The effects of infection with Plasmodium yoelii nigeriensis on the reproductive fitness of Anopheles stephensi

Plasmodium development in Anopheles: a tale of shared resources

Interactions between the Anopheles mosquito vector and Plasmodium parasites shape how malaria is transmitted in endemic regions. The long association of these two organisms has led to evolutionary processes that minimize fitness costs of infection and benefit both players through shared nutrient resources, parasite immune suppression, and mosquito tolerance to infection. In this review we explore recent data describing how Plasmodium falciparum, the deadliest malaria parasite, associates with one of its most important natural mosquito hosts, Anopheles gambiae, and we discuss the implications of these findings for parasite transmission and vector control strategies currently in development.

A mating-induced reproductive gene promotes Anopheles tolerance to Plasmodium falciparum infection

Anopheles mosquitoes have transmitted Plasmodium parasites for millions of years, yet it remains unclear whether they suffer fitness costs to infection. Here we report that the fecundity of virgin and mated females of two important vectors—Anopheles gambiae and Anopheles stephensi—is not affected by infection with Plasmodium falciparum, demonstrating that these human malaria parasites do not inflict this reproductive cost on their natural mosquito hosts. Additionally, parasite development is not impacted by mating status. However, in field studies using different P. falciparum isolates in Anopheles coluzzii, we find that Mating-Induced Stimulator of Oogenesis (MISO), a female reproductive gene strongly induced after mating by the sexual transfer of the steroid hormone 20-hydroxyecdysone (20E), protects females from incurring fecundity costs to infection. MISO-silenced females produce fewer eggs as they become increasingly infected with P. falciparum, while parasite development is not impacted by this gene silencing. Interestingly, previous work had shown that sexual transfer of 20E has specifically evolved in Cellia species of the Anopheles genus, driving the co-adaptation of MISO. Our data therefore suggest that evolution of male-female sexual interactions may have promoted Anopheles tolerance to P. falciparum infection in the Cellia subgenus, which comprises the most important malaria vectors.

Plasmodium falciparum, the deadliest form of human malaria, is transmitted when female Anopheles mosquitoes bite people and take a blood meal in order to develop eggs. To date, it is still poorly understood whether Anopheles mosquitoes that get infected with P. falciparum suffer fitness costs. Here, we find that the number of eggs produced by Anopheles gambiae and Anopheles stephensi females is not affected by P. falciparum infection, and that the mating status of the mosquitoes does not impact the parasite. However, in field experiments infecting a related species, Anopheles coluzzii, with P. falciparum using blood from donors in Burkina Faso, we find that interfering with the expression of a gene normally triggered by the sexual transfer of the steroid hormone 20-hydroxyecdysone induces increasing costs to egg development as females become more infected with P. falciparum, with no impacts on the parasite. The results of our study suggest that pathways triggered by mating may help Anopheles prevent reproductive costs associated with P. falciparum infection, providing new insights into evolutionary strategies adopted by anophelines in the face of a longstanding association with Plasmodium parasites.

Effect of Oral Infection of Mayaro Virus on Fitness Correlates and Expression of Immune Related Genes in Aedes aegypti

Mayaro virus is a mosquito-borne Alphavirus endemic to forests of tropical South America with a sylvatic cycle involving non-human primates and Haemagogus mosquitoes. Human infection with Mayaro virus causes a febrile illness and long-lasting arthralgia and cases are often associated with exposure to tropical forest habitats. Human movement between tropical forest habitats and urban settings may allow for imported cases and subsequent local transmission by domestic mosquito Aedes aegypti. The relative importance of Ae. aegypti as a vector of Mayaro virus may depend on the pathogenic effects of the virus on fitness correlates, especially those entomological parameters that relate to vectorial capacity. We performed mosquito infection studies and compared adult survival and fecundity of females from Brazilian and Floridian populations of Ae. aegypti following oral ingestion of uninfectious (control) and Mayaro virus infectious blood. Mayaro virus infected and refractory mosquitoes had similar or 30-50% lower fecundity than control (unexposed) mosquitoes, suggesting a reproductive cost to mounting an immune response or phenotypic expression of refractoriness. Survival of adult female mosquitoes and targeted gene expression in the Toll and IMD pathways were not altered by Mayaro virus infection. Adult lifespan and fecundity estimates were independent of measured viral titer in the bodies of mosquitoes. The lack of adverse effects of infection status on female survival suggests that Mayaro virus will not alter vectorial capacity mediated by changes in this parameter.

The Consequences of Mixed-Species Malaria Parasite Co-Infections in Mice and Mosquitoes for Disease Severity, Parasite Fitness, and Transmission Success

The distributions of human malaria parasite species overlap in most malarious regions of the world, and co-infections involving two or more malaria parasite species are common. Little is known about the consequences of interactions between species during co-infection for disease severity and parasite transmission success. Anti-malarial interventions can have disproportionate effects on malaria parasite species and may locally differentially reduce the number of species in circulation. Thus, it is important to have a clearer understanding of how the interactions between species affect disease and transmission dynamics. Controlled competition experiments using human malaria parasites are impossible, and thus we assessed the consequences of mixed-species infections on parasite fitness, disease severity, and transmission success using the rodent malaria parasite species Plasmodium chabaudi, Plasmodium yoelii, and Plasmodium vinckei. We compared the fitness of individual species within single species and co-infections in mice. We also assessed the disease severity of single vs. mixed infections in mice by measuring mortality rates, anemia, and weight loss. Finally, we compared the transmission success of parasites in single or mixed species infections by quantifying oocyst development in Anopheles stephensi mosquitoes. We found that co-infections of P. yoelii with either P. vinckei or P. chabaudi led to a dramatic increase in infection virulence, with 100% mortality observed in mixed species infections, compared to no mortality for P. yoelii and P. vinckei single infections, and 40% mortality for P. chabaudi single infections. The increased mortality in the mixed infections was associated with an inability to clear parasitaemia, with the non-P. yoelii parasite species persisting at higher parasite densities than in single infections. P. yoelii growth was suppressed in all mixed infections compared to single infections. Transmissibility of P. vinckei and P. chabaudi to mosquitoes was also reduced in the presence of P. yoelii in co-infections compared to single infections. The increased virulence of co-infections containing P. yoelii (reticulocyte restricted) and P. chabaudi or P. vinckei (predominantly normocyte restricted) may be due to parasite cell tropism and/or immune modulation of the host. We explain the reduction in transmission success of species in co-infections in terms of inter-species gamete incompatibility.

Steroid Hormone Function Controls Non-competitive Plasmodium Development in Anopheles

Transmission of malaria parasites occurs when a female Anopheles mosquito feeds on an infected host to acquire nutrients for egg development. How parasites are affected by oogenetic processes, principally orchestrated by the steroid hormone 20-hydroxyecdysone (20E), remains largely unknown. Here we show that Plasmodium falciparum development is intimately but not competitively linked to processes shaping Anopheles gambiae reproduction. We unveil a 20E-mediated positive correlation between egg and oocyst numbers; impairing oogenesis by multiple 20E manipulations decreases parasite intensities. These manipulations, however, accelerate Plasmodium growth rates, allowing sporozoites to become infectious sooner. Parasites exploit mosquito lipids for faster growth, but they do so without further affecting egg development. These results suggest that P. falciparum has adopted a non-competitive evolutionary strategy of resource exploitation to optimize transmission while minimizing fitness costs to its mosquito vector. Our findings have profound implications for currently proposed control strategies aimed at suppressing mosquito populations.

Killer bee molecules: antimicrobial peptides as effector molecules to target sporogonic stages of Plasmodium

A new generation of strategies is evolving that aim to block malaria transmission by employing genetically modified vectors or mosquito pathogens or symbionts that express anti-parasite molecules. Whilst transgenic technologies have advanced rapidly, there is still a paucity of effector molecules with potent anti-malaria activity whose expression does not cause detrimental effects on mosquito fitness. Our objective was to examine a wide range of antimicrobial peptides (AMPs) for their toxic effects on Plasmodium and anopheline mosquitoes. Specifically targeting early sporogonic stages, we initially screened AMPs for toxicity against a mosquito cell line and P. berghei ookinetes. Promising candidate AMPs were fed to mosquitoes to monitor adverse fitness effects, and their efficacy in blocking rodent malaria infection in Anopheles stephensi was assessed. This was followed by tests to determine their activity against P. falciparum in An. gambiae, initially using laboratory cultures to infect mosquitoes, then culminating in preliminary assays in the field using gametocytes and mosquitoes collected from the same area in Mali, West Africa. From a range of 33 molecules, six AMPs able to block Plasmodium development were identified: Anoplin, Duramycin, Mastoparan X, Melittin, TP10 and Vida3. With the exception of Anoplin and Mastoparan X, these AMPs were also toxic to an An. gambiae cell line at a concentration of 25 µM. However, when tested in mosquito blood feeds, they did not reduce mosquito longevity or egg production at concentrations of 50 µM. Peptides effective against cultured ookinetes were less effective when tested in vivo and differences in efficacy against P. berghei and P. falciparum were seen. From the range of molecules tested, the majority of effective AMPs were derived from bee/wasp venoms.

Next-generation site-directed transgenesis in the malaria vector mosquito Anopheles gambiae: self-docking strains expressing germline-specific phiC31 integrase

Diseases transmitted by mosquitoes have a devastating impact on global health and the situation is complicated due to difficulties with both existing control measures and the impact of climate change. Genetically modified mosquitoes that are refractory to disease transmission are seen as having great potential in the delivery of novel control strategies. The Streptomyces phage phiC31 integrase system has been successfully adapted for site-directed transgene integration in a range of insects, thus overcoming many limitations due to size constraints and random integration associated with transposon-mediated transformation. Using this technology, we previously published the first site-directed transformation of Anopheles gambiae, the principal vector of human malaria. Mosquitoes were initially engineered to incorporate the phiC31 docking site at a defined genomic location. A second phase of genetic modification then achieved site-directed integration of an anti-malarial effector gene. In the current publication we report improved efficiency and utility of the phiC31 integrase system following the generation of Anopheles gambiae self-docking strains. Four independent strains, with docking sites at known locations on three different chromosome arms, were engineered to express integrase under control of the regulatory regions of the nanos gene from Anopheles gambiae. The resulting protein accumulates in the posterior oocyte to provide integrase activity at the site of germline development. Two self-docking strains, exhibiting significantly different levels of integrase expression, were assessed for site-directed transgene integration and found to demonstrate greatly improved survival and efficiency of transformation. In the fight against malaria, it is imperative to establish a broad repertoire of both anti-malarial effector genes and tissue-specific promoters to regulate their expression, enabling those offering maximum effect with minimum fitness cost to be identified. The improved technology we describe here will facilitate comparative studies of effector transgenes, allowing informed choices to be made that potentially lead to transmission blockade.

Impact of Chikungunya virus on Aedes albopictus females and possibility of vertical transmission using the actors of the 2007 outbreak in Italy

We investigated the impact of CHIKV strains on some Aedes albopictus (Skuse) reproductive parameters and the possibility of vertical transmission. Two strains were collected in the area where the epidemic occurred in 2007, one isolated from mosquitoes, the other one isolated from a viraemic patient. Different types of blood meals, either infected or non-infected, were offered to Ae. albopictus females, that were then analyzed at increasing time post infection. The virus titre, measured by two RT-PCR methods in the blood meals, influenced the rate of infection and the rate of dissemination of CHIKV in Ae. albopictus body. We found individual variability with respect to the infection/dissemination rates and their latency both considering the female's body and appendages. The hatching rate was significantly lower for the eggs laid by the infected females than for the control eggs, while the mortality during the larval development (from first instar larva to adult emergence) was similar among the progeny of infected and non-infected female groups. Our findings seem to support the hypothesis that the vertical transmission is a rare event under our conditions, and that a certain time period is required in order to get the ovarioles infected. Field observations conducted during the Spring 2008 showed no evidence of the presence of infected overwintering progeny produced by Ae. albopictus females infected during the 2007 outbreak.

Dynamics of molecular markers linked to the resistance loci in a mosquito-Plasmodium system

Models on the evolution of resistance to parasitism generally assume fitness tradeoffs between the costs of being parasitized and the costs associated with resistance. This study tested this assumption using the yellow fever mosquito Aedes aegypti and malaria parasite Plasmodium gallinaceum system. Experimental mosquito populations were created by mixing susceptible and resistant strains in equal proportions, and then the dynamics of markers linked to loci for Plasmodium resistance and other unlinked neutral markers were determined over 12 generations. We found that when the mixed population was maintained under parasite-free conditions, the frequencies of alleles specific to the susceptible strain at markers closely linked to the loci for resistance (QTL markers) as well as other unlinked markers increased significantly in the first generation and then fluctuated around equilibrium frequencies for all six markers. However, when the mixed population was exposed to an infected blood meal every generation, allele frequencies at the QTL markers for resistance were not significantly changed. Small population size caused significant random fluctuations of allele frequencies at all marker loci. Consistent allele frequency changes in the QTL markers and other unlinked markers suggest that the reduced fitness in the resistant population has a genome-wide effect on the genetic makeup of the mixed population. Continuous exposure to parasites promoted the maintenance of alleles from the resistant Moyo-R strain in the mixed population. The results are discussed in relation to the proposed malaria control strategy through genetic disruption of vector competence.

Reproductive aspects of the mosquito Culex quinquefasciatus (Diptera:Culicidae) infected with Wuchereria bancrofti (Spirurida: Onchocercidae)

This study reports on the relationship between Wuchereria bancrofti infection and female body size, intake of blood and fecundity in the mosquito Culex quinquefasciatus, vector of this filarial parasite in Recife (Brazil). Adults from field collected larvae were infected via a membrane feeding procedure, using blood with parasitaemia ranging from 724-6,000 mf/ml. A positive correlation was observed between mosquito size (measured by wing length) and egg production in uninfected females. However, this relationship did not exist in W. bancrofti infected mosquitoes. This change is unlikely to be the result of changes in blood ingestion as no significant difference was found when infected and uninfected females were compared. Variation in egg production observed between trials could not be associated with parasite density in the blood. These results suggest infection with W. bancrofti may disrupt the relationship between mosquito size and egg production during the first gonotrophic cycle of C. quinquefasciatus such that fecundity is sometimes reduced. However, this overall affect is variable and many groups of mosquitoes do not respond in this way.

Design and activity of antimicrobial peptides against sporogonic-stage parasites causing murine malarias

Insects produce several types of peptides to combat a broad spectrum of invasive pathogenic microbes, including protozoans. However, despite this defense response, infections are often established. Our aim was to design novel peptides that produce high rates of mortality among protozoa of the genus Plasmodium, the malaria parasites. Using existing antimicrobial peptide sequences as templates, we designed and synthesized three short novel hybrids, designated Vida1 to Vida3. Each has a slightly different predicted secondary structure. The peptides were tested against sporogonic stages of the rodent malaria parasites Plasmodium berghei (in vitro and in vivo) and P. yoelii nigeriensis (in vitro). The level of activity varied for each peptide and according to the parasite stage targeted. Vida3 (which is predicted to have large numbers of beta sheets and coils but no alpha helices) showed the highest level of activity, killing the early sporogonic stages in culture and causing highly significant reductions in the prevalence and intensity of infection of P. berghei after oral administration or injection in Anopheles gambiae mosquitoes. The secondary structures of these peptides may play a crucial role in their ability to interact with and kill sporogonic forms of the malaria parasite.

MALARIA-INDUCED APOPTOSIS IN MOSQUITO OVARIES

Many insects are able to adjust their egg production according to physiological conditions such as nutrient supply and mating success. One way in which this is achieved is by resorption of some, or all, of the ovarian follicles at some stage during oogenesis. We have shown that the mosquito Anopheles stephensi responds in this manner when ookinetes of the malaria parasite Plasmodium yoelii nigeriensis first begin to invade the midgut. Little is known about the initiation and regulation of follicle resorption in any insect. Here, we demonstrate that there is a significant positive correlation between follicle resorption and the presence of follicular epithelial cells that are undergoing apoptosis. The parasite causes significantly more follicles to contain apoptotic cells from 16 h post-infection onwards. Injection of a caspase inhibitor immediately after feeding on an infective blood meal prevents parasite-induced resorption of follicles and thus demonstrates that apoptosis precedes resorption. Ultrastructural studies show that patches of follicular epithelial cells contain condensed nuclear chromatin, a characteristic of apoptosis, and that no patency develops in these cells. Our work suggests that apoptosis plays a role in malaria-initiated inhibition of mosquito oogenesis and that caspase is central to this process. Follicle resorption is one of the main factors contributing to malaria-induced fecundity reduction in mosquitoes.

The influence of host haematocrit on the blood feeding success of Anopheles stephensi: implications for enhanced malaria transmission

Two studies were carried out to determine the effect of the rodent malaria Plasmodium yoelii nigeriensis on the blood feeding success of Anopheles stephensi. Initially, pairs of mice with similar packed cell volume (PCV) (measured by haematocrit) were selected. Following infection of one of the pair its PCV gradually fell. At various times post-infection, a comparison was made of the bloodmeal size (haemoglobin content) of mosquitoes feeding on these mice. The bloodmeal sizes increased with parasite-induced fall in PCV down to a haematocrit of 43-44%, which occurred approximately 48 h post-infection. Bloodmeals were significantly reduced, however, when mosquitoes fed on mice with higher parasitaemias and a haematocrit of 15-35%. Thus, at early stages of infection, mosquitoes ingested a bloodmeal significantly greater than did the mosquitoes feeding on the control mice. However, mosquitoes were not able to compensate for severe infection-associated anaemia. To compensate for variation due to innate differences in the mice, a second experiment was performed. Mosquitoes were fed on the same mice before (control) and after infection. Again, the bloodmeal size increased with decreasing PCV down to haematocrits of 42-45%, but declined thereafter. In this host-parasite-vector system, haematocrits that maximized erythrocyte intake were produced when gametocytes, capable of exflagellation, were present.

Genetics of mosquito vector competence

Mosquito-borne diseases are responsible for significant human morbidity and mortality throughout the world. Efforts to control mosquito-borne diseases have been impeded, in part, by the development of drug-resistant parasites, insecticide-resistant mosquitoes, and environmental concerns over the application of insecticides. Therefore, there is a need to develop novel disease control strategies that can complement or replace existing control methods. One such strategy is to generate pathogen-resistant mosquitoes from those that are susceptible. To this end, efforts have focused on isolating and characterizing genes that influence mosquito vector competence. It has been known for over 70 years that there is a genetic basis for the susceptibility of mosquitoes to parasites, but until the advent of powerful molecular biological tools and protocols, it was difficult to assess the interactions of pathogens with their host tissues within the mosquito at a molecular level. Moreover, it has been only recently that the molecular mechanisms responsible for pathogen destruction, such as melanotic encapsulation and immune peptide production, have been investigated. The molecular characterization of genes that influence vector competence is becoming routine, and with the development of the Sindbis virus transducing system, potential antipathogen genes now can be introduced into the mosquito and their effect on parasite development can be assessed in vivo. With the recent successes in the field of mosquito germ line transformation, it seems likely that the generation of a pathogen-resistant mosquito population from a susceptible population soon will become a reality.