Population-level effects of fitness costs associated with repressible female-lethal transgene insertions in two pest insects

Population suppression by release of insects carrying a dominant sterile homing gene drive targeting doublesex in Drosophila

CRISPR homing gene drives can suppress pest populations by targeting female fertility genes, converting wild-type alleles into drive alleles in the germline of drive heterozygotes. fsRIDL (female-specific Release of Insects carrying a Dominant Lethal) is a self-limiting population suppression strategy involving continual release of transgenic males carrying female lethal alleles. Here, we propose an improved pest suppression system called "Release of Insects carrying a Dominant-sterile Drive" (RIDD), combining performance characteristics of homing drive and fsRIDL. We construct a split RIDD system in Drosophila melanogaster by creating a 3-gRNA drive disrupting the doublesex female exon. Drive alleles bias their inheritance in males, while drive alleles and resistance alleles formed by end-joining cause dominant female sterility. Weekly releases of RIDD males progressively suppressed and eventually eliminated cage populations. Modeling shows that RIDD is substantially stronger than SIT and fsRIDL. RIDD is also self-limiting, potentially allowing targeted population suppression.

New Insights into the Plutella xylostella Detoxifying Enzymes: Sequence Evolution, Structural Similarity, Functional Diversity, and Application Prospects of Glucosinolate Sulfatases

Brassica plants have glucosinolate (GLs)-myrosinase defense mechanisms to deter herbivores. However, Plutella xylostella specifically feeds on Brassica vegetables. The larvae possess three glucosinolate sulfatases (PxGSS1-3) that compete with plant myrosinase for shared GLs substrates and produce nontoxic desulfo-GLs (deGLs). Although PxGSSs are considered potential targets for pest control, the lack of a comprehensive review has hindered the development of PxGSSs-targeted pest control methods. Recent advances in integrative multi-omics analysis, substrate-enzyme kinetics, and molecular biological techniques have elucidated the evolutionary origin and functional diversity of these three PxGSSs. This review summarizes research progress on PxGSSs over the past 20 years, covering sequence properties, evolution, protein modification, enzyme activity, structural variation, substrate specificity, and interaction scenarios based on functional diversity. Finally, we discussed the potential applications of PxGSSs-targeted pest control technologies driven by artificial intelligence, including CRISPR/Cas9-mediated gene drive, transgenic plant-mediated RNAi, small-molecule inhibitors, and peptide inhibitors. These technologies have the potential to overcome current management challenges and promote the development and field application of PxGSSs-targeted pest control.

Phillips GC, Greene KK, Poletto M, Sperry BD, Warner SA, Rose NR, Frandsen GK, Verza NC, Gorman KJ, Matzen KJ. New self-sexing Aedes aegypti strain eliminates barriers to scalable and sustainable vector control for governments and communities in dengue-prone environments

For more than 60 years, efforts to develop mating-based mosquito control technologies have largely failed to produce solutions that are both effective and scalable, keeping them out of reach of most governments and communities in disease-impacted regions globally. High pest suppression levels in trials have yet to fully translate into broad and effective Aedes aegypti control solutions. Two primary challenges to date-the need for complex sex-sorting to prevent female releases, and cumbersome processes for rearing and releasing male adult mosquitoes-present significant barriers for existing methods. As the host range of Aedes aegypti continues to advance into new geographies due to increasing globalisation and climate change, traditional chemical-based approaches are under mounting pressure from both more stringent regulatory processes and the ongoing development of insecticide resistance. It is no exaggeration to state that new tools, which are equal parts effective and scalable, are needed now more than ever. This paper describes the development and field evaluation of a new self-sexing strain of Aedes aegypti that has been designed to combine targeted vector suppression, operational simplicity, and cost-effectiveness for use in disease-prone regions. This conditional, self-limiting trait uses the sex-determination gene doublesex linked to the tetracycline-off genetic switch to cause complete female lethality in early larval development. With no female progeny survival, sex sorting is no longer required, eliminating the need for large-scale mosquito production facilities or physical sex-separation. In deployment operations, this translates to the ability to generate multiple generations of suppression for each mosquito released, while being entirely self-limiting. To evaluate these potential benefits, a field trial was carried out in densely-populated urban, dengue-prone neighbourhoods in Brazil, wherein the strain was able to suppress wild mosquito populations by up to 96%, demonstrating the utility of this self-sexing approach for biological vector control. In doing so, it has shown that such strains offer the critical components necessary to make these tools highly accessible, and thus they harbour the potential to transition mating-based approaches to effective and sustainable vector control tools that are within reach of governments and at-risk communities who may have only limited resources.

The impact of transgenesis on mosquito fitness: A review

Transgenic mosquitoes developed by genetic manipulation, offer a promising strategy for the sustainable and effective control of mosquito-borne diseases. This strategy relies on the mass release of transgenic mosquitoes into the wild, where their transgene is expected to persist in the natural environment, either permanently or transiently, within the mosquito population. In such circumstances, the fitness of transgenic mosquitoes is an important factor in determining their survival in the wild. The impact of transgene expression, insertional mutagenesis, inbreeding depression related to laboratory adaptation, and the hitchhiking effect involved in developing homozygous mosquito lines can all have an effect on the fitness of transgenic mosquitoes. Therefore, real-time estimation of transgene-associated fitness cost is imperative for modeling and planning transgenic mosquito release programs. This can be achieved by directly comparing fitness parameters in individuals homozygous or hemizygous for the transgene and their wild-type counterparts, or by cage invasion experiments to monitor the frequency of the transgenic allele over multiple generations. Recent advancements such as site-specific integration systems and gene drives, provide platforms to address fitness issues in transgenic mosquitoes. More research on the fitness of transgenic individuals is required to develop transgenic mosquitoes with a low fitness cost.

Self-limiting fall armyworm: a new approach in development for sustainable crop protection and resistance management

BACKGROUND

The fall armyworm, Spodoptera frugiperda, is a significant and widespread pest of maize, sorghum, rice, and other economically important crops. Successful management of this caterpillar pest has historically relied upon application of synthetic insecticides and through cultivation of genetically engineered crops expressing insecticidal proteins (Bt crops). Fall armyworm has, however, developed resistance to both synthetic insecticides and Bt crops, which risks undermining the benefits delivered by these important crop protection tools. Previous modelling and empirical studies have demonstrated that releases of insecticide- or Bt-susceptible insects genetically modified to express conditional female mortality can both dilute insecticide resistance and suppress pest populations.

RESULTS

Here, we describe the first germline transformation of the fall armyworm and the development of a genetically engineered male-selecting self-limiting strain, OX5382G, which exhibits complete female mortality in the absence of an additive in the larval diet. Laboratory experiments showed that males of this strain are competitive against wild-type males for copulations with wild-type females, and that the OX5382G self-limiting transgene declines rapidly to extinction in closed populations following the cessation of OX5382G male releases. Population models simulating the release of OX5382G males in tandem with Bt crops and non-Bt 'refuge' crops show that OX5382G releases can suppress fall armyworm populations and delay the spread of resistance to insecticidal proteins.

CONCLUSIONS

This article describes the development of self-limiting fall armyworm designed to control this pest by suppressing pest populations, and population models that demonstrate its potential as a highly effective method of managing resistance to Bt crops in pest fall armyworm populations. Our results provide early promise for a potentially valuable future addition to integrated pest management strategies for fall armyworm and other pests for which resistance to existing crop protection measures results in damage to crops and impedes sustainable agriculture.

Engineered expression of the invertebrate-specific scorpion toxin AaHIT reduces adult longevity and female fecundity in the diamondback moth Plutella xylostella

BACKGROUND

Previous genetic pest management (GPM) systems in diamondback moth (DBM) have relied on expressing lethal proteins ('effectors') that are 'cell-autonomous', that is, they do not leave the cell in which they are expressed. To increase the flexibility of future GPM systems in DBM, we aimed to assess the use of a non-cell-autonomous, invertebrate-specific, neurotoxic effector - the scorpion toxin AaHIT. This AaHIT effector was designed to be secreted by expressing cells, potentially leading to effects on distant cells, specifically neuromuscular junctions.

RESULTS

Expression of AaHIT caused a 'shaking/quivering' phenotype that could be repressed by provision of an antidote (tetracycline): a phenotype consistent with the AaHIT mode-of-action. This effect was more pronounced when AaHIT expression was driven by the Hr5/ie1 promoter (82.44% of males, 65.14% of females) rather than Op/ie2 (57.35% of males, 48.39% of females). Contrary to expectations, the shaking phenotype and observed fitness costs were limited to adults in which they caused severe reductions in mean longevity (-81%) and median female fecundity (-93%). Quantitative polymerase chain reactions of AaHIT expression patterns and analysis of piggyBac-mediated transgene insertion sites suggest that restriction of the observed effects to the adult stages may be due to the influence of the local genomic environment on the tetO-AaHIT transgene.

CONCLUSION

We demonstrated the feasibility of using non-cell-autonomous effectors within a GPM context for the first time in Lepidoptera, one of the most economically damaging orders of insects. These findings provide a framework for extending this system to other pest Lepidoptera and to other secreted effectors. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Population-level multiplexing: A promising strategy to manage the evolution of resistance against gene drives targeting a neutral locus

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.

Identification and characterization of a Masculinizer homologue in the diamondback moth, Plutella xylostella

Recently, a novel sex-determination system was identified in the silkworm (Bombyx mori) in which a piwi-interacting RNA (piRNA) encoded on the female-specific W chromosome silences a Z-linked gene (Masculinizer) that would otherwise initiate male sex-determination and dosage compensation. Masculinizer provides various opportunities for developing improved genetic pest management tools. A pest lepidopteran in which a genetic pest management system has been developed, but which would benefit greatly from such improved designs, is the diamondback moth, Plutella xylostella. However, Masculinizer has not yet been identified in this species. Here, focusing on the previously described 'masculinizing' domain of B. mori Masculinizer, we identify P. xylostella Masculinizer (PxyMasc). We show that PxyMasc is Z-linked, regulates sex-specific alternative splicing of doublesex and is necessary for male survival. Similar results in B. mori suggest this survival effect is possibly through failure to initiate male dosage compensation. The highly conserved function and location of this gene between these two distantly related lepidopterans suggests a deep role for Masculinizer in the sex-determination systems of the Lepidoptera.

Genetic control of Plutella xylostella in omics era

Diamondback moth, Plutella xylostella (L.), is a specialist pest on cruciferous crops of economic importance. The large-scale use of chemical insecticides for the control of this insect pest has caused a number of challenges to agro-ecosystems. With the advent of the omics era, genetic pest management strategies are becoming increasingly feasible and show a powerful potential for pest control. Here, we review strategies for using transgenic plants and sterile insect techniques for genetic pest management and introduce the major advances in the control of P. xylostella using a female-specific RIDL (release of insects carrying a dominant lethal gene) strategy. Further, the advantages of gene drive developed in combination with sex determination and CRISPR/Cas9 systems are addressed, and the corresponding prospects and implementation issues are discussed. It is predictable that under the policy and regulation of professional committees, the genetic pest control strategy, especially for gene drive, will open a new avenue to sustainable pest management not only for P. xylostella but also for other insect pests.

Response to a Synthetic Pheromone Source by OX4319L, a Self-Limiting Diamondback Moth (Lepidoptera: Plutellidae) Strain, and Field Dispersal Characteristics of its Progenitor Strain

The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), is a global pest that infests vegetable and field crops within the Brassica family. A genetically engineered strain of P. xylostella, OX4319L, carrying a 'self-limiting' gene, has shown potential for managing P. xylostella populations, using sustained releases of OX4319L male moths. In order for such a strain to provide control, the transgenic individuals must exhibit attraction to female P. xylostella sex pheromone and adequate dispersal in the field. In this study, we tested these key traits. First, we compared the responses of the OX4319L male moths to a synthetic female sex pheromone source in wind tunnel trials to those of males from three other strains. We found that OX4319L males responded comparably to strains of non-engineered males, with all males flying upwind towards the pheromone source. Second, we used mark-release-recapture studies of a wildtype P. xylostella strain, from which the OX4319L strain was originally developed, to assess dispersal under field conditions. Released males were recaptured using both pheromone-baited and passive traps within a 2.83 ha circular cabbage field, with a recapture rate of 7.93%. Males were recaptured up to the boundary of the field at 95 m from the central release point. The median dispersal of males was 14 m. These results showed the progenitor strain of OX4319L retained its ability to disperse within a host field. The results of these experiments are discussed in relation to the potential for the effective use of engineered male-selecting P. xylostella strains under field conditions.

Daisy-chain gene drives for the alteration of local populations

If they are able to spread in wild populations, CRISPR-based gene-drive elements would provide new ways to address ecological problems by altering the traits of wild organisms, but the potential for uncontrolled spread tremendously complicates ethical development and use. Here, we detail a self-exhausting form of CRISPR-based drive system comprising genetic elements arranged in a daisy chain such that each drives the next. "Daisy-drive" systems can locally duplicate any effect achievable by using an equivalent self-propagating drive system, but their capacity to spread is limited by the successive loss of nondriving elements from one end of the chain. Releasing daisy-drive organisms constituting a small fraction of the local wild population can drive a useful genetic element nearly to local fixation for a wide range of fitness parameters without self-propagating spread. We additionally report numerous highly active guide RNA sequences sharing minimal homology that may enable evolutionarily stable daisy drive as well as self-propagating CRISPR-based gene drive. Especially when combined with threshold dependence, daisy drives could simplify decision-making and promote ethical use by enabling local communities to decide whether, when, and how to alter local ecosystems.

Aseptic Rearing and Infection with Gut Bacteria Improve the Fitness of Transgenic Diamondback Moth, Plutella xylostella

Mass insect rearing can have a range of applications, for example in biological control of pests. The competitive fitness of released insects is extremely important in a number of applications. Here, we investigated how to improve the fitness of a transgenic diamondback moth, which has shown variation in mating ability when reared in different insectaries. Specifically we tested whether infection with a gut bacteria, Enterobacter cloacae, and aseptic rearing of larvae could improve insect growth and male performance. All larvae were readily infected with E. cloacae. Under aseptic rearing, pupal weights were reduced and there was a marginal reduction in larval survival. However, aseptic rearing substantially improved the fitness of transgenic males. In addition, under aseptic rearing, inoculation with E. cloacae increased pupal weights and male fitness, increasing the proportion of transgenic progeny from 20% to 30% relative to uninfected insects. Aseptic conditions may improve the fitness of transgenic males by excluding microbial contaminants, while symbiont inoculation could further improve fitness by providing additional protection against infection, or by normalizing insect physiology. The simple innovation of incorporating antibiotic into diet, and inoculating insects with symbiotic bacteria that are resistant to that antibiotic, could provide a readily transferable tool for other insect rearing systems.

Simple paratransgenic mosquitoes models and their dynamics

To study the interactive dynamics of wild mosquitoes and mosquitoes carrying genetically-modified bacteria, we formulate continuous-time homogeneous and stage-structured models in this study. With appropriate transformations, complete results of the existence and stability of all boundary and positive equilibria for the homogeneous model are established and complete results of the existence and local stability of all boundary and positive equilibria for the stage-structured model are obtained as well. The outcomes from the homogeneous and the stage-structured models are similar. Based on the homogeneous model, we particularly investigate how the horizontal transmission of the transgenic bacteria, via the uptake rate of the transgenic bacteria, affects the interactive dynamics.

Combining the high-dose/refuge strategy and self-limiting transgenic insects in resistance management-A test in experimental mesocosms

The high-dose/refuge strategy has been the primary approach for resistance management in transgenic crops engineered with Bacillus thuringiensis toxins. However, there are continuing pressures from growers to reduce the size of Bt toxin-free refugia, which typically suffer higher damage from pests. One complementary approach is to release male transgenic insects with a female-specific self-limiting gene. This technology can reduce population sizes and slow the evolution of resistance by introgressing susceptible genes through males. Theory predicts that it could be used to facilitate smaller refugia or reverse the evolution of resistance. In this study, we used experimental evolution with caged insect populations to investigate the compatibility of the self-limiting system and the high-dose/refuge strategy in mitigating the evolution of resistance in diamondback moth, Plutella xylostella. The benefits of the self-limiting system were clearer at smaller refuge size, particularly when refugia were inadequate to prevent the evolution of resistance. We found that transgenic males in caged mesocosms could suppress population size and delay resistance development with 10% refugia and 4%-15% initial resistance allele frequency. Fitness costs in hemizygous transgenic insects are particularly important for introgressing susceptible alleles into target populations. Fitness costs of the self-limiting gene in this study (P. xylostella OX4139 line L) were incompletely dominant, and reduced fecundity and male mating competitiveness. The experimental evolution approach used here illustrates some of the benefits and pitfalls of combining mass release of self-limiting insects and the high-dose/refuge strategy, but does indicate that they can be complementary.

Maintaining Quality of Candidate Strains of Transgenic Mosquitoes for Studies in Containment Facilities in Disease Endemic Countries

Transgenic mosquitoes are being developed as novel components of area-wide approaches to vector-borne disease control. Best practice is to develop these in phases, beginning with laboratory studies, before moving to field testing and inclusion in control programs, to ensure safety and prevent costly field testing of unsuitable strains. The process of identifying and developing good candidate strains requires maintenance of transgenic colonies over many generations in containment facilities. By working in disease endemic countries with target vector populations, laboratory strains may be developed and selected for properties that will enhance intended control efficacy in the next phase, while avoiding traits that introduce unnecessary risks. Candidate strains aiming toward field use must consistently achieve established performance criteria, throughout the process of scaling up from small study colonies to production of sufficient numbers for field testing and possible open release. Maintenance of a consistent quality can be demonstrated by a set of insect quality and insectary operating indicators, measured over time at predetermined intervals. These indicators: inform comparability of studies using various candidate strains at different times and locations; provide evidence of conformity relevant to compliance with terms of approval for regulated use; and can be used to validate some assumptions related to risk assessments covering the contained phase and for release into the environment.

Genetics-based methods for agricultural insect pest management

The sterile insect technique is an area-wide pest control method that reduces agricultural pest populations by releasing mass-reared sterile insects, which then compete for mates with wild insects. Contemporary genetics-based technologies use insects that are homozygous for a repressible dominant lethal genetic construct rather than being sterilized by irradiation.Engineered strains of agricultural pest species, including moths such as the diamondback moth Plutella xylostella and fruit flies such as the Mediterranean fruit fly Ceratitis capitata, have been developed with lethality that only operates on females.Transgenic crops expressing insecticidal toxins are widely used; the economic benefits of these crops would be lost if toxin resistance spread through the pest population. The primary resistance management method is a high-dose/refuge strategy, requiring toxin-free crops as refuges near the insecticidal crops, as well as toxin doses sufficiently high to kill wild-type insects and insects heterozygous for a resistance allele.Mass-release of toxin-sensitive engineered males (carrying female-lethal genes), as well as suppressing populations, could substantially delay or reverse the spread of resistance. These transgenic insect technologies could form an effective resistance management strategy.We outline some policy considerations for taking genetic insect control systems through to field implementation.

Towards the genetic control of invasive species

Invasive species remain one of the greatest threats to global biodiversity. Their control would be enhanced through the development of more effective and sustainable pest management strategies. Recently, a novel form of genetic pest management (GPM) has been developed in which the mating behaviour of insect pests is exploited to introduce genetically engineered DNA sequences into wild conspecific populations. These 'transgenes' work in one or more ways to reduce the damage caused by a particular pest, for example reducing its density, or its ability to vector disease. Although currently being developed for use against economically important insect pests, these technologies would be highly appropriate for application against invasive species that threaten biodiversity. Importantly, these technologies have begun to advance in scope beyond insects to vertebrates, which include some of the world's worst invasives. Here we review the current state of this rapidly progressing field and, using an established set of eradication criteria, discuss the characteristics which make GPM technologies suitable for application against invasive pests.

Performance of a Genetically Modified Strain of the Mediterranean Fruit Fly (Diptera: Tephritidae) for Area-Wide Integrated Pest Management With the Sterile Insect Technique

The genetically modified strain of Ceratitis capitata (Wiedemann) VIENNA 8 1260 has two morphological markers that exhibit fluorescence in body and sperm. To assess the feasibility of its use in area-wide integrated pest management (AW-IPM) programs using the sterile insect technique, its rearing performance and quality control profile under small, medium, and large scales was evaluated, as well as in field cages. The VIENNA 8 1260 strain had a lower yield than the control strains, VIENNA 8 with D53 inversion (VIENNA 8) and without D53 inversion (VIENNA 8 D53-). At mass-rearing scale, yield gradually increased in three generations without reaching the control strain values. The VIENNA 8 1260 strain was stable in the genetic sexing mechanism (>99.9%) and expression of fluorescence (100%). In field cages, the VIENNA 8 1260 males reduced the mating potential of wild males in the same magnitude as the VIENNA 8, when evaluated in independent cage tests. However, the relative sterility index and the strain male relative performance index of VIENNA 8 1260 males were significantly lower than those of the VIENNA 8. There were no significant differences in longevity of these strains. The potential application of the VIENNA 8 1260 in AW-IPM programs is further discussed.

Resistance to genetic insect control: Modelling the effects of space

Genetic insect control, such as self-limiting RIDL² (Release of Insects Carrying a Dominant Lethal) technology, is a development of the sterile insect technique which is proposed to suppress wild populations of a number of major agricultural and public health insect pests. This is achieved by mass rearing and releasing male insects that are homozygous for a repressible dominant lethal genetic construct, which causes death in progeny when inherited. The released genetically engineered ('GE') insects compete for mates with wild individuals, resulting in population suppression. A previous study modelled the evolution of a hypothetical resistance to the lethal construct using a frequency-dependent population genetic and population dynamic approach. This found that proliferation of resistance is possible but can be diluted by the introgression of susceptible alleles from the released homozygous-susceptible GE males. We develop this approach within a spatial context by modelling the spread of a lethal construct and resistance trait, and the effect on population control, in a two deme metapopulation, with GE release in one deme. Results show that spatial effects can drive an increased or decreased evolution of resistance in both the target and non-target demes, depending on the effectiveness and associated costs of the resistant trait, and on the rate of dispersal. A recurrent theme is the potential for the non-target deme to act as a source of resistant or susceptible alleles for the target deme through dispersal. This can in turn have a major impact on the effectiveness of insect population control.

Comparison of Model Predictions and Laboratory Observations of Transgene Frequencies in Continuously-Breeding Mosquito Populations

The persistence of transgenes in the environment is a consideration in risk assessments of transgenic organisms. Combining mathematical models that predict the frequency of transgenes and experimental demonstrations can validate the model predictions, or can detect significant biological deviations that were neither apparent nor included as model parameters. In order to assess the correlation between predictions and observations, models were constructed to estimate the frequency of a transgene causing male sexual sterility in simulated populations of a malaria mosquito Anopheles gambiae that were seeded with transgenic females at various proportions. Concurrently, overlapping-generation laboratory populations similar to those being modeled were initialized with various starting transgene proportions, and the subsequent proportions of transgenic individuals in populations were determined weekly until the transgene disappeared. The specific transgene being tested contained a homing endonuclease gene expressed in testes, I-PpoI, that cleaves the ribosomal DNA and results in complete male sexual sterility with no effect on female fertility. The transgene was observed to disappear more rapidly than the model predicted in all cases. The period before ovipositions that contained no transgenic progeny ranged from as little as three weeks after cage initiation to as long as 11 weeks.

Genetically Engineered Ricin Suppresses Bactrocera dorsalis (Diptera: Tephritidae) based on Demographic Analysis of Group-Reared Life Table

The oriental fruit fly, Bactrocera dorsalis (Hendel), reduces the quantity and quality of many host fruits through the process of oviposition and larval feeding, and this insect has been considered a major insect pest in several Asian countries for decades. Using an earlier-developed, female-specific system that combines the toxicity of the ricin A chain (RTA) and the alternative RNA splicing property of doublesex ( Bddsx ), we show that transgenic male flies harboring the RTA-Bddsx transgene unevenly repress the pest population through inheritable effects. In age-stage, two-sex life-table analyses, high larval mortality and a delay in pupation were observed after introducing the transgene. The high male to female ratio in DsRed + flies demonstrates the lethal effect of ricin on females. The fitness of both the DsRed + - and DsRed - -transformed females was reduced as shown in the decrease of the net reproductive rate ( R0 ), intrinsic rate ( r ), and finite rate (λ) values compared with the wild-type populations. The integrity of the RTA-Bddsx transgene remained in more than 80% of DsRed + males after ten generations, supporting the stable inheritance of the transgene. All of the data from this study support the proposed RTA-Bddsx SIT approach, which provides a species-specific and environmentally friendly method of suppressing, rather than eradiating, B. dorsalis.

Evolutionary biology and genetic techniques for insect control

The requirement to develop new techniques for insect control that minimize negative environmental impacts has never been more pressing. Here we discuss population suppression and population replacement technologies. These include sterile insect technique, genetic elimination methods such as the release of insects carrying a dominant lethal (RIDL), and gene driving mechanisms offered by intracellular bacteria and homing endonucleases. We also review the potential of newer or underutilized methods such as reproductive interference, CRISPR technology, RNA interference (RNAi), and genetic underdominance. We focus on understanding principles and potential effectiveness from the perspective of evolutionary biology. This offers useful insights into mechanisms through which potential problems may be minimized, in much the same way that an understanding of how resistance evolves is key to slowing the spread of antibiotic and insecticide resistance. We conclude that there is much to gain from applying principles from the study of resistance in these other scenarios - specifically, the adoption of combinatorial approaches to minimize the spread of resistance evolution. We conclude by discussing the focused use of GM for insect pest control in the context of modern conservation planning under land-sparing scenarios.

Pest control and resistance management through release of insects carrying a male-selecting transgene

Background

Development and evaluation of new insect pest management tools is critical for overcoming over-reliance upon, and growing resistance to, synthetic, biological and plant-expressed insecticides. For transgenic crops expressing insecticidal proteins from the bacterium Bacillus thuringiensis (‘Bt crops’) emergence of resistance is slowed by maintaining a proportion of the crop as non-Bt varieties, which produce pest insects unselected for resistance. While this strategy has been largely successful, multiple cases of Bt resistance have now been reported.

One new approach to pest management is the use of genetically engineered insects to suppress populations of their own species. Models suggest that released insects carrying male-selecting (MS) transgenes would be effective agents of direct, species-specific pest management by preventing survival of female progeny, and simultaneously provide an alternative insecticide resistance management strategy by introgression of susceptibility alleles into target populations. We developed a MS strain of the diamondback moth, Plutella xylostella, a serious global pest of crucifers. MS-strain larvae are reared as normal with dietary tetracycline, but, when reared without tetracycline or on host plants, only males will survive to adulthood. We used this strain in glasshouse-cages to study the effect of MS male P. xylostella releases on target pest population size and spread of Bt resistance in these populations.

Results

Introductions of MS-engineered P. xylostella males into wild-type populations led to rapid pest population decline, and then elimination. In separate experiments on broccoli plants, relatively low-level releases of MS males in combination with broccoli expressing Cry1Ac (Bt broccoli) suppressed population growth and delayed the spread of Bt resistance. Higher rates of MS male releases in the absence of Bt broccoli were also able to suppress P. xylostella populations, whereas either low-level MS male releases or Bt broccoli alone did not.

Conclusions

These results support theoretical modeling, indicating that MS-engineered insects can provide a powerful pest population suppressing effect, and could effectively augment current Bt resistance management strategies. We conclude that, subject to field confirmation, MS insects offer an effective and versatile control option against P. xylostella and potentially other pests, and may reduce reliance on and protect insecticide-based approaches, including Bt crops.

Development and evaluation of male-only strains of the Australian sheep blowfly, Lucilia cuprina

The Australian sheep blowfly Lucilia cuprina (Wiedemann) is a major pest of sheep in Australia and New Zealand. From the 1960s to the 1980s there was a major effort to develop "field female killing" or FFK strains of L. cuprina that could be used for a cost-effective genetic control program. The FFK strains carried eye color mutations that were lethal to females in the field but not under conditions in the mass rearing facility. Males did not die in the field as normal copies of the eye color genes had been translocated to the Y chromosome and an autosome. Although the FFK strains showed some promise in field tests, a genetic control program in mainland Australia was never implemented for several reasons including instability of the FFK strains during mass rearing. A stable transgenic strain of L. cuprina that carried one or more dominant repressible female lethal genes offered the potential for efficient genetic control of blowfly populations. Here I review our research on tetracycline-repressible female lethal genetic systems, Lucilia germ-line transformation and sex determination genes that ultimately led to the successful development of transgenic "male-only" strains of L. cuprina. The technology developed for L. cuprina should be directly transferable to other blowfly livestock pests including L. sericata and the New World and Old World screwworm. 29