces pombe kill the gametes (spores) that do not inherit them, resulting in meiotic drive favoring the wtf allele. Other wtf genes act as suppressors of drive. However, the wtf gene family is diverse and only a small subset of the genes has been characterized. Here we analyze the functions of other members of this gene family and found eight new drivers as well as three new suppressors of drive. Surprisingly, we find that drive is relatively insensitive to changes in wtf gene sequence as highly diverged wtf genes execute gamete killing with similar efficiency. Finally, we also find that the expression and localization of some Wtf proteins are distinct from those of known drivers and suppressors, suggesting that these proteins may have non-meiotic drive functions.
Progress towards engineering gene drives for population control
7937R. R. Raban, J. M. Marshall and O. S. Akbari, The Journal of Experimental Biology, 223:jeb208181. 2020-02-07 20:21:50.
Vector-borne diseases, such as dengue, Zika and malaria, are a major cause of morbidity and mortality worldwide. These diseases have proven difficult to control and currently available management tools are insufficient to eliminate them in many regions. Gene drives have the potential to revolutionize vector-borne disease control. This suite of technologies has advanced rapidly in recent years as a result of the availability of new, more efficient gene editing technologies. Gene drives can favorably bias the inheritance of a linked disease-refractory gene, which could possibly be exploited (i) to generate a vector population incapable of transmitting disease or (ii) to disrupt an essential gene for viability or fertility, which could eventually eliminate a population. Importantly, gene drives vary in characteristics such as their transmission efficiency, confinability and reversibility, and their potential to develop resistance to the drive mechanism. Here, we discuss recent advancements in the gene drive field, and contrast the benefits and limitations of a variety of technologies, as well as approaches to overcome these limitations. We also discuss the current state of each gene drive technology and the technical considerations that need to be addressed on the pathway to field implementation. While there are still many obstacles to overcome, recent progress has brought us closer than ever before to genetic-based vector modification as a tool to support vector-borne disease elimination efforts worldwide.
Vector genetics, insecticide resistance and gene drives: an agent-based modeling approach to evaluate malaria transmission and elimination
7941P. Selvaraj, E. A. Wenger, D. Bridenbecker, N. Windbichler, J. R. Russell, J. Gerardin, C. A. Bever and M. Nikolov, bioRxiv, 2020.01.27.920421. 2020-02-06 20:32:18.
Vector control has been a key component in the fight against malaria for decades, and chemical insecticides are critical to the success of vector control programs worldwide. However, increasing resistance to insecticides threatens to undermine these efforts. Understanding the evolution and propagation of resistance is thus imperative to mitigating loss of intervention effectiveness. Additionally, accelerated research and development of new tools that can be deployed alongside existing vector control strategies is key to eradicating malaria in the near future. Methods such as gene drives that aim to genetically modify large mosquito populations in the wild to either render them refractory to malaria or impair their reproduction may prove invaluable tools. Mathematical models of gene flow in populations can offer invaluable insight into the behavior and potential impact of gene drives as well as the spread of insecticide resistance in the wild. Here, we present the first multi-locus, agent-based model of vector genetics that accounts for mutations and many-to-many mappings of genotypes to phenotypes to investigate gene flow and the propagation of gene drives in Anopheline populations. This model is embedded within a large scale individual-based model of malaria transmission representative of a high burden, high transmission setting characteristic of the Sahel. Results are presented for the selection of insecticide-resistant vectors and the spread of resistance through repeated deployment of insecticide treated nets (ITNs), in addition to scenarios where gene drives act in concert with existing vector control tools such as ITNs. The roles of seasonality, spatial distribution of vector habitat and feed sites, and existing vector control in propagating alleles that confer phenotypic traits via gene drives that result in reduced transmission are explored. The ability to model a spectrum of vector species with different genotypes and phenotypes in the context of malaria transmission allows us to test deployment strategies for existing interventions that reduce the deleterious effects of resistance and allows exploration of the impact of new tools being proposed or developed.Author summary Vector control interventions are essential to the success of global malaria control and elimination efforts but increasing insecticide resistance worldwide threatens to derail these efforts. Releasing genetically modified mosquitoes that use gene drives to pass on desired genes and their associated phenotypic traits to the entire population within a few generations has been proposed to address resistance and other issues such as transmission heterogeneity that can sustain malaria transmission indefinitely. While the ethics and safety of these methods are being debated, mathematical models offer an efficient way of predicting the behavior and estimating the efficacy of these interventions if deployed to specific regions facing challenges to reaching elimination. We have developed a detailed mathematical model of vector genetics where specific genomes code for physical attributes that influence transmission and are affected by the surrounding environment. This is the first model to incorporate an individual-based multi locus genetic model into a detailed individual-based model of malaria transmission. This model opens the door to investigate a number of subtle but important questions such as the effects of small numbers of mosquitoes in a region sustaining malaria transmission during the low transmission season, and the success of gene drives in regions where extant vector control interventions could kill off gene drive mosquitoes before establishment. Here, we investigate the reduced efficacy of current vector control measures in the presence of insecticide resistance and evaluate the likelihood of achieving local malaria elimination using gene drive mosquitoes released into a high transmission setting alongside other vector control measures.
Phased Conditional Approach for Mosquito Management Using Sterile Insect Technique
7351J. Bouyer, H. Yamada, R. Pereira, K. Bourtzis and M. J. B. Vreysen, Trends in Parasitology, 2020-02-05 21:36:11.
Mosquito-borne diseases represent a major threat to humankind. Recently, the incidence of malaria has stopped decreasing while that of dengue is increasing exponentially. Alternative mosquito-control methods are urgently needed. The sterile insect technique (SIT) has seen significant developments recently and may play an important role. However, testing and implementing SIT for vector control is challenging, and a phased conditional approach (PCA) is recommended, that is, advancement to the next phase depends on completion of activities in the previous one. We herewith present a PCA to test the SIT against mosquitoes within an area-wide-integrated pest-management programme, taking into account the experience gained with plant and livestock pests and the recent developments of the technique against mosquitoes.
Genetic Variation and Potential for Resistance Development to the tTA Overexpression Lethal System in Insects
7939K. E. Knudsen, W. R. Reid, T. M. Barbour, L. M. Bowes, J. Duncan, E. Philpott, S. Potter and M. J. Scott, G3: Genes|Genomes|Genetics, Early Online:g3.400990.2020. 2020-02-05 20:27:28.
Release of insect pests carrying the dominant lethal tetracycline transactivator (tTA) overexpression system has been proposed as a means for population suppression. High levels of the tTA transcription factor are thought to be toxic due to either transcriptional squelching or interference with protein ubiquitination. Here we utilized the Drosophila melanogaster Genetic Reference Panel (DGRP) to examine the influence of genetic variation on the efficacy of a female-specific tTA overexpression system. The level of female lethality between DGRP lines varied from 11 to 97% with a broad sense heritability of 0.89. A genome-wide association analysis identified 192 allelic variants associated with high or low lethality (p<10-5), although none were significant when corrected for multiple testing. 151 of the variants fell within 108 genes that were associated with several biological processes including transcription and protein ubiquitination. In four lines with high female lethality, tTA RNA levels were similar or higher than in the parental tTA overexpression strain. In two lines with low lethality, tTA levels were 2-fold lower than in the parental strain. However, in two other lines with low lethality, tTA levels were only 0-30% lower. RNAseq analysis identified genes that were up or downregulated in the low female lethal lines compared to the four high lethal lines. For example, genes associated with RNA processing and rRNA maturation were significantly upregulated in low lethal lines. Our data suggest that standing genetic variation in an insect population could provide multiple mechanisms for resistance to the tTA overexpression system.
Engineered symbionts activate honey bee immunity and limit pathogens
19387P. Leonard Sean, J. E. Powell, J. Perutka, P. Geng, C. Heckmann Luke, D. Horak Richard, W. Davies Bryan, D. Ellington Andrew, E. Barrick Jeffrey and A. Moran Nancy, Science, 367:573-576. 2020-01-31 17:14:03.
Honey bees are essential pollinators threatened by colony losses linked to the spread of parasites and pathogens. Here, we report a new approach for manipulating bee gene expression and protecting bee health. We engineered a symbiotic bee gut bacterium, Snodgrassella alvi, to induce eukaryotic RNA interference (RNAi) immune responses. We show that engineered S. alvi can stably recolonize bees and produce double-stranded RNA to activate RNAi and repress host gene expression, thereby altering bee physiology, behavior, and growth. We used this approach to improve bee survival after a viral challenge, and we show that engineered S. alvi can kill parasitic Varroa mites by triggering the mite RNAi response. This symbiont-mediated RNAi approach is a tool for studying bee functional genomics and potentially for safeguarding bee health.
Gene technologies in weed management: a technical feasibility analysis
7948N. Kumaran, A. Choudhary, M. Legros, A. W. Sheppard, L. G. Barrett, D. M. Gardiner and S. Raghu, Current Opinion in Insect Science, 38:6-14. 2020-01-30 20:44:15.
With the advent of new genetic technologies such as gene silencing and gene drive, efforts to develop additional management tools for weed management is gaining significant momentum. These technologies promise novel ways to develop sustainable weed control options because gene silencing can switch-off genes mediating adaptation (e.g. growth, herbicide resistance), and gene drive can be used to spread modified traits and to engineer wild populations with reduced fitness. However, applying gene silencing and/or gene drive is expected to be inherently complex as their application is constrained by several methodological and technological difficulties. In this review we explore the challenges of these technologies, and discuss strategies and resources accessible to accelerate the development of gene-tech based tools for weed management. We also highlight how gene technologies can be integrated into existing management tactics such as classical biological control, and their possible interactions.
