Effects of food variability on growth and reproduction of Aedes aegypti

Despite a large body of knowledge about the evolution of life histories, we know little about how variable food availability during an individual's development affects its life history. We measured the effects of manipulating food levels during early and late larval development of the mosquito Aedes aegypti on its growth rate, life history and reproductive success. Switching from low to high food led to compensatory growth: individuals grew more rapidly during late larval development and emerged at a size close to that of mosquitoes consistently reared at high food. However, switching to high food had very little effect on longevity, and fecundity and reproductive success were considerably lower than in consistently well‐fed mosquitoes. Changing from high to low food led to adults with similar size as in consistently badly nourished mosquitoes, but even lower fecundity and reproductive success. A rapid response of growth to changing resources can thus have unexpected effects in later life and in lifetime reproductive success. More generally, our study emphasizes the importance of varying developmental conditions for the evolutionary pressures underlying life‐history evolution.     

Heterogeneous genetic invasions of three insecticide resistance mutations in Indo‐Pacific populations of Aedes aegypti (L.)

Nations throughout the Indo‐Pacific region use pyrethroid insecticides to control Aedes aegypti, the mosquito vector of dengue, often without knowledge of pyrethroid resistance status of the pest or origin of resistance. Two mutations (V1016G + F1534C) in the sodium channel gene (Vssc) of Ae. aegypti modify ion channel function and cause target‐site resistance to pyrethroid insecticides, with a third mutation (S989P) having a potential additive effect. Of 27 possible genotypes involving these mutations, some allelic combinations are never seen whereas others predominate. Here, five allelic combinations common in Ae. aegypti from the Indo‐Pacific region are described and their geographical distributions investigated using genome‐wide SNP markers. We tested the hypothesis that resistance allele combinations evolved de novo in populations versus the alternative that dispersal of Ae. aegypti between populations facilitated genetic invasions of allele combinations. We used latent factor mixed‐models to detect SNPs throughout the genome that showed structuring in line with resistance allele combinations and compared variation at SNPs within the Vssc gene with genome‐wide variation. Mixed‐models detected an array of SNPs linked to resistance allele combinations, all located within or in close proximity to the Vssc gene. Variation at SNPs within the Vssc gene was structured by resistance profile, whereas genome‐wide SNPs were structured by population. These results demonstrate that alleles near to resistance mutations have been transferred between populations via linked selection. This indicates that genetic invasions have contributed to the widespread occurrence of Vssc allele combinations in Ae. aegypti in the Indo‐Pacific region, pointing to undocumented mosquito invasions between countries.     

Wolbachia pipientis occurs in Aedes aegypti populations in New Mexico and Florida,USA

The mosquitoes Aedes aegypti (L.) and Ae. albopictus Skuse are the major vectors of dengue, Zika, yellow fever, and chikungunya viruses worldwide. Wolbachia, an endosymbiotic bacterium present in many insects, is being utilized in novel vector control strategies to manipulate mosquito life history and vector competence to curb virus transmission. Earlier studies have found that Wolbachia is commonly detected in Ae. albopictus but rarely detected in Ae. aegypti. In this study, we used a two‐step PCR assay to detect Wolbachia in wild‐collected samples of Ae. aegypti. The PCR products were sequenced to validate amplicons and identify Wolbachia strains. A loop‐mediated isothermal amplification (LAMP) assay was developed and used for detecting Wolbachia in selected mosquito specimens as well. We found Wolbachia in 85/148 (57.4%) wild Ae. aegypti specimens from various cities in New Mexico, and in 2/46 (4.3%) from St. Augustine, Florida. Wolbachia was not detected in 94 samples of Ae. aegypti from Deer Park, Harris County, Texas. Wolbachia detected in Ae. aegypti from both New Mexico and Florida was the wAlbB strain of Wolbachia pipientis. A Wolbachia‐positive colony of Ae. aegypti was established from pupae collected in Las Cruces, New Mexico, in 2018. The infected females of this strain transmitted Wolbachia to their progeny when crossed with males of Rockefeller strain of Ae. aegypti, which does not carry Wolbachia. In contrast, none of the progeny of Las Cruces males mated to Rockefeller females were infected with Wolbachia.     

Population structure of a vector of human diseases: Aedes aegypti in its ancestral range,Africa

Aedes aegypti, the major vector of dengue, yellow fever, chikungunya, and Zika viruses, remains of great medical and public health concern. There is little doubt that the ancestral home of the species is Africa. This mosquito invaded the New World 400‐500 years ago and later, Asia. However, little is known about the genetic structure and history of Ae. aegypti across Africa, as well as the possible origin(s) of the New World invasion. Here, we use ~17,000 genome‐wide single nucleotide polymorphisms (SNPs) to characterize a heretofore undocumented complex picture of this mosquito across its ancestral range in Africa. We find signatures of human‐assisted migrations, connectivity across long distances in sylvan populations, and of local admixture between domestic and sylvan populations. Finally, through a phylogenetic analysis combined with the genetic structure analyses, we suggest West Africa and especially Angola as the source of the New World's invasion, a scenario that fits well with the historic record of 16th‐century slave trade between Africa and Americas.     

Temporal genetic stability of Stegomyia aegypti (= Aedes aegypti) populations

The mosquito Stegomyia aegypti (= Aedes aegypti) (Diptera: Culicidae) is the primary vector of viruses that cause yellow fever, dengue and Chikungunya fever. In the absence of effective vaccines, the reduction of these diseases relies on vector control strategies. The success of these strategies is tightly linked to the population dynamics of target populations. In the present study, 14 collections from St. aegypti populations separated by periods of 1–13 years were analysed to determine their temporal genetic stability. Although temporal structure is discernible in most populations, the degree of temporal differentiation is dependent on the population and does not obscure the geographic structure of the various populations. The results suggest that performing detailed studies in the years prior to and after population reduction‐ or modification‐based control interventions at each target field site may be useful in assessing the probability of success.     

Sunshine versus gold: The effect of population age on genetic structure of an invasive mosquito

The genetic diversity and structure of invasive species are affected by the time since invasion, but it is not well understood how. We compare likely the oldest populations of Aedes aegypti in continental North America with some of the newest to illuminate the range of genetic diversity and structure that can be found within the invasive range of this important disease vector. Aedes aegypti populations in Florida have probably persisted since the 1600‐1700s, while populations in southern California derive from new invasions that occurred in the last 10 years. For this comparison, we genotyped 1,193 individuals from 28 sites at 12 highly variable microsatellites and a subset of these individuals at 23,961 single nucleotide polymorphisms (SNPs). This is the largest sample analyzed for genetic structure for either region, and it doubles the number of southern California populations previously analyzed. As predicted, the older populations (Florida) showed fewer indicators of recent founder effect and bottlenecks; in particular, these populations have dramatically higher genetic diversity and lower genetic structure. Geographic distance and driving distance were not good predictors of genetic distance in either region, especially southern California. Additionally, southern California had higher levels of genetic differentiation than any comparably sized documented region throughout the worldwide distribution of the species. Although population age and demographic history are likely driving these differences, differences in climate and transportation practices could also play a role.     

Worldwide patterns of genetic differentiation imply multiple 'domestications' of Aedes aegypti, a major vector of human diseases

Understanding the processes by which species colonize and adapt to human habitats is particularly important in the case of disease-vectoring arthropods. The mosquito species Aedes aegypti, a major vector of dengue and yellow fever viruses, probably originated as a wild, zoophilic species in sub-Saharan Africa, where some populations still breed in tree holes in forested habitats. Many populations of the species, however, have evolved to thrive in human habitats and to bite humans. This includes some populations within Africa as well as almost all those outside Africa. It is not clear whether all domestic populations are genetically related and represent a single 'domestication' event, or whether association with human habitats has developed multiple times independently within the species. To test the hypotheses above, we screened 24 worldwide population samples of Ae. aegypti at 12 polymorphic microsatellite loci. We identified two distinct genetic clusters: one included all domestic populations outside of Africa and the other included both domestic and forest populations within Africa. This suggests that human association in Africa occurred independently from that in domestic populations across the rest of the world. Additionally, measures of genetic diversity support Ae. aegypti in Africa as the ancestral form of the species. Individuals from domestic populations outside Africa can reliably be assigned back to their population of origin, which will help determine the origins of new introductions of Ae. aegypti.     

Effects of intraspecific larval competition on adult longevity in the mosquitoes Aedes aegypti and Aedes albopictus

Abstract Larval competition is common in container‐breeding mosquitoes. The impact of competition on larval growth has been thoroughly examined and findings that larval competition can lead to density‐dependent effects on adult body size have been documented. The effects of larval competition on adult longevity have been less well explored. The effects of intraspecific larval densities on the longevity of adults maintained under relatively harsh environmental conditions were tested in the laboratory by measuring the longevity of adult Aedes aegypti (L.) and Aedes albopictus (Skuse) (Diptera: Culicidae) that had been reared under a range of larval densities and subsequently maintained in high‐ or low‐humidity regimes (85% or 35% relative humidity [RH], respectively) as adults. We found significant negative effects of competition on adult longevity in Ae. aegypti, but not in Ae. albopictus. Multivariate analysis of variance suggested that the negative effect of the larval environment on the longevity of Ae. aegypti adults was most strongly associated with increased development time and decreased wing length as adults. Understanding how larval competition affects adult longevity under a range of environmental conditions is important in establishing the relationship between models of mosquito population regulation and epidemiological models of vector‐borne disease transmission.     

An elusive endosymbiont: Does Wolbachia occur naturally in Aedes aegypti?

Wolbachia are maternally inherited endosymbiotic bacteria found within many insect species. Aedes mosquitoes experimentally infected with Wolbachia are being released into the field for Aedes‐borne disease control. These Wolbachia infections induce cytoplasmic incompatibility which is used to suppress populations through incompatible matings or replace populations through the reproductive advantage provided by this mechanism. However, the presence of naturally occurring Wolbachia in target populations could interfere with both population replacement and suppression programs depending on the compatibility patterns between strains. Aedes aegypti were thought to not harbor Wolbachia naturally but several recent studies have detected Wolbachia in natural populations of this mosquito. We therefore review the evidence for natural Wolbachia infections in A. aegypti to date and discuss limitations of these studies. We draw on research from other mosquito species to outline the potential implications of natural Wolbachia infections in A. aegypti for disease control. To validate previous reports, we obtained a laboratory population of A. aegypti from New Mexico, USA, that harbors a natural Wolbachia infection, and we conducted field surveys in Kuala Lumpur, Malaysia, where a natural Wolbachia infection has also been reported. However, we were unable to detect Wolbachia in both the laboratory and field populations. Because the presence of naturally occurring Wolbachia in A. aegypti could have profound implications for Wolbachia‐based disease control programs, it is important to continue to accurately assess the Wolbachia status of target Aedes populations.     

Higher genetic diversity on mountain tops: the role of historical and contemporary processes in shaping genetic variation in the bank vole

Glacial phases during the Pleistocene caused remarkable changes in species range distributions, with inevitable genetic consequences. Specifically, during interglacial phases, when the ice melted and new habitats became suitable again, species could recolonize regions that were previously covered by ice, such as high latitudes and elevations. Based on theoretical models and empirical data, a decrease in genetic variation is predicted along recolonization routes as a result of the consecutive founder effects that characterize the recolonization process. In the present study, we assessed the relative importance of historical and contemporary processes in shaping genetic diversity and differentiation of bank vole (Myodes glareolus) populations at different elevations in the Swiss Alps. By contrast to expectations, we found that genetic variation increased with elevation. Estimates of recent migration rates and a contrasting pattern of genetic differentiation observed at the mitochondrial cytochrome b gene and nuclear microsatellites support the hypothesis that higher genetic diversity at high elevation results from contemporary gene flow. Although historical recolonization processes can have marked effects on the genetic structure of populations, the present study provides an example where contemporary processes along an environmental gradient can reverse predicted patterns of genetic variation.     

Isolation and variability of polymorphic microsatellite loci in Aedes aegypti,the vector of dengue viruses

We isolated five microsatellite sequences from an enriched‐(CAA)_n library of 5000 recombinant clones in Aedes aegypti. Two polymorphic microsatellites from our library and four from other sequence databases were tested: we investigated their polymorphism and Mendelian inheritance in mosquito populations. Our results indicate that trinucleotide repeat markers could be used to differentiate Ae. aegypti populations, making them valuable tools for the study of population genetic structure.     

白纹伊蚊和埃及伊蚊经卵传递登革病毒的研究

白纹伊蚊和埃及伊蚊通过吸食病毒液或叮吸有病毒血症的小鸡血后,能感染登革１－４型病毒,并能在蚊体内增殖,对感染雌蚊了１和子２代幼虫,雌性或雄性成虫４５５９只,分１０１批进行了病毒检测,白纹伊蚊子１代的批阳性率;登革１型为１０％（１／１０）２型为２２．２２％（２／９）３型为３３．３３％（４／１２）,４型为２８．９５％（１１／３８）登革１～４型的最低子代感染率依次主国０．２０％,０．７１％,０．７０％和     

Aedes aegypti lachesin protein binds to the domain III of envelop protein of Dengue virus‐2 and inhibits viral replication

Dengue virus (DENV) comprises of four serotypes (DENV‐1 to ‐4) and is medically one of the most important arboviruses (arthropod‐borne virus). DENV infection is a major human health burden and is transmitted between humans by the insect vector, Aedes aegypti. Ae. aegypti ingests DENV while feeding on infected humans, which traverses through its gut, haemolymph and salivary glands of the mosquito before being injected into a healthy human. During this process of transmission, DENV must interact with many proteins of the insect vector, which are important for its successful transmission. Our study focused on the identification and characterisation of interacting protein partners in Ae. aegypti to DENV. Since domain III (DIII) of envelope protein (E) is exposed on the virion surface and is involved in virus entry into various cells, we performed phage display library screening against domain III of the envelope protein (EDIII) of DENV‐2. A peptide sequence showing similarity to lachesin protein was found interacting with EDIII. The lachesin protein was cloned, heterologously expressed, purified and used for in vitro interaction studies. Lachesin protein interacted with EDIII and also with DENV. Further, lachesin protein was localised in neuronal cells of different organs of Ae. aegypti by confocal microscopy. Blocking of lachesin protein in Ae. aegypti with anti‐lachesin antibody resulted in a significant reduction in DENV replication.     

Paternal phylogeography and genetic diversity of East Asian goats

This study was a first analysis of paternal genetic diversity for extensive Asian domestic goats using SRY gene sequences. Sequencing comparison of the SRY 3′‐untranslated region among 210 Asian goats revealed four haplotypes (Y1A, Y1B, Y2A and Y2B) derived from four variable sites including a novel substitution detected in this study. In Asian goats, the predominant haplotype was Y1A (62%) and second most common was Y2B (30%). Interestingly, the Y2B was a unique East Asian Y chromosomal variant, which differentiates eastern and western Eurasian goats. The SRY geographic distribution in Myanmar and Cambodia indicated predominant the haplotype Y1A in plains areas and a high frequency of Y2B in mountain areas. The results suggest recent genetic infiltration of modern breeds into South‐East Asian goats and an ancestral SRY Y2B haplotype in Asian native goats.     

Production of the bioinsecticide Bacillus thuringiensis subsp. israelensis with deltamethrin increases toxicity towards mosquito larvae

Bacillus thuringiensis subsp. israelensis is a bioinsecticide used for larval mosquito control and it represents a safe alternative to chemical insecticides. Despite its environmental safety, it is less efficient and persistent than chemical insecticides. To bypass these limitations, we propose to combine the advantages of chemical and biological insecticides by producing Bti in a medium supplemented with a chemical insecticide (DDT, deltamethrin, permethrin, propoxur or temephos). Among the investigated insecticides, the addition of deltamethrin in the medium induced a higher toxicity (over 6·72‐fold) of the composite deltamethrin‐Bti towards mosquito larvae as compared to Bti alone. This was mainly due to the insertion of deltamethrin into the membranes of Bti spores, as evidenced by a quantification of membrane‐extracted deltamethrin by HPLC. This composite larvicide is a promising tool to decrease the quantity of chemicals dispersed in the environment, to increase the efficacy of Bti and to facilitate its widespread use as a transition between chemical and biological insecticides. Further experiments are required to characterize the mechanisms that underline the incorporation of deltamethrin into Bti to optimize the production and the toxicity of this composite larvicide.

Significance and Impact of the Study

This study is the first report of an increased efficacy of the mosquitocidal bioinsecticide Bacillus thuringiensis subsp. israelensis (Bti) when produced with a chemical insecticide. The results clearly demonstrate that deltamethrin is able to synergize the insecticidal activity of Bti through inclusion into spore membranes, reducing off‐target and nonspecific toxicity occurring when the chemical is used alone as sprays. This new composite chemical–biological insecticide can become an invaluable tool as an intermediate between single chemical usage and the widespread use of Bti, notably in developing countries with limited financial resources for intensive mosquito control campaigns.     

Genetic differentiation of the dengue vector, Aedes aegypti (Ho Chi Minh City, Vietnam) using microsatellite markers

Dengue haemorrhagic fever emerged in the 1950s and has become a major public health concern in most Asian countries. In Vietnam, little is known about the intraspecific variation of the vector and its consequences on vectorial capacity. Here we report the use of microsatellite markers to differentiate Aedes aegypti populations in Ho Chi Minh City, a typical, overcrowded Asian city. Six microsatellite loci, with 5-14 alleles per locus, were scored in 20 mosquito samples collected in 1998 in Ho Chi Minh City. We found substantial differentiation among Ae. aegypti populations from the outskirts, whereas populations from the centre of the city showed less differentiation. These results are consistent with the hypothesis that populations of Ae. aegypti in central Ho Chi Minh City are panmictic because there are abundant larval breeding sites and an abundance of humans for adults to feed upon. In contrast, populations on the outskirts become differentiated largely through the processes of genetic drift because larval breeding sites are not as abundant. These findings implicate human activities associated with urbanization, as factors shaping the genetic structure of Ae. aegypti populations.     

Lessons of Aedes aegypti control in Thailand

Abstract. The incidence of dengue haemorrhagic fever (DHF) in Thailand has increased cyclically since the first recognized outbreak in 1958. Without an effective vaccine against dengue, and considering the clinical difficulty of treating DHF cases, vector control is needed to prevent dengue transmission. Since the establishment of the WHO Aedes Research Unit in 1964, continued since 1973 as the WHO Collaborating Centre at the Department of Medical Research in Bangkok, much operational research has been carried out in Thailand on the bionomics and control of dengue vectors: Aedes aegypti and Ae.albopictus. This review shows that, as in most other countries, dengue vector control programmes in Thailand make little use of the procedures arising from research, nor have they reduced the upward trend of dengue or prevented DHF outbreaks. Implications of the reluctance to use results of operational research on vector control are considered and remedial suggestions made.     

Laboratory evalution of a translocation double heterozygote for genetic control of Aedes aegypti

Summary Two pure translocation homozygote stocks, T₁/T₁ and T₃/T₃, were used to produce a double translocation heterozygote system designated T₁/T₃, employing T₁/T₁ as the male and T₃/T₃ as the female parent. The double heterozygote showed 73 % sterility when mated to wild females. Tests on mating competitiveness, recombination frequency in the differential segment, insemination rate and inheritance of sterility after release, for four generations in laboratory cages, have been carried out to evaluate the efficiency of this strain as an agent for a population control programme.