Genetic Variation and Potential for Resistance Development to the tTA Overexpression Lethal System in Insects

Flying toward a plastic-free world: Can Drosophila serve as a model organism to develop new strategies of plastic waste management?

The last century was dominated by the widespread use of plastics, both in terms of invention and increased usage. The environmental challenge we currently face is not just about reducing plastic usage but finding new ways to manage plastic waste. Recycling is growing but remains a small part of the solution. There is increasing focus on studying organisms and processes that can break down plastics, offering a modern approach to addressing the environmental crisis. Here, we provide an overview of the organisms associated with plastics biodegradation, and we explore the potential of harnessing and integrating their genetic and biochemical features into a single organism, such as Drosophila melanogaster. The remarkable genetic engineering and microbiota manipulation tools available for this organism suggest that multiple features could be amalgamated and modeled in the fruit fly. We outline feasible genetic engineering and gut microbiome engraftment strategies to develop a new class of plastic-degrading organisms and discuss of both the potential benefits and the limitations of developing such engineered Drosophila melanogaster strains.

Effects of antibiotics on the in vitro expression of tetracycline-off constructs and the performance of Drosophila suzukii female-killing strains

Genetic control strategies such as the Release of Insects Carrying a Dominant Lethal (RIDL) gene and Transgenic Embryonic Sexing System (TESS) have been demonstrated in the laboratory and/or deployed in the field. These strategies are based on tetracycline-off (Tet-off) systems which are regulated by antibiotics such as Tet and doxycycline (Dox). Here, we generated several Tet-off constructs carrying a reporter gene cassette mediated by a 2A peptide. Different concentrations (0.1, 10, 100, 500, and 1,000 μg/mL) and types (Tet or Dox) of antibiotics were used to evaluate their effects on the expression of the Tet-off constructs in the Drosophila S2 cells. One or both of the two concentrations, 100 and 250 μg/mL, of Tet or Dox were used to check the influence on the performances of a Drosophila suzukii wild-type strain and female-killing (FK) strains employing TESS. Specifically, the Tet-off construct for these FK strains contains a Drosophila suzukii nullo promoter to regulate the tetracycline transactivator gene and a sex-specifically spliced pro-apoptotic gene hid ^Ala4 to eliminate females. The results suggested that the in vitro expression of the Tet-off constructs was controlled by antibiotics in a dose-dependent manner. ELISA experiments were carried out identifying Tet at 34.8 ng/g in adult females that fed on food supplemented with Tet at 100 μg/mL. However, such method did not detect Tet in the eggs produced by antibiotic-treated flies. Additionally, feeding Tet to the parents showed negative impact on the fly development but not the survival in the next generation. Importantly, we demonstrated that under certain antibiotic treatments females could survive in the FK strains with different transgene activities. For the strain V229_M4f1 which showed moderate transgene activity, feeding Dox to fathers or mothers suppressed the female lethality in the next generation and feeding Tet or Dox to mothers generated long-lived female survivors. For the strain V229_M8f2 which showed weak transgene activity, feeding Tet to mothers delayed the female lethality for one generation. Therefore, for genetic control strategies employing the Tet-off system, the parental and transgenerational effects of antibiotics on the engineered lethality and insect fitness must be carefully evaluated for a safe and efficient control program.

Modeling-informed Engineered Genetic Incompatibility strategies to overcome resistance in the invasive Drosophila suzukii

Engineered Genetic Incompatibility (EGI) is an engineered extreme underdominance genetic system wherein hybrid animals are not viable, functioning as a synthetic speciation event. There are several strategies in which EGI could be leveraged for genetic biocontrol of pest populations. We used an agent-based model of Drosophila suzukii (Spotted Wing Drosophila) to determine how EGI would fare with high rates of endemic genetic resistance alleles. We discovered a surprising failure mode wherein field-generated females convert an incompatible male release program into a population replacement gene drive. Local suppression could still be attained in two seasons by tailoring the release strategy to take advantage of this effect, or alternatively in one season by altering the genetic design of release agents. We show in this work that data from modeling can be utilized to recognize unexpected emergent phenomena and a priori inform genetic biocontrol treatment design to increase efficacy.

The sterile insect technique is protected from evolution of mate discrimination

Background

The sterile insect technique (SIT) has been used to suppress and even extinguish pest insect populations. The method involves releasing artificially reared insects (usually males) that, when mating with wild individuals, sterilize the broods. If administered on a large enough scale, the sterility can collapse the population. Precedents from other forms of population suppression, especially chemicals, raise the possibility of resistance evolving against the SIT. Here, we consider resistance in the form of evolution of female discrimination to avoid mating with sterile males. Is resistance evolution expected?

Methods

We offer mathematical models to consider the dynamics of this process. Most of our models assume a constant-release protocol, in which the same density of males is released every generation, regardless of wild male density. A few models instead assume proportional release, in which sterile releases are adjusted to be a constant proportion of wild males.

Results

We generally find that the evolution of female discrimination, although favored by selection, will often be too slow to halt population collapse when a constant-release implementation of the SIT is applied appropriately and continually. The accelerating efficacy of sterile males in dominating matings as the population collapses works equally against discriminating females as against non-discriminating females, and rare genes for discrimination are too slow to ascend to prevent the loss of females that discriminate. Even when migration from source populations sustains the treated population, continued application of the SIT can prevent evolution of discrimination. However, periodic premature cessation of the SIT does allow discrimination to evolve. Likewise, use of a 'proportional-release' protocol is also prone to escape from extinction if discriminating genotypes exist in the population, even if those genotypes are initially rare. Overall, the SIT is robust against the evolution of mate discrimination provided care is taken to avoid some basic pitfalls. The models here provide insight for designing programs to avoid those pitfalls.

Evaluation of Additional Drosophila suzukii Male-Only Strains Generated Through Remobilization of an FL19 Transgene

Drosophila suzukii (D. suzukii) (Matsumura, 1931; Diptera: Drosophilidae), also known as spotted wing Drosophila, is a worldwide pest of fruits with soft skins such as blueberries and cherries. Originally from Asia, D. suzukii is now present in the Americas and Europe and has become a significant economic pest. Growers largely rely on insecticides for the control of D. suzukii. Genetic strategies offer a species-specific environmentally friendly way for suppression of D. suzukii populations. We previously developed a transgenic strain of D. suzukii that produced only males on a diet that did not contain tetracycline. The strain carried a single copy of the FL19 construct on chromosome 3. Repeated releases of an excess of FL19 males led to suppression of D. suzukii populations in laboratory cage trials. Females died as a consequence of overexpression of the tetracycline transactivator (tTA) and tTA-activated expression of the head involution defective proapoptotic gene. The aim of this study was to generate additional male-only strains that carried two copies of the FL19 transgene through crossing the original line with a piggyBac jumpstarter strain. Males that carried either two chromosome 3 or a singleX-linked transgene were identified through stronger expression of the red fluorescent protein marker gene. The brighter fluorescence of the X-linked lines was likely due to dosage compensation of the red fluorescent protein gene. In total, four X-linked lines and eleven lines with two copies on chromosome 3 were obtained, of which five were further examined. All but one of the strains produced only males on a diet without tetracycline. When crossed with wild type virgin females, all of the five two copy autosomal strains examined produced only males. However, the single copy X-linked lines did not show dominant female lethality. Five of the autosomal lines were further evaluated for productivity (egg to adult) and male competition. Based on these results, the most promising lines have been selected for future population suppression experiments with strains from different geographical locations.

Genetically engineered insects with sex-selection and genetic incompatibility enable population suppression

Engineered Genetic Incompatibility (EGI) is a method to create species-like barriers to sexual reproduction. It has applications in pest control that mimic Sterile Insect Technique when only EGI males are released. This can be facilitated by introducing conditional female-lethality to EGI strains to generate a sex-sorting incompatible male system (SSIMS). Here, we demonstrate a proof of concept by combining tetracycline-controlled female lethality constructs with a pyramus-targeting EGI line in the model insect Drosophila melanogaster. We show that both functions (incompatibility and sex-sorting) are robustly maintained in the SSIMS line and that this approach is effective for population suppression in cage experiments. Further we show that SSIMS males remain competitive with wild-type males for reproduction with wild-type females, including at the level of sperm competition.

A conditional female lethal system for genetic suppression of the global fruit crop pest Drosophila suzukii

BACKGROUND

Drosophila suzukii (Matsumura, 1931, Diptera: Drosophilidae) is a global pest of soft-skinned fruits such as blueberries, cherries and raspberries. Also known as spotted-wing drosophila, D. suzukii is native to Asia but is now widely distributed in the Americas and Europe, and presents a serious challenge for growers. Genetic control strategies offer an environmentally friendly approach for the control of D. suzukii.

RESULTS

In this study, we developed transgenic strains of D. suzukii that carry dominant conditional female lethal transgenes. When raised in the absence of tetracycline, female D. suzukii die. We show that repeated releases of an excess of transgenic males can suppress D. suzukii populations in laboratory cage trials.

CONCLUSION

Our data suggest that the transgenic strain could provide an effective approach for control of this invasive pest of soft-skinned fruits.

A transgenic female killing system for the genetic control of Drosophila suzukii

The spotted wing Drosophila (Drosophila suzukii) is an invasive pest of soft-skinned fruit crops. It is rapidly transmitted in Europe and North America, causing widespread agricultural losses. Genetic control strategies such as the sterile insect technique (SIT) have been proposed as environment-friendly and species-restricted approaches for this pest. However, females are inefficient agents in SIT programs. Here we report a conditional female-killing (FK) strategy based on the tetracycline-off system. We assembled sixteen genetic constructs for testing in vitro and in vivo. Twenty-four independent transgenic strains of D. suzukii were generated and tested for female-specific lethality. The strongest FK effect in the absence of tetracycline was achieved by the construct containing D. suzukii nullo promoter for early gene expression, D. suzukii pro-apoptotic gene hidAla4 for lethality, and the transformer gene intron from the Mediterranean fruit fly Ceratitis capitata for female-specific splicing. One strain carrying this construct eliminated 100% of the female offspring during embryogenesis and produced only males. However, homozygous females from these FK strains were not viable on a tetracycline-supplemented diet, possibly due to the basal expression of hidAla4. Potential improvements to the gene constructs and the use of such FK strains in an SIT program are discussed.

Genetic pest management and the background genetics of release strains

Genetic pest management (GPM) methods involve releasing modified versions of a pest species to mate with wild pests in the target area. Proposed for a wide range of applications in public health, agriculture and conservation, most progress has been made with pest insects. Offspring of the released modified insects and wild pests carry the modification-which might be transgenes, artificially introduced Wolbachia or genetic damage from radiation, for example-but they also carry a complete haploid genome from their laboratory-reared parent, as well as one from their wild parent. Unless these F₁ hybrids are completely unable to reproduce, further mating will lead to introgression of DNA sequences from the release strain into the wild population. We discuss issues around strain selection and the potential consequences of such introgression. We conclude that such introgression is probably harmless in almost all circumstances, and could, in theory, provide specific additional benefits to the release programme. We outline population monitoring approaches that could be used, going forward, to determine how background genetics may affect GPM. This article is part of the theme issue 'Novel control strategies for mosquito-borne diseases'.

Building a transgenic sexing strain for genetic control of the Australian sheep blow fly Lucilia cuprina using two lethal effectors

Background

The sterile insect technique (SIT) has been successfully used in many pest management programs worldwide. Some SIT programs release both sexes due to the lack of genetic sexing strains or efficient sex separation methods but sterile females are ineffective control agents. Transgenic sexing strains (TSS) using the tetracycline-off control system have been developed in a variety of insect pests, from which females die by either of two commonly used lethal effectors: overexpression of the transcription factor tetracycline transactivator (tTA) or ectopic expression of a proapoptotic gene, such as head involution defective (hid). The lethality from tTA overexpression is thought to be due to “transcriptional squelching”, while hid causes lethality by induction of apoptosis. This study aims to create and characterize a TSS of Lucilia cuprina, which is a major pest of sheep, by combining both lethal effectors in a single transgenic strain.

Results

Here a stable TSS of L. cuprina (DH6) that carries two lethal effectors was successfully generated, by crossing FL3#2 which carries a female-specific tTA overexpression cassette, with EF1#12 which carries a tTA-regulated Lshid^Ala2 cassette. Females with one copy of the FL3#2 transgene are viable but up to 99.8% of homozygous females die at the pupal stage when raised on diet that lacks tetracycline. Additionally, the female lethality of FL3#2 was partially repressed by supplying tetracycline to the parental generation. With an additional Lshid^Ala2 effector, the female lethality of DH6 is 100% dominant and cannot be repressed by maternal tetracycline. DH6 females die at the late-larval stage. Several fitness parameters important for mass rearing such as hatching rate, adult emergence and sex ratio were comparable to those of the wild type strain.

Conclusions

Compared to the parental FL3#2 strain, the DH6 strain shows stronger female lethality and lethality occurs at an earlier stage of development. The combination of two tTA-dependent lethal effectors could improve strain stability under mass rearing and could reduce the risk of resistance in the field if fertile males are released. Our approach could be easily adapted for other pest species for an efficient, safe and sustainable genetic control program.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00947-y.

Genetic breakdown of a Tet-off conditional lethality system for insect population control

Genetically modified conditional lethal strains have been created to improve the control of insect pest populations damaging to human health and agriculture. However, understanding the potential for the genetic breakdown of lethality systems by rare spontaneous mutations, or selection for inherent suppressors, is critical since field release studies are in progress. This knowledge gap was addressed in a Drosophila tetracycline-suppressible embryonic lethality system by analyzing the frequency and structure of primary-site spontaneous mutations and second-site suppressors resulting in heritable survivors from 1.2 million zygotes. Here we report that F₁ survivors due to primary-site deletions and indels occur at a 5.8 × 10⁻⁶ frequency, while survival due to second-site maternal-effect suppressors occur at a ~10⁻⁵ frequency. Survivors due to inherent lethal effector suppressors could result in a resistant field population, and we suggest that this risk may be mitigated by the use of dual redundant, albeit functionally unrelated, lethality systems.

Insect population control using conditional lethal systems could break down due to spontaneous mutations that render the system ineffective. Here the authors analyse the structure and frequency of such mutations in Drosophila and suggest the use of dual lethality systems to mitigate their survival.

Development and testing of a novel killer-rescue self-limiting gene drive system in Drosophila melanogaster

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.