Costs and benefits of Wolbachia infection in immature Aedes albopictus depend upon sex and competition level

Bacterial endosymbionts induce various effects on hosts and can dramatically impact host fitness and development. An example is provided by obligate, maternally-inherited Wolbachia, which infect a broad range of invertebrates. Wolbachia are capable of altering host reproduction, thereby promoting infection spread. Wolbachia also pose direct physiological costs and benefits to hosts, complicating their categorization as parasites or mutualists. This study examines for an effect of Wolbachia infection in intra-specific larval competition by Aedes albopictus mosquitoes, with the goal of examining for an impact of Wolbachia infection in mixed populations. Similar to prior work examining for an influence of Wolbachia infection on the fitness of A. albopictus in adults, the results presented here support the hypothesized impact of Wolbachia across all life stages, including immatures. The differential competitiveness of infected larvae detected in our experiments indicates that Wolbachia infected A. albopictus females are less competitive relative to uninfected females when competing under highly competitive conditions. In contrast, under low competitive pressures, infected females experience higher survivorship. Thus, Wolbachia infection shifts from parasitism to mutualism as a function of developmental conditions. Results are discussed in relation to the invasion and persistence of Wolbachia in A. albopictus populations.The results are important to the evolution of stable Wolbachia symbioses, including Wolbachia invasion of an uninfected population. The resulting infection dynamics that occur in an infected population are discussed.     

Differential Tolerance to Direct and Indirect Density-Dependent Costs of Viral Infection in Arabidopsis thaliana

Population density and costs of parasite infection may condition the capacity of organisms to grow, survive and reproduce, i.e. their competitive ability. In host–parasite systems there are different competitive interactions: among uninfected hosts, among infected hosts, and between uninfected and infected hosts. Consequently, parasite infection results in a direct cost, due to parasitism itself, and in an indirect cost, due to modification of the competitive ability of the infected host. Theory predicts that host fitness reduction will be higher under the combined effects of costs of parasitism and competition than under each factor separately. However, experimental support for this prediction is scarce, and derives mostly from animal–parasite systems. We have analysed the interaction between parasite infection and plant density using the plant-parasite system of Arabidopsis thaliana and the generalist virus Cucumber mosaic virus (CMV). Plants of three wild genotypes grown at different densities were infected by CMV at various prevalences, and the effects of infection on plant growth and reproduction were quantified. Results demonstrate that the combined effects of host density and parasite infection may result either in a reduction or in an increase of the competitive ability of the host. The two genotypes investing a higher proportion of resources to reproduction showed tolerance to the direct cost of infection, while the genotype investing a higher proportion of resources to growth showed tolerance to the indirect cost of infection. Our findings show that the outcome of the interaction between host density and parasitism depends on the host genotype, which determines the plasticity of life-history traits and consequently, the host capacity to develop different tolerance mechanisms to the direct or indirect costs of parasitism. These results indicate the high relevance of host density and parasitism in determining the competitive ability of a plant, and stress the need to simultaneously consider both factors to understand the selective pressures that drive host–parasite co-evolution.     

Stability of the wMel Wolbachia Infection following Invasion into Aedes aegypti Populations

The wMel infection of Drosophila melanogaster was successfully transferred into Aedes aegypti mosquitoes where it has the potential to suppress dengue and other arboviruses. The infection was subsequently spread into two natural populations at Yorkeys Knob and Gordonvale near Cairns, Queensland in 2011. Here we report on the stability of the infection following introduction and we characterize factors influencing the ongoing dynamics of the infection in these two populations. While the Wolbachia infection always remained high and near fixation in both locations, there was a persistent low frequency of uninfected mosquitoes. These uninfected mosquitoes showed weak spatial structure at both release sites although there was some clustering around two areas in Gordonvale. Infected females from both locations showed perfect maternal transmission consistent with patterns previously established pre-release in laboratory tests. After >2 years under field conditions, the infection continued to show complete cytoplasmic incompatibility across multiple gonotrophic cycles but persistent deleterious fitness effects, suggesting that host effects were stable over time. These results point to the stability of Wolbachia infections and their impact on hosts following local invasion, and also highlight the continued persistence of uninfected individuals at a low frequency most likely due to immigration.     

Sex‐dependent infection causes nonadditive effects on kissing bug fecundity

The influence of parasites on host reproduction has been widely studied in natural and experimental conditions. Most studies, however, have evaluated the parasite impact on female hosts only, neglecting the contribution of males for host reproduction. This omission is unfortunate as sex‐dependent infection may have important implications for host–parasite associations. Here, we evaluate for the first time the independent and nonindependent effects of gender infection on host reproductive success using the kissing bug Mepraia spinolai and the protozoan Trypanosoma cruzi as model system. We set up four crossing treatments including the following: (1) both genders infected, (2) both genders uninfected, (3) males infected—females uninfected, and (4) males uninfected—females infected, using fecundity measures as response variables. Interactive effects of infection between sexes were prevalent. Uninfected females produced more and heavier eggs when crossed with uninfected than infected males. Uninfected males, in turn, sired more eggs and nymphs when crossed with uninfected than infected females. Unexpectedly, infected males sired more nymphs when crossed with infected than uninfected females. These results can be explained by the effect of parasitism on host body size. As infection reduced size in both genders, infection on one sex only creates body size mismatches and mating constraints that are not present in pairs with the same infection status. Our results indicate the fitness impact of parasitism was contingent on the infection status of genders and mediated by body size. As the fecundity impact of parasitism cannot be estimated independently for each gender, inferences based only on female host infection run the risk of providing biased estimates of parasite‐mediated impact on host reproduction.     

Variation of clutch size and trophic egg proportion in a ladybird with and without male-killing bacterial infection

Maternally inherited bacterial endosymbionts can kill male embryos of their arthropod hosts to enhance the transmission efficiency of the endosymbionts. The resources from killed male eggs can be reallocated to infected female hatchlings as additional maternal investment. As a result, the number of offspring per patch and the maternal investment per offspring are expected to differ from the original optimal values for the host mother. Thus, in response to infection, these trait values should be adjusted to maximize the lifetime reproductive success of host females and the fitness of inherited endosymbionts as well. Here, we examined clutch size, egg size, and the proportion of trophic eggs (i.e., production of unhatched eggs, a maternal phenotype) per clutch of host mothers infected with male-killing bacteria. First, we developed a mathematical model to predict the optimal clutch size and trophic egg proportion in uninfected and infected females. Next, we experimentally compared these life-history traits in a ladybird, Harmonia yedoensis, between females infected or uninfected with male-killing Spiroplasma bacteria. Consistent with our predictions, clutch size was larger, egg size was smaller, and trophic egg proportion was lower in infected H. yedoensis females, compared with uninfected females. To our knowledge, this is the first empirical demonstration of variation in these life-history traits depending on infection with bacterial endosymbionts.     

Factors Affecting the Strength of Cardinium-Induced Cytoplasmic Incompatibility in the Parasitic Wasp Encarsia pergandiella (Hymenoptera: Aphelinidae)

Bacteria that cause cytoplasmic incompatibility (CI) are among the most common maternally transmitted parasites of insects. In CI, uninfected females produce few or no offspring when they mate with infected males and, as a result, are often at a reproductive disadvantage relative to infected females. Two different bacteria are known to cause CI, Wolbachia and Cardinium. CI Cardinium was discovered more recently and has been little studied. Here, factors that could influence the reduction in reproductive output in a CI cross, or CI “strength,” were explored in the parasitic wasp Encarsia pergandiella. Cardinium in this wasp exhibits variable CI strength. Experiments tested the effect of male age, male size, male host species, Cardinium density, and male development time on CI strength. We found a striking effect of male development time, with males that took longer to develop exhibiting stronger CI when mated to uninfected females. Male age had little effect; although in one experiment, the oldest males exhibited stronger CI. Male size, host species, and bacterial density had no effect on the strength of CI. Identifying the factors that control CI are crucial for understanding the dynamics of infection, as well as the success of strategies that aim to use CI microbes to control insect pests and disease vectors.     

Malaria infection increases bird attractiveness to uninfected mosquitoes

The epidemiology of vector‐borne pathogens is largely determined by the host‐choice behaviour of their vectors. Here, we investigate whether a Plasmodium infection renders the host more attractive to host‐seeking mosquitoes. For this purpose, we work on a novel experimental system: the avian malaria parasite Plasmodium relictum, and its natural vector, the mosquito Culex pipiens. We provide uninfected mosquitoes with a choice between an uninfected bird and a bird undergoing either an acute or a chronic Plasmodium infection. Mosquito choice is assessed by microsatellite typing of the ingested blood. We show that chronically infected birds attract significantly more vectors than either uninfected or acutely infected birds. Our results suggest that malaria parasites manipulate the behaviour of uninfected vectors to increase their transmission. We discuss the underlying mechanisms driving this behavioural manipulation, as well as the broader implications of these effects for the epidemiology of malaria.     

Physiological consequences of immune response by Drosophila melanogaster (Diptera: Drosophilidae) against the parasitoid Asobara tabida (Hymenoptera: Braconidae)

Abstract Parasites can exert a wide range of negative effects on their hosts. Consequently, hosts that can resist infection should have a selective advantage over nonresistant conspecifics. Yet, host populations remain susceptible to some parasites. Could genetic heterogeneity in the host's ability to resist parasites reflect costs of mounting an immune response? Previous work on Drosophila melanogaster establishes that maintaining the ability to mount an immune response decreases larval competitive ability. Moreover, mounting an immune response decreases fitness. I report on the impact of mounting an immune response on fitness of D. melanogaster survived parasitism by Asobara tabida. I used isofemale lines to determine whether genotype influences the costs of immune response. I examined fitness consequences both to larvae and adults. Survivors of parasitism show no measurable decrease in larval fitness (development time) but have decreased adult fitness (population growth rates), probably because of their smaller size.     

Sexually transmitted parasites and host mating behavior in the decorated cricket

Sexually transmitted diseases play a potentially important rolein the ecology and evolution of host mating behavior. Here,we use a sexually transmitted nematode-cricket (Mehdinema alii–Gryllodessigillatus) system to examine the effects of parasitism on hostmating activity and female choice. Previous work has shown thatinfected male crickets produce a significantly smaller nuptialgift (spermatophylax) than uninfected males. This is expectedto result in reduced spermatophylax feeding duration and earlyampulla removal. Here, we hypothesize that the parasite-mediatedreduction in spermatophylax size will consequently shorten femaleintercopulatory interval. We predict that females mated to infectedmales will exhibit a shorter intercopulatory interval than femalesmated to uninfected males. To test this hypothesis, we experimentallymeasured the behavioral responses of females mated to uninfectedand infected males. We found no significant difference betweenfemale handling of the spermatophylax and ampulla from infectedversus uninfected males. Although the duration of spermatophylaxconsumption is positively correlated with the duration of ampullaattachment, neither of these variables is correlated with femaleintercopulatory interval. Intercopulatory intervals for femalespreviously mated with uninfected versus infected males are notstatistically different. We conclude that parasitism in maleG. sigillatus does not influence female intercopulatory intervalor male mating success. We found no evidence for female matechoice based on male infection status. The lack of female choiceis consistent with theoretical predictions involving parasitesthat are sexually transmitted.     

Inhibition of Dirofilaria immitis in gregarine-infected Aedes aegypti: preliminary observations.

This study examined some of the effects of Ascocystis culicis infections in Aedes aegypti mosquitoes and on the development of dog heartworm, Dirofilaria immitis, in A. culicis-infected A. aegypti. Infections of 100 or more gregarines had an adverse effect on mosquito larvae and the resulting pupae weighed 1 to 2 mg less than the gregarine-free stock mosquitoes. Intensity of gregarine-induced pathology of adult mosquitoes was best estimated by counting the cells of the Malpighian tubules. Larvae infected with approximately 100 gregarines per larva had at least half the number of cells in their tubules destroyed, and development of D. immitis was greatly reduced. Thus, gregarine infections may be useful in reducing populations of aedine mosquitoes and, in areas endemic for heartworm, may interrupt transmission of D. immitis.     

Cost of resistance: relationship between reduced fertility and increased resistance in a snail—schistosome host—parasite system

Natural host populations often exhibit genetic variability in resistance to parasitism. One possible mechanism for maintaining such diversity is a trade-off between fitness costs associated with resistance and fitness costs associated with parasitism. However, little is known about the nature or magnitude of these costs in animal populations. Using artificial selection experiments in a Biomphalaria glabrata–Schistosoma mansoni host–parasite system, we demonstrated that resistance and susceptibility to infection are heritable. We then investigated whether resistance had any associated costs in terms of snail reproductive success. Susceptible-selected snail lines showed significantly higher fertility (number of offspring produced) than resistant-selected or unselected control snail lines, irrespective of current infection status. There were no consistent differences between snail lines in fecundity, proportion of abnormal egg masses produced, or mean number of eggs per egg mass. Mortality rate was higher among infected than uninfected snails. These results are consistent with snails incurring costs of resistance to schistosome infection in the absence of the parasite.     

Avian malaria infection intensity influences mosquito feeding patterns

Pathogen-induced host phenotypic changes are widespread phenomena that can dramatically influence host–vector interactions. Enhanced vector attraction to infected hosts has been reported in a variety of host–pathogen systems, and has given rise to the parasite manipulation hypothesis whereby pathogens may adaptively modify host phenotypes to increase transmission from host to host. However, host phenotypic changes do not always favour the transmission of pathogens, as random host choice, reduced host attractiveness and even host avoidance after infection have also been reported. Thus, the effects of hosts’ parasitic infections on vector feeding behaviour and on the likelihood of parasite transmission remain unclear. Here, we experimentally tested how host infection status and infection intensity with avian Plasmodium affect mosquito feeding patterns in house sparrows (Passer domesticus). In separate experiments, mosquitoes were allowed to bite pairs containing (i) one infected and one uninfected bird and (ii) two infected birds, one of which treated with the antimalarial drug, primaquine. We found that mosquitoes fed randomly when exposed to both infected and uninfected birds. However, when mosquitoes were exposed only to infected individuals, they preferred to bite the non-treated birds. These results suggest that the malarial parasite load rather than the infection itself plays a key role in mosquito attraction. Our findings partially support the parasite manipulation hypothesis, which probably operates via a reduction in defensive behaviour, and highlights the importance of considering parasite load in studies on host–vector–pathogen interactions.     

Infertility and fecundity loss of Wolbachia-infected Aedes aegypti hatched from quiescent eggs is expected to alter invasion dynamics

The endosymbiotic bacterium Wolbachia shows viral blocking in its mosquito host, leading to its use in arboviral disease control. Releases with Wolbachia strains wMel and wAlbB infecting Aedes aegypti have taken place in several countries. Mosquito egg survival is a key factor influencing population persistence and this trait is also important when eggs are stored prior to releases. We therefore tested the viability of mosquitoes derived from Wolbachia wMel and wAlbB-infected as well as uninfected eggs after long-term storage under diurnal temperature cycles of 11–19°C and 22–30°C. Eggs stored at 11–19°C had higher hatch proportions than those stored at 22–30°C. Adult Wolbachia density declined when they emerged from eggs stored for longer, which was associated with incomplete cytoplasmic incompatibility (CI) when wMel-infected males were crossed with uninfected females. Females from stored eggs at both temperatures continued to show perfect maternal transmission of Wolbachia, but storage reduced the fecundity of both wMel and wAlbB-infected females relative to uninfected mosquitoes. Furthermore, we found a very strong negative impact of the wAlbB infection on the fertility of females stored at 22–30°C, with almost 80% of females hatching after 11 weeks of storage being infertile. Our findings provide guidance for storing Wolbachia-infected A. aegypti eggs to ensure high fitness adult mosquitoes for release. Importantly, they also highlight the likely impact of egg quiescence on the population dynamics of Wolbachia-infected populations in the field, and the potential for Wolbachia to suppress mosquito populations through cumulative fitness costs across warm and dry periods, with expected effects on dengue transmission.     

Histopathology of Aedes aegypti (Diptera: Culicidae) larvae parasitized by Reesimermis nielseni (Nematoda: Mermithidae)

Histological observations were made of Aedes aegypti larvae parasitized for 2, 4, and 6 days by Reesimermis nielseni. Little difference was detected between the tissues of uninfected and nematode-parasitized larvae 4 days after infection, at which time most hosts were in the early fourth instar and their fat bodies were well developed containing abundant storage materials. Nematodes grew most rapidly between day 4 and day 6 of parasitism, depleting host metabolites, reducing the fat body and other host storage tissues while accumulating storage material in their trophosomes. Development of host imaginal discs was inhibited during this period. The severity of the nematodes upon host tissues depended upon intensity of infection. Dry weight measurements of nematode and host supported histological observations that the nematode developed most rapidly 4–6 days post-infection, thus causing most serious effects upon the host at that time.     

Plasmodium infection brings forward mosquito oviposition

Invertebrate hosts often bring forward their reproductive effort in response to a parasitic infection. This is widely interpreted as a host-driven response aimed at compensating for the expected losses in future fitness as a result of parasitism. Here we report that mosquitoes bring forward their oviposition schedule when they are infected with Plasmodium, a parasite known to severely curtail mosquito fecundity. This response could aim at compensating for a negative time-dependent effect of the parasite on mosquito fitness, as infected mosquitoes seem to display a strong and progressive decrease in the quality of the eggs they lay. In addition, we show that this shift in oviposition date is dependent on mosquito strain: a comparison of several isogenic mosquitoes strains, one insecticide-susceptible and two insecticide-resistant ones, reveals that only the former shift their oviposition strategy when infected. This pattern suggests the existence of a costly host-driven response to parasitism, as insecticide-resistant mosquitoes have been shown to be in generally poorer condition.     

Pathogen exposure reduces sexual dimorphism in a host’s upper thermal limits

The climate is warming at an unprecedented rate, pushing many species toward and beyond the upper temperatures at which they can survive. Global change is also leading to dramatic shifts in the distribution of pathogens. As a result, upper thermal limits and susceptibility to infection should be key determinants of whether populations continue to persist, or instead go extinct. Within a population, however, individuals vary in both their resistance to both heat stress and infection, and their contributions to vital growth rates. No more so is this true than for males and females. Each sex often varies in their response to pathogen exposure, thermal tolerances, and particularly their influence on population growth, owing to the higher parental investment that females typically make in their offspring. To date, the interplay between host sex, infection, and upper thermal limits has been neglected. Here, we explore the response of male and female Daphnia to bacterial infection and static heat stress. We find that female Daphnia, when uninfected, are much more resistant to static heat stress than males, but that infection negates any advantage that females are afforded. We discuss how the capacity of a population to cope with multiple stressors may be underestimated unless both sexes are considered simultaneously.     

Parasitism of the grasshopper Melanoplus sanguinipes by a sarcophagid fly, Blaesoxipha atlanis: influence of solitary and gregarious development on host and parasitoid

Blaesoxipha atlanis (Aldrich) (Diptera: Sarcophagidae) is a common parasitoid of the grasshopper Melanoplus sanguinipes (F.) (Orthoptera: Acrididae) in western Canada. We tested the hypothesis that B. atlanis can develop as either a solitary or a gregarious parasitoid, and assessed the influence of parasitism on the growth and survival of infected grasshoppers. Males and females of M. sanguinipes were parasitized manually with one, two, or three first-instar larvae of B. atlanis in the laboratory. Parasitism was more deleterious to males than females of M. sanguinipes; females are larger than males. Host survival and longevity declined with the number of larvae per host in a sex-specific manner. In females, 39%, 24%, and 8% of hosts containing, respectively, one, two, and three sarcophagid larvae survived parasitism. Although 41% of single-parasitized males survived, all males containing more than one larva died. Variations in host quality as measured by dry mass explained much of the response to parasitism in male, but not female, hosts. Parasitoid larvae, apparently, did not cause significant physical damage to host organs and tissues but instead functioned as a metabolic sink. The greater metabolic activity associated with egg production could account for the relatively higher tolerance to parasitism of female, as opposed to male, grasshoppers. Developmental time, adult size, and percentage survival of B. atlanis declined with the intensity of parasitism, especially in parasitoids developing in male hosts. Females developing gregariously contained fewer ovarioles at eclosion than counterparts developing as solitary larvae. The mean body size of field-collected B. atlanis did not differ from that of laboratory-reared parasitoids developing singly in a host. Gregarious development is an alternative strategy to solitary development that may enable B. atlanis to maintain population numbers during periods of grasshopper scarcity.     

Sexual selection in an isopod with Wolbachia‐induced sex reversal: males prefer real females

A variety of genetic elements encode traits beneficial to their own transmission. Despite their ‘selfish’ behaviour, most of these elements are often found at relatively low frequencies in host populations. This is the case of intracytoplasmic Wolbachia bacteria hosted by the isopod Armadillidium vulgare that distort the host sex ratio towards females by feminizing the genetic males they infect. Here we tested the hypothesis that sexual selection against Wolbachia‐infected females could maintain a polymorphism of the infection in populations. The infected neo‐females (feminized males) have lower mating rates and received less sperm relative to uninfected females. Males exhibited an active choice: they interacted more with uninfected females and made more mating attempts. A female behavioural difference was also observed in response to male mating attempts: infected neo‐females more often exhibited behaviours that stop the mating sequence. The difference in mating rate was significant only when males could choose between the two female types. This process could maintain a polymorphism of the infection in populations. Genetic females experimentally infected with Wolbachia are not exposed to the same sexual selection pressure, so the infection alone cannot explain these differences.     

Brugia pahangi: effects upon the flight capability of Aedes aegypti.

Aedes aegypti mosquitoes were adversely affected by infections of the filarial worm Brugia pahangi. Infected mosquitoes flew significantly shorter distances and showed marked reductions in total flight time during 24-hr flight mill tests compared to uninfected controls. Total flight range and duration flown by infected mosquitoes remained relatively constant throughout the infection process, while control mosquitoes flew further and longer with increasing time after their blood meal. Furthermore, a significantly greater number of infected mosquitoes either died or were rendered incapable of flight. Of flying and nonflying mosquitoes with 6-day-old or older infections dissected for parasite burdens, the nonflying group contained significantly more worms. Results of this study indicate that developing filarial larvae within this mosquito vector reduce its ability to survive and to transmit its infection by reducing its flight capabilities. Conclusions from this study relate only to A. aegypti homozygous for the gene fm which is fully susceptible to this filarial parasite.     

Both sexes suffer increased parasitism and reduced energy storage as costs of reproduction in the brown anole,Anolis sagrei

Sexual selection theory proposes that males suffer reduced immune function and increased parasitism as costs of expressing sexual signals. Life‐history theory proposes that females suffer the same costs because of inherent trade‐offs between reproduction and self‐maintenance. Mechanistically, each theory invokes an energetic trade‐off, although few experiments have directly compared these costs of reproduction between the sexes as a result of fundamental sex differences in the nature of reproductive investment and a tendency for each theory to focus on a single sex. To test whether males and females experience comparable costs of reproduction in terms of energetics, immune function, and parasitism, we used gonadectomy to eliminate most aspects of reproductive investment in wild brown anole lizards (Anolis sagrei) of both sexes. We compared these nonreproductive males and females with intact, reproductive controls with respect to stored energy (fat bodies), immune function (swelling response to phytohemagglutinin), and the prevalence and intensity of infection by four types of parasite (gastric nematodes, intestinal nematodes, faecal coccidia, and ectoparasitic mites). Gonadectomized anoles experienced dramatic increases in fat storage that were accompanied by decreases in the prevalence of intestinal nematodes and in the intensity of coccidia infection. These costs of reproduction were comparable between males and females, although neither sex exhibited the predicted increase in immune function after gonadectomy. Our results suggest that, despite fundamental sex differences in the nature of reproductive investment, both male and female anoles experience similar costs of reproduction with respect to energy storage and some aspects of parasitism.