Quantifying the efficacy of genetic shifting in control of mosquito‐borne diseases

Abstract

Many of the world's most prevalent diseases are transmitted by animal vectors such as dengue transmitted by mosquitoes. To reduce these vector‐borne diseases, a promising approach is “genetic shifting”: selective breeding of the vectors to be more resistant to pathogens and releasing them to the target populations to reduce their ability to transmit pathogens, that is, lower their vector competence. The efficacy of genetic shifting will depend on possible counterforces such as natural selection against low vector competence. To quantitatively evaluate the potential efficacy of genetic shifting, we developed a series of coupled genetic–demographic models that simulate the changes of vector competence during releases of individuals with low vector competence. We modeled vector competence using different genetic architectures, as a multilocus, one‐locus, or two‐locus trait. Using empirically determined estimates of model parameters, the model predicted a reduction of mean vector competence of at least three standard deviations after 20 releases, one release per generation, and 10% of the size of the target population released each time. Sensitivity analysis suggested that release efficacy depends mostly on the vector competence of the released population, release size, release frequency, and the survivorship of the released individuals, with duration of the release program less important. Natural processes such as density‐dependent survival and immigration from external populations also strongly influence release efficacy. Among different sex‐dependent release strategies, releasing blood‐fed females together with males resulted in the highest release efficacy, as these females mate in captivity and reproduce when released, thus contributing a greater proportion of low‐vector‐competence offspring. Conclusions were generally consistent across three models assuming different genetic architectures of vector competence, suggesting that genetic shifting could generally apply to various vector systems and does not require detailed knowledge of the number of loci contributing to vector competence.

Genetic Variation in Insect Vectors: Death of Typology?

“…Because no new genetic material is introduced, just increasing frequencies of already existing refractory alleles, this method has been called “genetic shifting”. Recent modeling has indicated that size and number of releases needed to significantly lower the ability of the target population to transmit are realistically achievable [77]. Because the release strain is identical to genotypes already present, mating discrimination is less likely to evolve and if it does, new strains for release could be re-selected from the target population.…”

Increasing Effectiveness of Genetically Modifying Mosquito Populations: Risk Assessment of Releasing Blood-Fed Females

“…We assume the released mosquitoes are 5 days old when starting the simulation (i.e., time spent in the laboratory/facility before being released), and that they have a lower VC ( Table 2 ). 7 Blood-fed mosquito females enter the simulation at the beginning of the oviposition phase, in contrast to the non-fed and wild mosquitoes which begin in the biting phase ( Figure 1B ). Although we recognize that Ae.…”

“…For example, Robert et al 6 showed that bi-sex or female-only releases have higher efficiency at reducing mosquito population density. Recently, Xia et al 7 modeled a proposed scheme, first made by Powell and Tabachnick, 8 and named “genetic shifting,” to genetically modify natural mosquito populations by releasing mosquitoes selected in the laboratory to be refractory to transmission using classical artificial selection methods. The capacity of transmitting diseases in mosquitoes is at least partially genetically determined, 9 , 10 so releasing refractory mosquitoes could alter the gene pools in the target mosquitoes and “shift” the subsequent generations to be less competent.…”

“…The capacity of transmitting diseases in mosquitoes is at least partially genetically determined, 9 , 10 so releasing refractory mosquitoes could alter the gene pools in the target mosquitoes and “shift” the subsequent generations to be less competent. 8 Xia et al 7 considered several release strategies, including the standard one of releasing only males, but also a scheme to release females that had been blood-fed before being released along with males. This new releasing scheme was shown to greatly increase the efficiency of the genetic modification of the target population.…”

“…This is due to both doubling the release numbers by including both sexes and pre-fed females being primed to directly contribute on the order of 50–100 eggs to the target population, multiplying the effect of each released female. 7 …”