Camilla Beech

Field performance of engineered male mosquitoes

Dengue is the most medically important arthropod-borne viral disease, with 50–100 million cases reported annually worldwide. As no licensed vaccine or dedicated therapy exists for dengue, the most promising strategies to control the disease involve targeting the predominant mosquito vector, Aedes aegypti. However, the current methods to do this are inadequate. Various approaches involving genetically engineered mosquitoes have been proposed, including the release of transgenic sterile males. However, the ability of laboratory-reared, engineered male mosquitoes to effectively compete with wild males in terms of finding and mating with wild females, which is critical to the success of these strategies, has remained untested. We report data from the first open-field trial involving a strain of engineered mosquito. We demonstrated that genetically modified male mosquitoes, released across 10 hectares for a 4-week period, mated successfully with wild females and fertilized their eggs. These findings suggest the feasibility of this technology to control dengue by suppressing field populations of A. aegypti.

Successful suppression of a field mosquito population by sustained release of engineered male mosquitoes

Successful suppression of a field mosquito population by sustained release of engineered male mosquitoes

Open Field Release of Genetically Engineered Sterile Male Aedes aegypti in Malaysia

Background Dengue is the most important mosquito-borne viral disease. In the absence of specific drugs or vaccines, control focuses on suppressing the principal mosquito vector, Aedes aegypti, yet current methods have not proven adequate to control the disease. New methods are therefore urgently needed, for example genetics-based sterile-male-release methods. However, this requires that lab-reared, modified mosquitoes be able to survive and disperse adequately in the field. Methodology/Principal Findings Adult male mosquitoes were released into an uninhabited forested area of Pahang, Malaysia. Their survival and dispersal was assessed by use of a network of traps. Two strains were used, an engineered ‘genetically sterile’ (OX513A) and a wild-type laboratory strain, to give both absolute and relative data about the performance of the modified mosquitoes. The two strains had similar maximum dispersal distances (220 m), but mean distance travelled of the OX513A strain was lower (52 vs. 100 m). Life expectancy was similar (2.0 vs. 2.2 days). Recapture rates were high for both strains, possibly because of the uninhabited nature of the site. Conclusions/Significance After extensive contained studies and regulatory scrutiny, a field release of engineered mosquitoes was safely and successfully conducted in Malaysia. The engineered strain showed similar field longevity to an unmodified counterpart, though in this setting dispersal was reduced relative to the unmodified strain. These data are encouraging for the future testing and implementation of genetic control strategies and will help guide future field use of this and other engineered strains.

Genetic control of Aedes mosquitoes.

Abstract Aedes mosquitoes include important vector species such as Aedes aegypti, the major vector of dengue. Genetic control methods are being developed for several of these species, stimulated by an urgent need owing to the poor effectiveness of current methods combined with an increase in chemical pesticide resistance. In this review we discuss the various genetic strategies that have been proposed, their present status, and future prospects. We focus particularly on those methods that are already being tested in the field, including RIDL and Wolbachia-based approaches.

Appropriate regulation of GM insects.

After many years of open discussion and development [1]–[8], the first genetically modified (GM) insect strains are entering field trials [9], [10]. A key engineered trait renders the insects “genetically sterile”, such that some or all of their offspring die [11]–[14]; the insects additionally carry a fluorescent marker gene for easy identification. Such “genetic sterility” transgenes (in genetic terms, conditional dominant lethal genes) are not able to establish or spread in the wild due to their high fitness cost; such self-limiting strategies are widely viewed as the lowest risk category (e.g., [15]). Other genetic strategies are in development, including more invasive genetic systems. Some of these, such as those based on artificial infection with Wolbachia [16], may not be covered by regulations narrowly focused on the use of recombinant DNA technology despite having many similar properties [17].

Oral Ingestion of Transgenic RIDL Ae. aegypti Larvae Has No Negative Effect on Two Predator Toxorhynchites Species

Dengue is the most important mosquito-borne viral disease. No specific treatment or vaccine is currently available; traditional vector control methods can rarely achieve adequate control. Recently, the RIDL (Release of Insect carrying Dominant Lethality) approach has been developed, based on the sterile insect technique, in which genetically engineered ‘sterile’ homozygous RIDL male insects are released to mate wild females; the offspring inherit a copy of the RIDL construct and die. A RIDL strain of the dengue mosquito, Aedes aegypti, OX513A, expresses a fluorescent marker gene for identification (DsRed2) and a protein (tTAV) that causes the offspring to die. We examined whether these proteins could adversely affect predators that may feed on the insect. Aedes aegypti is a peri-domestic mosquito that typically breeds in small, rain-water-filled containers and has no specific predators. Toxorhynchites larvae feed on small aquatic organisms and are easily reared in the laboratory where they can be fed exclusively on mosquito larvae. To evaluate the effect of a predator feeding on a diet of RIDL insects, OX513A Ae. aegypti larvae were fed to two different species of Toxorhynchites (Tx. splendens and Tx. amboinensis) and effects on life table parameters of all life stages were compared to being fed on wild type larvae. No significant negative effect was observed on any life table parameter studied; this outcome and the benign nature of the expressed proteins (tTAV and DsRed2) indicate that Ae. aegypti OX513A RIDL strain is unlikely to have any adverse effects on predators in the environment.

Assessment of the Impact of Potential Tetracycline Exposure on the Phenotype of Aedes aegypti OX513A: Implications for Field Use

Background Aedes aegypti is the primary vector of dengue fever, a viral disease which has an estimated incidence of 390 million infections annually. Conventional vector control methods have been unable to curb the transmission of the disease. We have previously reported a novel method of vector control using a tetracycline repressible self-limiting strain of Ae. aegypti OX513A which has achieved >90% suppression of wild populations. Methodology/Principal Findings We investigated the impact of tetracycline and its analogues on the phenotype of OX513A from the perspective of possible routes and levels of environmental exposure. We determined the minimum concentration of tetracycline and its analogues that will allow an increased survivorship and found these to be greater than the maximum concentration of tetracyclines found in known Ae. aegypti breeding sites and their surrounding areas. Furthermore, we determined that OX513A parents fed tetracycline are unable to pre-load their progeny with sufficient antidote to increase their survivorship. Finally, we studied the changes in concentration of tetracycline in the mass production rearing water of OX513A and the developing insect. Conclusion/Significance Together, these studies demonstrate that potential routes of exposure of OX513A individuals to tetracycline and its analogues in the environment are not expected to increase the survivorship of OX513A.

Irritant and Repellent Behavioral Responses of Aedes aegypti Male Populations Developed for RIDL Disease Control Strategies

ABSTRACT Behavioral responses of Aedes aegypti male populations developed for Release of Insects Carrying a Dominant Lethal (RIDL) technology and a Malaysian wild-type population of two age groups (4–5 and 8–10 d old) were tested under laboratory conditions against chemical irritants and repellents using the high-throughput screening system device. Results indicate that all male Ae. aegypti test populations showed significant (P < 0.01) behavioral escape responses when exposed to alphacypermethrin, DDT, and deltamethrin at the test dose of 25 nmol/cm2. In addition, all populations showed significant (P < 0.05) spatial repellent responses to DDT, whereas alphacypermethrin and deltamethrin elicited no directional movement in the assay. These data suggest that genetic modification has not suppressed expected irritancy and repellency behavior. Age effects were minimal in both contact irritant and spatial repellent assays. The magnitude of irritant response, based on percentage responding, was stronger in the RIDL test cohorts as compared with the wild-type Malaysian population, but the impact, if any, that this increased behavioral sensitivity might have on the success of a RIDL strategy has yet to be defined. Information of the type reported in the current study is vital in defining the effects of genetic modification on vector behavior and understanding how these behaviors may influence the success of RIDL technology as they relate to other vector control interventions implemented in the same disease-endemic locale.

Genetically Engineered Insects – Regulatory Progress and Challenges

Genetically engineered insects (GE) represent a potentially valuable new tool in the control of insect pests both in agriculture and public health.

Exposure to genetically engineered olive fly ( Bactrocera oleae ) has no negative impact on three non-target organisms

Bactrocera oleae (Diptera: Tephritidae) remains a major pest of olive fruit production worldwide. Current pest management programs largely depend on chemical insecticides, resulting in high economic and environmental costs. Alternative pest control approaches are therefore highly desirable. We have created a conditional female-specific self-limiting strain of B. oleae (OX3097D-Bol) that could be applied for sustainable pest control. OX3097D-Bol olive fly carries a fluorescent marker (DsRed2) for identification and a self-limiting genetic trait that is repressed by tetracycline. In the absence of tetracycline, the tetracycline transactivator (tTAV) accumulates, resulting in female death at larvae and early pupal stages. The aim of this study was to evaluate the impact of genetically engineered OX3097D-Bol olive fly on three non-target organisms that either predate or parasitize olive flies, one from the guild of parasitoids (Psyttalia concolor) and two from the guild of predators (Pardosa spider species and the rove beetle Aleochara bilineata). No significant negative effect was observed on life history parameters, mortality and reproductive capacity of the non-target organisms studied. These results suggest that potential exposure to DsRed2 and tTAV gene products (e.g. mRNA and encoded proteins) would have a negligible impact on on-target organisms in the guilds or predators and parasitoids.