Interspecific hybridization yields strategy for South Pacific filariasis vector elimination

Abstract

Background: Lymphatic filariasis (LF) is a leading cause of disability in South Pacific regions, where >96% of the 1.7 million population are at risk of LF infection. As part of current global campaign, mass drug administration (MDA) has effectively reduced lymphatic filiariasis prevalence, but mosquito vector biology can complicate the MDA strategy. In some regions, there is evidence that the goal of LF elimination cannot be attained via MDA alone. Obligate vector mosquitoes provide additional targets for breaking the LF transmission cycle, but existing methods are ineffective for controlling the primary vector throughout much of the South Pacific, Aedes polynesiensis.

Methodology/principal findings: Here we demonstrate that interspecific hybridization and introgression results in an A. polynesiensis strain ('CP' strain) that is stably infected with the endosymbiotic Wolbachia bacteria from Aedes riversi. The CP strain is bi-directionally incompatible with naturally infected mosquitoes, resulting in female sterility. Laboratory assays demonstrate that CP males are equally competitive, resulting in population elimination when CP males are introduced into wild type A. polynesiensis populations.

Conclusions/significance: The findings demonstrate strategy feasibility and encourage field tests of the vector elimination strategy as a supplement to ongoing MDA efforts.

Figure 1. Introgression Strategy and PCR confirmation of Wolbachia infection type.

(A) Wolbachia infection type is indicated by symbol shading: gray-filled symbols represent the B type Wolbachia infection from A. riversi; the black-filled symbol represents the A type Wolbachia infection from A. polynesiensis (AP strain); unshaded symbols represent aposymbiotic individuals (APT strain). The theoretical percentage of A. polynesiensis genotype is shown as a percentage below symbols. The Wolbachia infection is maternally inherited, while the genotype is inherited from both parents. Repeated introgression of hybrid females with APT males results in the CP strain. (B) Wolbachia type-specific primers demonstrate that the CP strain infection type is the same as that of the original A. riversi strain (AR) and differs from the infection that naturally occurs in A. polynesiensis (AP). Wolbachia type-specific primers used in amplification reactions are indicated above the horizontal lines. STD is the molecular weight marker: 1kb DNA Ladder Plus (Fermentas Inc., Hanover, MD).

Figure 2. Microsatellite allelic frequencies.

Panels represent the allelic frequencies of the six microsatellite loci (Loci 1-6), which are examined for each mosquito strain (AP, AR and CP). In each panel, the Y-axis denotes allelic frequencies and the x-axis denotes the size of each fragment in base pairs. Loci 1-6 correspond to previously reported microsatellite loci AP1-AP6 (naming modified here to avoid ambiguity with mosquito strains).

Figure 3. A. polynesiensis suppression strategy and CP male competitiveness.

Based upon the observed egg hatch rate, females are scored as either ‘compatible mating’ ( = eggs hatching) or ‘incompatible mating’ ( = eggs not hatching). The percent compatible females (compatible females/total females) is determined for each cage replicate (10 females/cage; 5 cage replicates/treatment). Circles and bars indicate the mean±standard deviation for each treatment (i.e., male ratio). The trend line (solid line) with 95% confidence intervals (dotted lines) are generated based upon the observed values. Predicted values (dashed line) are calculated assuming equal competitiveness of CP and AP males. Below the graph, egg hatch rates are based upon the combined oviposition of females within cages. Differing superscripted letters indicate significant differences (Two-tailed, Mann-Whitney, P<0.01, Bonferroni corrected). R² value is shown for the trend line fitted to observations.