Late-acting dominant lethal genetic systems and mosquito control

BACKGROUND: Reduction or elimination of vector populations will tend to reduce or eliminate transmission of vector-borne diseases. One potential method for environmentally-friendly, species-specific population control is the Sterile Insect Technique (SIT). SIT has not been widely used against insect disease vectors such as mosquitoes, in part because of various practical difficulties in rearing, sterilization and distribution. Additionally, vector populations with strong density-dependent effects will tend to be resistant to SIT-based control as the population-reducing effect of induced sterility will tend to be offset by reduced density-dependent mortality. RESULTS: We investigated by mathematical modeling the effect of manipulating the stage of development at which death occurs (lethal phase) in an SIT program against a density-dependence-limited insect population. We found late-acting lethality to be considerably more effective than early-acting lethality. No such strains of a vector insect have been described, so as a proof-of-principle we constructed a strain of the principal vector of the dengue and yellow fever viruses, Aedes (Stegomyia) aegypti, with the necessary properties of dominant, repressible, highly penetrant, late-acting lethality. CONCLUSION: Conventional SIT induces early-acting (embryonic) lethality, but genetic methods potentially allow the lethal phase to be tailored to the program. For insects with strong density-dependence, we show that lethality after the density-dependent phase would be a considerable improvement over conventional methods. For density-dependent parameters estimated from field data for Aedes aegypti, the critical release ratio for population elimination is modeled to be 27% to 540% greater for early-acting rather than late-acting lethality. Our success in developing a mosquito strain with the key features that the modeling indicated were desirable demonstrates the feasibility of this approach for improved SIT for disease control

Engineered Repressible Lethality for Controlling the Pink Bollworm, a Lepidopteran Pest of Cotton

<div><p>The sterile insect technique (SIT) is an environmentally friendly method of pest control in which insects are mass-produced, irradiated and released to mate with wild counterparts. SIT has been used to control major pest insects including the pink bollworm (<em>Pectinophora gossypiella</em> Saunders), a global pest of cotton. Transgenic technology has the potential to overcome disadvantages associated with the SIT, such as the damaging effects of radiation on released insects. A method called RIDL (Release of Insects carrying a Dominant Lethal) is designed to circumvent the need to irradiate insects before release. Premature death of insects’ progeny can be engineered to provide an equivalent to sterilisation. Moreover, this trait can be suppressed by the provision of a dietary antidote. In the pink bollworm, we generated transformed strains using different DNA constructs, which showed moderate-to-100% engineered mortality. In permissive conditions, this effect was largely suppressed. Survival data on cotton in field cages indicated that field conditions increase the lethal effect. One strain, called OX3402C, showed highly penetrant and highly repressible lethality, and was tested on host plants where its larvae caused minimal damage before death. These results highlight a potentially valuable insecticide-free tool against pink bollworm, and indicate its potential for development in other lepidopteran pests.</p> </div

A dominant lethal genetic system for autocidal control of the Mediterranean fruitfly.

The Sterile Insect Technique (SIT) used to control insect pests relies on the release of large numbers of radiation-sterilized insects. Irradiation can have a negative impact on the subsequent performance of the released insects and therefore on the cost and effectiveness of a control program. This and other problems associated with current SIT programs could be overcome by the use of recombinant DNA methods and molecular genetics. Here we describe the construction of strains of the Mediterranean fruit fly (medfly) harboring a tetracycline-repressible transactivator (tTA) that causes lethality in early developmental stages of the heterozygous progeny but has little effect on the survival of the parental transgenic tTA insects. We show that these properties should prove advantageous for the implementation of insect pest control programs

Schematic representation of the RIDL cassette (markers and <i>piggyBac</i> sequences omitted) of constructs used for transformation of pink bollworm: (a) OX1124; (b) OX3347; (c) OX3400; and (d) OX3402.

<p>Schematic representation of the RIDL cassette (markers and <i>piggyBac</i> sequences omitted) of constructs used for transformation of pink bollworm: (a) OX1124; (b) OX3347; (c) OX3400; and (d) OX3402.</p

Survival to adulthood of transgenic pink bollworm, heterozygous for an OX1124 insertion (line A, C, D or E), reared on larval diet with chlortetracycline (grey bars) or without chlortetracycline (white bars).

<p>Survival is expressed relative to that of each group’s wild-type siblings. Error bars indicate 95% confidence intervals.</p

Southern blot analysis of OX3402C.

<p>From left to right, lanes are: Dig-labelled DNA molecular weight marker II (M), OX3402C genomic DNA digested with <i>Bgl</i>II (A), <i>Nhe</i>I (B) and <i>Xho</i>I (C).</p

Impact of OX3402C larvae on cotton bolls in the laboratory.

<p>(a) typical marks on exterior of cotton bolls (indicated by arrows) caused by OX3402C larvae, and (b) minor damage to lint caused by a OX3402C larva that had penetrated the boll (indicated by arrow) but remained stunted at approximately 1 mm in length. Equivalent wild-type larvae had pupated at this time, after reaching approximately 10 mm in length.</p

Survival to adulthood of transgenic pink bollworm, heterozygous for an OX3402 insertion (line A, C, M, P, T or U), reared on larval diet with chlortetracycline (grey bars) or without chlortetracycline (not visible as all data points are zero).

<p>Survival is expressed relative to that of each group’s wild-type siblings. Error bars indicate 95% confidence intervals.</p