General R&R dynamics.

<p>Dynamics of an <i>Ae. aegypti</i> population when continuous male-only R&R releases occur for one year (<i>T</i> = 365). (A) Relative female population density for releases occurring at four different release ratios: <i>r</i> = 1 (blue), <i>r</i> = 2 (green), <i>r</i> = 3 (brown), and <i>r</i> = 4 (black). Dashed lines indicate the relative density of competent vectors, and solid lines represent the relative density of the total adult female population. Solid lines also indicate the relative density of the total (and thus competent) adult female population during FK releases. Density is relative to the density of the total adult female population before releases begin. Note the vertical axis is on a log scale. (B) Juvenile FK (yellow) and AP (purple) allele frequencies for a 2∶1 (<i>r</i> = 2) release. For both panels, the first vertical dashed line represents the first day of release (<i>t</i> = 30) and the second vertical dashed line represents the last day of release (<i>t</i> = 395). All other parameter values are the default values listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0073233#pone-0073233-t002" target="_blank">Table 2</a>.</p

Releases including females.

<p>Relative female population density when releases are conducted at a 1∶1 (<i>r</i> = 1) release ratio for <i>T</i> = 100 days with releases of only males (blue), males and females (brown), and females only (black). (A) Relative density of competent vectors. (B) Relative density of total adult females (including released females). All other parameter values are the default values listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0073233#pone-0073233-t002" target="_blank">Table 2</a>. Note the vertical axis is on a log scale.</p

Fitness cost.

<p>Dynamics of an <i>Ae. aegypti</i> population subject to continuous male-only R&R releases at a 2∶1 (<i>r</i> = 2) release ratio for one year (<i>T</i> = 365) when there is an additive fitness cost associated with the AP allele. The fitness costs considered here are <i>c</i>_A = 0 (black), <i>c</i>_A = 0.1 (yellow), and <i>c</i>_A = 0.2 (green), where <i>c</i>_A is the fitness cost of carrying two AP alleles. (A) Relative density of competent vectors. Note the vertical axis is on a log scale. (B) AP allele frequency in the juvenile population. All other parameter values are the default values listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0073233#pone-0073233-t002" target="_blank">Table 2</a>.</p

A Reduce and Replace Strategy for Suppressing Vector-Borne Diseases: Insights from a Stochastic, Spatial Model

<div><p>Two basic strategies have been proposed for using transgenic <i>Aedes aegypti</i> mosquitoes to decrease dengue virus transmission: population reduction and population replacement. Here we model releases of a strain of <i>Ae. aegypti</i> carrying both a gene causing conditional adult female mortality and a gene blocking virus transmission into a wild population to assess whether such releases could reduce the number of competent vectors. We find this “reduce and replace” strategy can decrease the frequency of competent vectors below 50% two years after releases end. Therefore, this combined approach appears preferable to releasing a strain carrying only a female-killing gene, which is likely to merely result in temporary population suppression. However, the fixation of anti-pathogen genes in the population is unlikely. Genetic drift at small population sizes and the spatially heterogeneous nature of the population recovery after releases end prevent complete replacement of the competent vector population. Furthermore, releasing more individuals can be counter-productive in the face of immigration by wild-type mosquitoes, as greater population reduction amplifies the impact wild-type migrants have on the long-term frequency of the anti-pathogen gene. We expect the results presented here to give pause to expectations for driving an anti-pathogen construct to fixation by relying on releasing individuals carrying this two-gene construct. Nevertheless, in some dengue-endemic environments, a spatially heterogeneous decrease in competent vectors may still facilitate decreasing disease incidence.</p></div

Release duration.

<p>Relative adult female population density when continuous male-only R&R releases occur at a 2∶1 (<i>r</i> = 2) release ratio for different release durations. Dashed lines indicate the relative density of the competent vectors, and solid lines indicate the relative density of the total adult female population. Each release begins on day 30, and release durations are <i>T</i> = 120 (yellow), <i>T</i> = 240 (green), and <i>T</i> = 360 (purple) days. The black vertical dashed line marks the beginning of releases, and the end of each release is indicated by a vertical dashed line of corresponding color. All other parameter values are the default values listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0073233#pone-0073233-t002" target="_blank">Table 2</a>. Note that the vertical axis is on a log scale.</p

Parameter values calibrated to Iquitos, Peru. For details, see [21].

<p>Parameter values calibrated to Iquitos, Peru. For details, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081860#pone.0081860-Legros2" target="_blank">[21]</a>.</p

The frequency distribution across model runs for adult females carrying an anti-pathogen gene when adult male transgenic mosquitoes were released in equal numbers at all sites.

<p>Bars facing left describe results for an anti-pathogen gene that lacks a fitness cost; bars facing right describe results for an anti-pathogen gene that carries a fitness cost (5% reduction in the probability of survival per gene). For both left and right-facing bars, the frequency of adult females carrying an anti-pathogen gene increases vertically. Model runs resulting in population extinction are not shown on the histogram. Each panel describes the frequencies resulting from release programs with weekly releases at all sites for (A) a single year of male-only releases, (B) three years of male-only releases, and (C) a single year with both male and female releases. Lines between the frequency bars represent the average frequency of adult females carrying an anti-pathogen gene across runs that do not result in extinction. The first number at the top of each frequency bar represents the proportion (out of 30) of runs resulting in population extinctions without fitness costs associated with the anti-pathogen gene; the second number represents the proportion of runs (out of 30) resulting in population extinction with fitness costs associated with the anti-pathogen gene. For extinction frequencies larger than 0.1, the values have been rounded to one decimal place. Unlike the model runs illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081860#pone-0081860-g002" target="_blank">Figures 2</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081860#pone-0081860-g003" target="_blank">3</a>, each model run represents a different, randomized spatial configuration of sites. Releasing one male and one female per site per week for three years always resulted in extinction. For all release strategies, increasing the number of individuals released can increase the average frequency of carriers of the anti-pathogen gene, but also increases the variability across model runs. The total release numbers range from approximately 250,000 to 1.5 million adult mosquitoes for single year releases and from approximately 750,000 to 2.3 million adult mosquitoes for three year releases (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081860#pone-0081860-t004" target="_blank">Table 4</a>).</p

Feasible Introgression of an Anti-pathogen Transgene into an Urban Mosquito Population without Using Gene-Drive

<div><p>Background</p><p>Introgressing anti-pathogen constructs into wild vector populations could reduce disease transmission. It is generally assumed that such introgression would require linking an anti-pathogen gene with a selfish genetic element or similar technologies. Yet none of the proposed transgenic anti-pathogen gene-drive mechanisms are likely to be implemented as public health measures in the near future. Thus, much attention now focuses instead on transgenic strategies aimed at mosquito population suppression, an approach generally perceived to be practical. By contrast, aiming to replace vector competent mosquito populations with vector incompetent populations by releasing mosquitoes carrying a single anti-pathogen gene without a gene-drive mechanism is widely considered impractical.</p><p>Methodology/Principal Findings</p><p>Here we use Skeeter Buster, a previously published stochastic, spatially explicit model of <i>Aedes aegypti</i> to investigate whether a number of approaches for releasing mosquitoes with only an anti-pathogen construct would be efficient and effective in the tropical city of Iquitos, Peru. To assess the performance of such releases using realistic release numbers, we compare the transient and long-term effects of this strategy with two other genetic control strategies that have been developed in <i>Ae. aegypti</i>: release of a strain with female-specific lethality, and a strain with both female-specific lethality and an anti-pathogen gene. We find that releasing mosquitoes carrying only an anti-pathogen construct can substantially decrease vector competence of a natural population, even at release ratios well below that required for the two currently feasible alternatives that rely on population reduction. Finally, although current genetic control strategies based on population reduction are compromised by immigration of wild-type mosquitoes, releasing mosquitoes carrying only an anti-pathogen gene is considerably more robust to such immigration.</p><p>Conclusions/Significance</p><p>Contrary to the widely held view that transgenic control programs aimed at population replacement require linking an anti-pathogen gene to selfish genetic elements, we find releasing mosquitoes in numbers much smaller than those considered necessary for transgenic population reduction can result in comparatively rapid and robust population replacement. In light of this non-intuitive result, directing efforts to improve rearing capacity and logistical support for implementing releases, and reducing the fitness costs of existing recombinant technologies, may provide a viable, alternative route to introgressing anti-pathogen transgenes under field conditions.</p></div

The effect of immigration on the frequency of adult females carrying an anti-pathogen gene two years after releases end under a point-source release of adult male transgenic mosquitoes with weekly releases of (A) 80 males released per site, with the anti-pathogen gene carrying a fitness cost, (B) 80 males released per site, with no fitness cost, (C) 160 males released per site, with the anti-pathogen gene carrying a fitness cost, and (D) 160 males released per site, with no fitness cost.

<p>These release numbers represent values that resulted in relatively low (80 males per site each week) and high (160 males released per site each week) levels of variability across replicate simulations in the frequencies of the anti-pathogen gene (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081860#pone-0081860-g005" target="_blank">Fig. 5A</a>). We focused our analysis of the effects of immigration on release numbers where population extinction never occurred. These release numbers correspond to total release numbers of approximately 1 million (A–B) to 2 million (C–D) transgenic adult males over a single year. Only gravid wild-type females are assumed to migrate into the system. Although removing the fitness cost can reduce the impact of immigration, increasing the number of mosquitoes released amplifies the ability of immigration to counteract the replacement strategy. With immigration, none of the model runs result in extinction across the simulated region. As in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081860#pone-0081860-g004" target="_blank">figures 4</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081860#pone-0081860-g006" target="_blank">6</a>, each model run represents a different, randomized spatial configuration of sites.</p

Model parameters.

<p>Description of model parameters with default values and references for default values.</p