The Effect of Virus-Blocking Wolbachia on Male Competitiveness of the Dengue Vector Mosquito,Aedes aegypti

Background

The bacterial endosymbiont Wolbachia blocks the transmission of dengue virus by its vector mosquito Aedes aegypti, and is currently being evaluated for control of dengue outbreaks. Wolbachia induces cytoplasmic incompatibility (CI) that results in the developmental failure of offspring in the cross between Wolbachia-infected males and uninfected females. This increases the relative success of infected females in the population, thereby enhancing the spread of the beneficial bacterium. However, Wolbachia spread via CI will only be feasible if infected males are sufficiently competitive in obtaining a mate under field conditions. We tested the effect of Wolbachia on the competitiveness of A. aegypti males under semi-field conditions.

Methodology/Principal Findings

In a series of experiments we exposed uninfected females to Wolbachia-infected and uninfected males simultaneously. We scored the competitiveness of infected males according to the proportion of females producing non-viable eggs due to incompatibility. We found that infected males were equally successful to uninfected males in securing a mate within experimental tents and semi-field cages. This was true for males infected by the benign wMel Wolbachia strain, but also for males infected by the virulent wMelPop (popcorn) strain. By manipulating male size we found that larger males had a higher success than smaller underfed males in the semi-field cages, regardless of their infection status.

Conclusions/Significance

The results indicate that Wolbachia infection does not reduce the competitiveness of A. aegypti males. Moreover, the body size effect suggests a potential advantage for lab-reared Wolbachia-males during a field release episode, due to their better nutrition and larger size. This may promote Wolbachia spread via CI in wild mosquito populations and underscores its potential use for disease control.     

Female discrimination against sworded males in a poeciliid fish

Female poeciliid fish of the sister genera Xiphophorus and Priapella share a mating preference favouring a sword, despite phylogenetic evidence that the sword arose in Xiphophorus. This conclusion is based on the presence of a preference favouring a sword in three unsworded species and one sworded species. Here I investigate the response to a sword in the poeciliid fish Heterandria bimaculata by testing females with conspecific males that differ in the presence or absence of a sword. Female H. bimaculata were found to prefer unsworded males, thus sworded males were discriminated against. I also tested the strength of female response; females did not show a differential response to males based on sword length. The preference for unsworded males could indicate that when new male traits arise, biases might oppose trait evolution, even in cases in which similar male traits in related taxa are favoured. Explanations for the female preference for unsworded males are discussed.     

Wolbachia-Based Population Control Strategy Targeting Culex quinquefasciatus Mosquitoes Proves Efficient under Semi-Field Conditions

In mosquitoes, the maternally inherited bacterial Wolbachia induce a form of embryonic lethality called cytoplasmic incompatibility (CI). This property can be used to reduce the density of mosquito field populations through inundative releases of incompatible males in order to sterilize females (Incompatible Insect Technique, or IIT, strategy). We have previously constructed the LR[wPip(Is)] line representing a good candidate for controlling field populations of the Culex quinquefasciatus mosquito in the islands of the south-western Indian Ocean. The main purpose of the present study was to fill the gap between laboratory experiments and field implementation, i.e. assessing mating competitiveness of these incompatible males under semi-field conditions. In a first set of experiments, we analyzed crossing relationships between LR[wPip(Is)] males and La Réunion field females collected as larvae in 19 distinct localities throughout the island. This investigation revealed total embryonic mortality, confirming the strong sterilizing capacity of LR[wPip(Is)] males. Subsequently, mating competitiveness of LR[wPip(Is)] males was assessed under semi-field conditions in the presence of field males and females from La Réunion. Confrontations were carried out in April and December using different ratios of LR[wPip(Is)] to field males. The results indicated that the LR[wPip(Is)] males successfully compete with field males in mating with field females, displaying even higher competitiveness than field males in April. Our results support the implementation of small-scale field tests in order to assess the feasibility of IIT against Cx. quinquefasciatus in the islands of southwestern Indian Ocean where this mosquito species is a proven competent vector for human pathogens.     

Competitive habitat utilization in the damselfly,Mnais nawai (Zygoptera: Calopterygidae) coexisting with a related species,Mnais pruinosa

Reproductive behaviors related to habitat utilization were studied in males of the damsefly,Mnais nawai, which has two male forms, territorial orange-winged males (nawai) and non-territorial pale-orange-winged males (sahoi), at the upper part of a mountain stream where they partiallycoexist with a related species,Mnais pruinosa, which also has two male forms, territorial orange-winged males (esakii) and non-territorial hyaline-winged males (strigata). These two species showed parapatric distribution; the lower part of the stream was occupied byM. nawai, and the upper part byM. pruinosa. In the present study, cross-matings occurred between bothMnais species, although normal intraspecific matings occurred more frequently than cross-matings. Territorial males of both species copulated with conspecific females that entered their territory and guarded the ovipositing females, probably to avoid sperm displacement resulting from subsequent copulations. Severe competition for oviposition sites by territorial males even occurred between the two species. On the other hand, non-territorial males of both species have alternative mating strategies (including several tactics such as sneaking, takeover and interception). The possible benefits from conflict among territorial males of both species is discussed.     

Mate guarding and alternative reproductive tactics in the ant Hypoponera opacior

In many animals, males have evolved weapons, elaborate courtship displays, or costly ornaments to increase their reproductive success. Ants, in contrast, commonly mate during nuptial flights, in which males do not profit from fighting or attempting to monopolize females. However, where mating occurs in the nest, males can use other reproductive tactics. We found that wingless (apterous) males of Hypoponera opacior sat on top of queen cocoons, inserted their genitalia into the cocoons and remained in copula with cocooned queens for up to 40 h. These males were tolerant of each other; fighting was never recorded. Our observations therefore suggest that wingless males of H. opacior ensure reproduction by copulatory mate guarding. This strategy, although time consuming, presumably reduces the likelihood of subsequent inseminations by other males. Apterous H. opacior males have only a limited amount of sperm available: histological preparations showed that, in contrast to Cardiocondyla fighter males, the testes degenerate in early adult life. Males ofH. opacior have relatively few mating opportunities. Although some wingless males were reproductively active for more than 3 weeks, we observed a maximum of only six matings per male, with a mean slightly above one. SomeH. opacior males used an alternative reproductive tactic of dispersal and outbreeding. We found colonies headed by single, dealate queens, which did not rear wingless sexuals but presumably reproduced through winged reproductives that mate in nuptial flights. The social structure of those colonies contrasted with nests containing wingless reproductives, which were highly polygynous and polydomous.     

Sexual Selection, Epistasis and Species Boundaries in Sympatric Hawaiian Picture-winged Drosophila

The Hawaiian picture-winged flies in the genus Drosophila are a spectacular example of rapid evolutionary diversification in which sexual selection is considered an important mechanism for reproductive isolation and speciation. We investigated the behavioral reproductive isolation of two closely related and sympatric Hawaiian picture-winged Drosophila species, D. silvestris and D. heteroneura, which are known to hybridize in nature and produce viable and fertile hybrids. We compared the mating success of parental, F1 and backcross males in pairings with D. heteroneura females. The F1 males were produced by mating D. heteroneura males with D. silvestris females, and the backcross males were produced by mating F1 females with D. heteroneura males. The mating success of backcross males paired with D. heteroneura females were significantly reduced relative to that of parental and F1 males. This reduced mating success occurred primarily at a late stage of courtship where female choice of mate may be important. Two- and three-gene models demonstrate that epistasis involving a few genes could account for the observed variation in male mating success. These results are consistent with negative epistasis in the backcross generation and support the importance of sexual selection and negative epistasis in the evolution and maintenance of these species.     

Sex-specific differences and the role of predation in the interaction between the hermit crab, Pagurus longicarpus, and its epibiont, Hydractinia symbiolongicarpus

The interaction between the hermit crab, Pagurus longicarpus, and the shell epibiont, Hydractinia symbiolongicarpus, varies from mutualism to parasitism based on the environmental context. We tested the hypothesis that this interaction also varies as a function of hermit crab sex. Given that recent work showed a negative effect of Hydractinia on female reproduction, we predicted a greater frequency of males in Hydractinia shells in the field and a stronger preference by males than females for shells with Hydractinia. Field collections documented a significantly greater proportion of males than females occupying shells with Hydractinia, and a significantly greater proportion of ovigerous females than non-ovigerous females in shells with Hydractinia. In laboratory shell-switching experiments, a greater proportion of males than females chose to enter shells with Hydractinia, but there was no difference in the proportions of males and females that vacated shells with Hydractinia.We examined whether the presence of Hydractinia influenced predation rates. Blue crabs fed on more than twice as many hermit crabs in shells with Hydractinia as compared to bare shells, but there was no significant difference for stone crabs. Laboratory experiments showed that the force required to crush shells was significantly greater for shells without Hydractinia. Thus, the lower occupancy and preference exhibited by females than males for shells with Hydractinia appears to result both from the decreased reproduction shown in past studies and an increase in predation risk.     

Bigger Is Not Always Better: Females Prefer Males of Mean Body Size in Philautus odontotarsus

Most species are believed to evolve larger body sizes over evolutionary time. Previous studies have suggested that sexual selection, through male-male competition and female choice, favors larger males. However, there is little evidence of selection against large size. The female serrate-legged small treefrogs (Philautus odontotarsus) must carry passive males from leks to breeding grounds over relatively long distances after amplexus to find a suitable place to lay eggs. The costs of large male size may therefore decrease mating success due to reduced agility and/or higher energy requirements. Thus, we hypothesized that selection would not favor larger males in P. odontotarsus. Females can assess male body size on the basis of the dominant frequency of male calls in frogs. To assess female P. odontotarsus preferences for a potential mate’s body size, male calls of high, average and low dominant frequency were played back to the females in phonotaxis experiments. Results showed that most females prefer the advertisement call with average dominant frequency. In addition, we compared the body mass distribution of amplectant males with that of single males in nature. The body masses of amplectant males are more narrowly distributed in the intermediate range than that of single males. The phonotaxis results and the data of actual female preferences in the field show that females strongly prefer potential mates of mean body sizes, consistent with the view that, in this species at least, larger males are not always perceived as better by females. In the present study, P. odontotarsus provides an example of an amphibian species in which large size does not have an advantage in mating success for males. Instead, our results provide evidences that stabilizing selection favors the optimal intermediate size of males.     

Inseparability of X-Heterochromatic Functions Responsible for X:Y Pairing, Meiotic Drive, and Male Fertility in Drosophila melanogaster

Deficiencies encompassing part or all of the X heterochromatin of Drosophila melanogaster have been linked to three abnormalities in male meiosis and spermatogenesis: X-Y nondisjunction, skewed sperm recovery ratios favoring sperm with reduced chromatin content, and sterility in males carrying either a Y-autosome translocation or mal ⁺Y. In this study, 18 X heterochromatic deficiencies of varying sizes were tested in XY males for their spermatogenic phenotypes. All 18 proved to be either mutant for all three phenotypes or wild type for all three. Although variable among mutant deficiencies, expression levels of all three phenotypes were strongly correlated. Deficiencies that cause high levels of nondisjunction also cause severe recovery ratio distortion and are completely sterile in conjunction with mal⁺ Y. Low nondisjunction deficiencies cause comparable mild effects for the other phenotypes. The same deficiencies were also tested in males carrying a large heterochromatic free X duplication Dp(1; f)3. For all deficiencies which induce nondisjunction in XY males, the Y and free duplication pair regularly and the X fails to pair in XYDp males. Drive levels are constant across deficiencies in these males. Thus elimination of variability in the pairing phenotype also eliminates variability in sperm recovery ratios.     

Internal architecture of pampas deer antlers differs in males allocated with and without females

Bone mineralization of antlers and the depth of the antler seal (the basal surface of the cast antlers) are positively related to testosterone concentrations. Pampas deer males that are in permanent contact with females have greater, heavier, and darker antlers than males that are isolated from them. The objectives were to determine if antler compact/spongy bone ratio, antler seal depth, and compact bone darkness are greater in pampas deer males permanently allocated with females than those in males isolated from them. Antlers from males permanently allocated with or without females were cut transversally in seven points and scanned, and the compact/spongy ratio was calculated. The pixel intensity of each image was determined with a software for image analysis. The coronet of the antler was cut longitudinally, and the height of the most prominent protrusion was measured. The compact/spongy ratio was greater in antlers from males that were in contact with females in the second tine (P?=?0.02) and tended to be great in the two other tines (P?=?0.06 and P?=?0.1, respectively). Compact bone pixel color was darker in males in contact with females than that in males isolated from females in two points (P?=?0.02 and P?=?0.05, respectively) and tended to do so in two more (P?=?0.055 and P?=?0.1, respectively). Antler seal convexity was also greater in antlers from males in contact with females (P?=?0.006). We concluded that permanent contact with females stimulated pampas deer males increasing compact bone portion of the antler tines, the seal depth size, and the darkness of the compact bone.     

Sex Attraction and Mating in Bursaphelenchus okinawaensis and B. xylophilus

The fungal feeding, hermaphroditic Bursaphelenchus okinawaensis is a laboratory model to understand the biology of Bursaphelenchus. The extent to which B. okinawaensis can be used to model Bursaphelenchus xylophilus mating was investigated. A chemotaxis assay was conducted to examine whether B. xylophilus and B. okinawaensis produce and respond to volatile sex attractants. Unmated B. xylophilus females were found to attract B. xylophilus males. Similarly, old (sperm depleted) but not young (sperm repleted) B. okinawaensis hermaphrodites attract B. okinawaensis males. Thus, in both species, sperm status corresponds to its ability to attract males. B. xylophilus males also produce a volatile pheromone that attracts both mated and unmated females. A second assay, in which the behavior of males on petri plates in the presence of different females or hermaphrodites of Bursaphelenchus was observed, revealed that B. xylophilus unmated females attract B. okinawaensis males, and B. okinawaensis old hermaphrodites attract B. xylophilus males. These observations suggested that the pheromones of Bursaphelenchus work to some extent across species. Mating behavior through spicule insertion occurs across species, suggesting that postcopulatory mechanisms prevent production of interspecific progeny. The hermaphroditic B. okinawaensis will be a useful model to conduct genetic studies for the understanding of the molecular mechanisms underlying mating behavior in Bursaphelenchus nematodes.     

How cross-modality effects during intraspecific interactions of jumping spiders differ depending on whether a female-choice or mutual-choice mating system is adopted

An important prediction from game theory is that the value of a resource influences the level to which conflict escalates. Here we use jumping spiders (Salticidae) to consider this prediction in the context of species adopting different mating systems (‘female mate-choice’ and ‘mutual mate-choice’). Our experiments are designed for determining whether the odour of conspecific females, more than the odour of heterospecific females, primes males to escalate conflict with a potential same-sex rival and also whether the odour of conspecific males, more than the odour of heterospecific males, primes females to escalate conflict with a potential same-sex rival. Four species were studied: Evarcha culicivora, a species in which mutual mate-choice is pronounced, and Portia fimbriata, Portia africana, and Jacksonoides queenslandicus, more conventional salticids in which female mate-choice and male–male competition appear to be dominant. Our hypothesis is that, for all four species, there is strong competition between males for access to females and that, for E. culicivora, but not for the other three species, there is also strong competition between females for access to males. Our findings are consistent with this hypothesis, as we show that, although the odour of conspecific females primes escalation of vision-based male–male conflict for all four species, E. culicivora is the only species for which there is evidence of odour from conspecific males priming the escalation of female–female conflict.     

Mark-release-recapture of males of Bactrocera cucurbitae and B. dorsalis (Diptera: Tephritidae) in two residential areas of Honolulu

This paper describes a mark-release-recapture study involving males of two economically important tephritid fruit flies (Diptera: Tephritidae), Bactrocera cucurbitae (Coquillett) and B. dorsalis (Hendel), conducted over a 2-year period in Honolulu, Hawaii. In each of two residential neighborhoods, we placed two traps, one baited with cue lure and the other with methyl eugenol (male attractants for B. cucurbitae and B. dorsalis, respectively), in a single tree. Dyed, mature males from recently established laboratory colonies were released 100 or 500 m from the traps along the four compass directions in both winter and summer seasons. In each neighborhood, a total of 5,600 flight-able males of each species were released 100 m from the traps (14 dates × 4 directions × 100 males/release) and 56,000 flight-able males of each species were released 500 m from the traps (14 dates × 4 directions × 1,000 males/release). Within each study area, the number of males trapped did not vary significantly with direction or season for either species for either the 100- or 500-m releases. Significantly higher numbers of B. dorsalis males were captured than B. cucurbitae males for both the 100-m (16 versus 8 males/release) and 500-m (7 versus 2 males/release) releases (average values computed over both study areas). Also, following their release, B. dorsalis males were, in general, trapped more quickly than B. cucurbitae males. Given the strong attractancy of methyl eugenol, trap captures for B. dorsalis were lower than expected, and possible explanations are discussed.     

Intraspecific shape variation in horseshoe crabs: The importance of sexual and natural selection for local adaptation

A morphometric analysis of the body shape of three species of horseshoe crabs was undertaken in order to infer the importance of natural and sexual selection. It was expected that natural selection would be most intense, leading to highest regional differentiation, in the American species Limulus polyphemus, which has the largest climatic differences between different populations. Local adaptation driven by sexual selection was expected in males but not females because horseshoe crab mating behaviour leads to competition between males, but not between females. Three hundred fifty-nine horseshoe crabs from nine populations, representing three species, were analyzed using a digitizer to position sixty morphometric landmarks in a three-dimensional space. Discriminant analysis revealed strong regional structuring within a species, which suggests strong philopatry, and showed the existence of geographically-based intraspecific variation. An admixture analysis showed regional intraspecific differentiation for males and females of L. polyphemus and males of the Asian horseshoe crab Carcinoscorpius rotundicauda, but not for females of C. rotundicauda and another Asian horseshoe crab, Tachypleus gigas. Differences in shape variation between sexes were tested with F-tests, which showed lower intrapopulation morphometric variation in males than females. These results indicate a lower degree of local adaptation on body shape in C. rotundicauda and T. gigas than in L. polyphemus and a lower degree of local adaptation in females than in males.     

Effect of Drosophila melanogaster Female Size on Male Mating Success

In this study, we examined the influence of female size on mating success in Drosophila melanogaster. The results that were obtained from experiments performed in mating chambers allowed us to confirm the results of previous studies, demonstrating higher mating success of larger D. melanogaster males, and to conclude that female size also affects mating success, either when considering a single male or two competing males. We observed that the advantage for larger males depends on their size relative to that of the female, demonstrating a previously unknown role for female size in mating behavior studies. This effect of female size on mating success depends on various factors: males take longer to initiate courtship toward larger females, large females receive more wing vibrations from males prior to mating, and large females tend to keep moving for longer periods during male courtship. The importance of this finding is discussed in the context of recent reports on sexual conflict in D. melanogaster, in which males were observed to depress fitness in females as a result of intercourse.     

Mating strategies in two species of dart-poison frogs: a comparative study

Comparative field studies of species of dart-poison frogs in the genus Dendrobates were carried out to test predictions from two hypotheses that attempt to explain female-female competition for mates in species of Dendrobates with male parental care. The sex role reversal hypothesis proposes that males invest so much time and energy in parental care that receptive males are rare relative to receptive females, and females compete to find and mate with receptive males. The parental quality hypothesis proposes that females compete to monopolize the parental effort of particular males, because they potentially suffer a cost when their mates care for the offspring of other females. Comparisons between species with male parental care (Dendrobates leucomelas) and female parental care (Dendrobates histrionicus) contradicted prediction of the sex role reversal hypothesis, but were consistent with predictions of the parental quality hypothesis. Male D. histrionicus did not compete for mates more aggressively than male D. leucomelas, and male D. leucomelas were not more selective about mating than male D. histrionicus. Female D. leucomelas and D. histrionicus were both selective about mating; female D. leucomelas associated with and competed for particular males, whereas female D. histrionicus did not.     

The mechanism of the fecundity reduction in Callosobruchus maculatus caused by Callosobruchus chinensis males

Male promiscuity sometimes results in interspecific reproductive interaction, also known as reproductive interference. Reproductive interference entails costs for the individuals involved and affects the community structure by reducing the population growth rate. However, our understanding of the mechanisms generating reproductive interference is still insufficient. Two congeneric bean weevils, Callosobruchus chinensis and C. maculatus, show asymmetric reproductive interference; only C. chinensis males reduce the fecundity of the other species. Here we investigated the mechanism of reproductive interference by C. chinensis males on C. maculatus females in terms of lifetime fecundity. Callosobruchus chinensis males with ablated genitals, which could harass C. maculatus females but not copulate with them, did not reduce the C. maculatus fecundity, suggesting that interspecific copulation was necessary. However, a single interspecific copulation did not affect C. maculatus fecundity as long as the females also copulated with a conspecific male. Exposure to C. chinensis males for 24 h prior to oviposition significantly reduced C. maculatus fecundity, and fecundity was negatively correlated with the number of C. chinensis males the females were exposed to. Additionally, C. maculatus females experienced more interspecific copulations when they were housed with more C. chinensis males. Together these findings suggest that multiple interspecific copulations by C. chinensis males reduce the fecundity of C. maculatus females. Thus in general, even if a single interspecific copulation is apparently harmless, repeated interspecific copulations can be costly for the individuals involved. Furthermore, only by quantifying reproductive success were we able to identify the precise mechanism of reproductive interference.     

Females lay fewer eggs for males with greater courtship success in a lekking Drosophila

The evolutionary basis for female mate choice in lek mating systems has been a common subject of research in animal behaviour. Because males apparently provide only gametes to females in lekking species, most research has focused on possible indirect (genetic) benefits that females might gain by discriminating among males. Despite the emphasis on indirect benefits, it has been recognized that females in non-resource-based systems such as leks could potentially gain direct benefits via mate choice if males varied in fertilization abilities, for example. Previous evidence has shown that females of a lekking Hawaiian Drosophila, D. grimshawi, vary in fecundity when mated to certain males, and that females possess preferences for vigorously courting males. This study tests the hypothesis that D. grimshawi females gain direct benefits by preferentially mating with more sexually vigorous males. Male courtship vigour (performance of wing and head-under-wing displays) and the consequences of female choice on offspring production were evaluated separately using different females. Unexpectedly, matings involving more vigorously courting males resulted in fewer offspring being produced. Reduced offspring number resulted because females laid fewer eggs when mating with males having greater courtship success. These results are discussed in light of sexual conflict and possible multiple mating by females. Females also demonstrated considerable variation in mating behaviour and behavioural variation was correlated with mating benefits. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Wolbachia Age-Sex-Specific Density in Aedes albopictus: A Host Evolutionary Response to Cytoplasmic Incompatibility?

Background

Wolbachia bacteria have invaded many arthropod species by inducing Cytoplasmic Incompatibility (CI). These symbionts represent fascinating objects of study for evolutionary biologists, but also powerful potential biocontrol agents. Here, we assess the density dynamics of Wolbachia infections in males and females of the mosquito Aedes albopitcus, an important vector of human pathogens, and interpret the results within an evolutionary framework.

Methodology/Principal Findings

Wolbachia densities were measured in natural populations and in age controlled mosquitoes using quantitative PCR. We show that the density dynamics of the wAlbA Wolbachia strain infecting Aedes albopictus drastically differ between males and females, with a very rapid decay of infection in males only.

Conclusions/Significance

Theory predicts that Wolbachia and its hosts should cooperate to improve the transmission of infection to offspring, because only infected eggs are protected from the effects of CI. However, incompatible matings effectively lower the fertility of infected males, so that selection acting on the host genome should tend to reduce the expression of CI in males, for example, by reducing infection density in males before sexual maturation. The rapid decay of one Wolbachia infection in Aedes albopictus males, but not in females, is consistent with this prediction. We suggest that the commonly observed reduction in CI intensity with male age reflects a similar evolutionary process. Our results also highlight the importance of monitoring infection density dynamics in both males and females to assess the efficiency of Wolbachia-based control strategies.