Comparative studies of stenogamous behaviour in the mosquito Culex pipiens complex

Are the Culex pipiens biotypes pipiens, molestus and their hybrids competent vectors of avian Plasmodium?

The common house mosquito Culex pipiens s.l., widely distributed in Europe, Africa, and North America has two recognized biotypes, Cx. pipiens biotype pipiens and Cx. pipiens biotype molestus, which hybridize. Despite their morphological similarities, these biotypes may exhibit ecological differences. This complex ecological mosaic may affect the interaction of Cx. pipiens with pathogens like avian Plasmodium, which is transmitted to wildlife. Although the vector competence for Cx. pipiens biotype molestus has been well studied, there is a lack of studies comparing the vector competence of Cx. pipiens biotype pipiens and their hybrids for the transmission of avian Plasmodium. Here, we evaluated the vector competence of the Cx. pipiens biotypes pipiens, molestus and their hybrids for the transmission of two avian Plasmodium species. Mosquitoes were allowed to feed on blood of wild infected birds and the presence of DNA of Plasmodium in head-thorax and saliva of mosquitoes was molecularly evaluated at 13 day-post exposure. The transmission rates (i.e., the detection of parasite DNA in mosquito saliva) for Plasmodium cathemerium were similar for the two biotypes of Cx. pipiens and their hybrids while Plasmodium relictum DNA was only found in the saliva of Cx. pipiens biotype pipiens. In addition, P. cathemerium was significantly more prevalent than P. relictum in the saliva of Cx. pipiens biotype pipiens. Our results suggest that avian Plasmodium is transmitted by both Cx. pipiens biotypes and their hybrids although differences could be found depending of the parasite species studied. Differences in the abundance of each biotype and their hybrids within areas characterized by distinct environmental conditions, along with variations in their blood-feeding patterns and the parasites infecting birds, could ultimately determine differences in the relevance of each Cx. pipiens biotype in the transmission of avian Plasmodium.

Pupal Exuviae of Culex Pipiens L. (Diptera: Culicidae) Can be Utilised as a Non-Invasive Method of Biotype Differentiation

BACKGROUND

Culex pipiens L. is a principal vector of zoonotic arboviruses in Europe, acting in both an amplification role in enzootic transmission between avian hosts and as a bridge vector between avian hosts and mammals. The species consists of two forms which are indistinguishable using morphological methods but possess varying ecological and physiological traits that influence their vector capacity. In this study we validate methods that can be used to extract trace DNA from single pupal exuviae of Cx. pipiens for use in molecular speciation of samples. These DNA extraction methods are compared using measurement of the total yield and successful identification using a real-time polymerase chain reaction (PCR) assay.

RESULTS

Genomic DNA was initially extracted from colony-derived individuals using an ethanol precipitation method, two commercially available DNA extraction kits: DNeasy® Blood & Tissue Kit (Qiagen, UK) and Wizard® SV Genomic DNA Purification System (Promega, UK) and a direct real-time PCR method. Time elapsed between eclosion and processing of pupae significantly influenced Cx. pipiens form identification as nucleic acid concentration and PCR amplification success decreased with increased time elapsed. Real-time PCR amplification success, however, was not shown to vary significantly between the three extraction methods, with all methods successfully identifying all samples, but the direct real-time PCR method achieved a lesser amplification success rate of 70% (n = 20 for each treatment). More variable results were produced when field-derived exuviae were used, with no significant difference in real-time PCR amplification success found across the four methods and a lower overall rate of successful identification of 55-80%.

CONCLUSIONS

This study shows that both colony and field derived Cx. pipiens pupal exuviae can be a useful non-invasive source of trace DNA permitting accurate biotype differentiation for at least twenty-four hours post-eclosion. The significance and utility of this technique in ecological and behavioural studies of Cx. pipiens is discussed and recommendations made for use according to experimental scenario.

Transcriptome profiling reveals sex-specific gene expressions in pupal and adult stages of the mosquito Culex pipiens

Understanding the development process of male and female mosquitoes provides important basic information for sterile insect release programmes and is important for improving other vector control strategies. However, little is known about the molecular mechanisms that distinguish male from female-specific developmental processes in this species. We used IlluminaRNA-seq to identify sex-specific genes during pupal and adult stages. One hundred and forty-seven genes were expressed only in pupal males, 56 genes were expressed in adult males and another 82 genes were commonly expressed in both male samples. In addition, 26 genes were expressed only in the pupal females, 163 genes were found in the adult females and only one gene was expressed in both female samples. A further quantitative real-time PCR validation of selected genes from the RNA sequencing (RNA-seq) analysis confirmed upregulation of those genes in a sex-specific manner, including: fibrinogen and fibronectin, a zinc finger protein, phospholipase A(2) and a serine protein for female pupae; venom allergen 3, a perlecan, testis-specific serine/threonine-protein kinase 1, testis-specific serine/threonine-protein kinase 6 and cytochrome c-2 for male pupae; a salivary protein, D7 protein precursor, trypsin 7 precursor, D7 protein and nanos for female adults; and tetraspanin F139, cytosol aminopeptidase, testis-specific serine/threonine-protein kinase 1, a testis-specific serine/threonine-protein kinase 6 and a C-type lectin for male adults. These findings provide insight into the development and physiology of Culex mosquitoes, which will help in the development of more effective control methods for these disease vectors.

Differential Gene Expression in the Heads of Behaviorally Divergent Culex pipiens Mosquitoes

Host preferences of Cx. pipiens, a bridge vector for West Nile virus to humans, have the potential to drive pathogen transmission dynamics. Yet much remains unknown about the extent of variation in these preferences and their molecular basis. We conducted host choice assays in a laboratory setting to quantify multi-day human and avian landing rates for Cx. pipiens females. Assayed populations originated from five above-ground and three below-ground breeding and overwintering habitats. All three below-ground populations were biased toward human landings, with rates of human landing ranging from 69-85%. Of the five above-ground populations, four had avian landing rates of >80%, while one landed on the avian host only 44% of the time. Overall response rates and willingness to alternate landing on the human and avian hosts across multiple days of testing also varied by population. For one human- and one avian-preferring population, we examined patterns of differential expression and splice site variation at genes expressed in female heads. We also compared gene expression and splice site variation within human-seeking females in either gravid or host-seeking physiological states to identify genes that may regulate blood feeding behaviors. Overall, we identified genes with metabolic and regulatory function that were differentially expressed in our comparison of gravid and host-seeking females. Differentially expressed genes in our comparison of avian- and human-seeking females were enriched for those involved in sensory perception. We conclude with a discussion of specific sensory genes and their potential influence on the divergent behaviors of avian- and human-seeking Cx. pipiens.

Fitness characteristics of the malaria vector Anopheles funestus during an attempted laboratory colonization

BACKGROUND

The malaria vector Anopheles funestus is increasingly recognized as a dominant vector of residual transmission in many African settings. Efforts to better understand its biology and control are significantly impeded by the difficulties of colonizing it under laboratory conditions. To identify key bottlenecks in colonization, this study compared the development and fitness characteristics of wild An. funestus from Tanzania (FUTAZ) and their F₁ offspring during colonization attempts. The demography and reproductive success of wild FUTAZ offspring were compared to that of individuals from one of the only An. funestus strains that has been successfully colonized (FUMOZ, from Mozambique) under similar laboratory conditions.

METHODS

Wild An. funestus (FUTAZ) were collected from three Tanzanian villages and maintained inside an insectary at 70-85% RH, 25-27 °C and 12 h:12 h photoperiod. Eggs from these females were used to establish three replicate F₁ laboratory generations. Larval development, survival, fecundity, mating success, percentage pupation and wing length were measured in the F₁ -FUTAZ offspring and compared with wild FUTAZ and FUMOZ mosquitoes.

RESULTS

Wild FUTAZ laid fewer eggs (64.1; 95% CI [63.2, 65.0]) than FUMOZ females (76.1; 95% CI [73.3, 79.1]). Survival of F₁-FUTAZ larvae under laboratory conditions was low, with an egg-to-pupae conversion rate of only 5.9% compared to 27.4% in FUMOZ. The median lifespan of F₁-FUTAZ females (32 days) and males (33 days) was lower than FUMOZ (52 and 49 for females and males respectively). The proportion of female F₁-FUTAZ inseminated under laboratory conditions (9%) was considerably lower than either FUMOZ (72%) or wild-caught FUTAZ females (92%). This resulted in nearly zero viable F₂-FUTAZ eggs produced. Wild FUTAZ wings appear to be larger compared to the lab reared F₁-FUTAZ and FUMOZ.

CONCLUSIONS

This study indicates that poor larval survival, mating success, low fecundity and shorter survival under laboratory conditions all contribute to difficulties in colonizing of An. funestus. Future studies should focus on enhancing these aspects of An. funestus fitness in the laboratory, with the biggest barrier likely to be poor mating.

Transcript Assembly and Quantification by RNA-Seq Reveals Significant Differences in Gene Expression and Genetic Variants in Mosquitoes of the Culex pipiens (Diptera: Culicidae) Complex

The taxonomy of Culex pipiens complex of mosquitoes is still debated, but in North America it is generally regarded to include Culex pipiens pipiens, Culex pipiens molestus, and Culex quinquefasciatus (or Culex pipiens quinquefasciatus). Although these mosquitoes have very similar morphometry, they each have unique life strategies specifically adapted to their ecological niche. Differences include the capability for overwintering diapause, bloodmeal preference, mating behaviors, and reliance on blood meals to produce eggs. Here, we used RNA-seq transcriptome analysis to investigate the differential gene expression and nucleotide polymorphisms that may link to the divergent traits specifically between Cx. pipiens pipiens and Cx. pipiens molestus.

The mosquito taste system and disease control

Mosquitoes are a widely diverse group of organisms, comprising ∼3,500 species that live in an enormous range of habitats. Some species are vectors of diseases that afflict hundreds of millions of people each year. Although understanding of mosquito olfaction has progressed dramatically in recent years, mosquito taste remains greatly understudied. Since taste is essential to feeding, egg laying, and mating decisions in insects, improved understanding of taste in mosquitoes could provide new mechanistic insight into many aspects of their behavior. We provide a guide to current knowledge in the field, and we suggest a wealth of opportunities for research that are now enabled by recent scientific and technological advances. We also propose means by which taste might be exploited in new strategies for mosquito control, which may be urgently needed as the geographical ranges of vector species increase with climate change.

Genomic differentiation and intercontinental population structure of mosquito vectors Culex pipiens pipiens and Culex pipiens molestus

Understanding the population structure and mechanisms of taxa diversification is important for organisms responsible for the transmission of human diseases. Two vectors of West Nile virus, Culex pipiens pipiens and Cx. p. molestus, exhibit epidemiologically important behavioral and physiological differences, but the whole-genome divergence between them was unexplored. The goal of this study is to better understand the level of genomic differentiation and population structures of Cx. p. pipiens and Cx. p. molestus from different continents. We sequenced and compared the whole genomes of 40 individual mosquitoes from two locations in Eurasia and two in North America. Principal Component, ADMIXTURE, and neighbor joining analyses of the nuclear genomes identified two major intercontinental, monophyletic clusters of Cx. p. pipiens and Cx. p. molestus. The level of genomic differentiation between the subspecies was uniform along chromosomes. The ADMIXTURE analysis determined signatures of admixture in Cx. p. pipens populations but not in Cx. p. molestus populations. Comparison of mitochondrial genomes among the specimens showed a paraphyletic origin of the major haplogroups between the subspecies but a monophyletic structure between the continents. Thus, our study identified that Cx. p. molestus and Cx. p. pipiens represent different evolutionary units with monophyletic origin that have undergone incipient ecological speciation.

QTL Determining Diel Flight Activity in Male Culex pipiens Mosquitoes

Members of the Culex pipiens complex differ in physiological traits that facilitate their survival in diverse environments. Assortative mating within the complex occurs in some regions where autogenous (the ability to lay a batch of eggs without a blood meal) and anautogenous populations are sympatric, and differences in mating behaviors may be involved. For example, anautogenous populations mate in flight/swarms, while autogenous populations often mate at rest. Here, we characterized flight activity of males and found that anautogenous strain males were crepuscular, while autogenous strain males were crepuscular and nocturnal, with earlier activity onset times. We conducted quantitative trait locus (QTL) mapping to explore the genetic basis of circadian chronotype (crepuscular vs. crepuscular and nocturnal) and time of activity onset. One major-effect QTL was identified for chronotype, while 3 QTLs were identified for activity onset. The highest logarithm of the odds (LOD) score for the chronotype QTL coincides with a chromosome 3 marker that contains a 15-nucleotide indel within the coding region of the canonical clock gene, cryptochrome 2. Sequencing of this locus in 7 different strains showed that the C-terminus of CRY2 in the autogenous forms contain deletions not found in the anautogenous forms. Consequently, we monitored activity in constant darkness and found males from the anautogenous strain exhibited free running periods of ~24 h while those from the autogenous strain were ~22 h. This study provides novel insights into the genetic basis of flight behaviors that likely reflect adaptation to their distinct ecological niches.

Proper direction of male genitalia is prerequisite for copulation in Drosophila, implying cooperative evolution between genitalia rotation and mating behavior

Animal morphology and behavior often appear to evolve cooperatively. However, it is difficult to assess how strictly these two traits depend on each other. The genitalia morphologies and courtship behaviors in insects, which vary widely, may be a good model for addressing this issue. In Diptera, phylogenetic analyses of mating positions suggested that the male-above position evolved from an end-to-end one. However, with this change in mating position, the dorsoventral direction of the male genitalia became upside down with respect to that of the female genitalia. It was proposed that to compensate for this incompatibility, the male genitalia rotated an additional 180° during evolution, implying evolutionary cooperativity between the mating position and genitalia direction. According to this scenario, the proper direction of male genitalia is critical for successful mating. Here, we tested this hypothesis using a Drosophila Myosin31DF (Myo31DF) mutant, in which the rotation of the male genitalia terminates prematurely, resulting in various deviations in genitalia direction. We found that the proper dorsoventral direction of the male genitalia was a prerequisite for successful copulation, but it did not affect the other courtship behaviors. Therefore, our results suggested that the male genitalia rotation and mating position evolved cooperatively in Drosophila.