Comparison of spermatheca morphology between reproductive and non-reproductive females in social wasps

Social wasps show an obvious evolution of the differentiation in behavior and external size between reproductive and non-reproductive females, with no clear differences in the Stenogastrinae, via overlap in the Polistinae, to clear differences in the Vespinae. In this study, we examined the morphological appearance of the spermatheca in representative species of these three subfamilies. The general anatomical organization of the spermatheca comprises a reservoir, a duct and two spermathecal glands, and is in line with its common structure in other social Hymenoptera. All examined wasp species have a spermathecal reservoir with uniform wall thickness, which is similar to the situation in the bees, but differentiates them from the ants. Within the wasps, the shape of the reservoir, the shape of the spermathecal glands and their attachment site to the spermatheca differs among the Stenogastrinae, Polistinae and Vespinae. The reservoir wall is thick in the Polistinae and Vespinae, while in the Stenogastrinae, it varies from thin in Parischnogaster to thick in Eustenogaster, with an intermediate situation in Liostenogaster. In all examined species, we found no differences in the spermathecal development between reproductive and non-reproductive wasps.     

Genetic support for the evolutionary theory of reproductive transactions in social wasps

Recent evolutionary models of reproductive partitioning within animal societies (known as 'optimal skew', 'concessions' or 'transactional' models) predict that a dominant individual will often yield some fraction of the group's reproduction to a subordinate as an incentive to stay in the group and help rear the dominant's offspring. These models quantitatively predict how the magnitude of the subordinate's 'staying incentive' will vary with the genetic relatedness between dominant and subordinate, the overall expected group output and the subordinate's expected output if it breeds solitarily. We report that these predictions accord remarkably well with the observed reproductive partitioning between conesting dominant and subordinate queens in the social paper wasp Polistes fuscatus. In particular, the theory correctly predicts that (i) the dominant's share of reproduction, i.e. the skew, increases as the colony cycle progresses and (ii) the skew is positively associated both with the colony's productivity and with the relatedness between dominant and subordinate. Moreover, aggression between foundresses positively correlated with the skew, as predicted by transactional but not alternative tug-of-war models of societal evolution. Thus, our results provide the strongest (quantitative support yet for a unifying model of social evolution.     

Visual sensitivity and foraging in social wasps

Summary While there is a distinction between that intensity of illumination which permits social wasps to forage, and that to which a sessile worker can respond, nevertheless illumination is the most critical of the environmental factors which control the activity of wasps. Low temperatures, high winds, and heavy rain all reduce activity but unless exceptionally severe do not wholly stop it. At dawn, when the critical level of illumination is attained, workers leave the nest, but at dusk they will not leave should the same critical level be due in the course of the foraging flight, after which they could not return.The three species of wasp,Vespula vulgaris, V. rufa, andV. germanica have a common threshold of illumination, although the hornet,Vespa crabro can forage in moonlight at an altogether lower illumination. Honey-bees normally need a still higher illumination than do wasps.In all these species, the thresholds of illumination are related to the length of the compound eyes, so that species with large eyes need less light by which to forage. Moreover, there is a slight difference between the threshold at dawn when workers leave the nest, and that at dusk, when they must needs have sufficient light by which to return. This difference is almost constant for each species, when, as is customary, one measures it on a logarithmic scale.Lastly, the estimates, which these experiments provide, of the threshold illuminations depend stochastically on the number of workers foraging. A correction for this bias is given.

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Sommaire Parmi les facteurs du milieu qui contrôlent l'activité des guêpes, celui de l'intensité de lumière est le plus important; toutefois on note une différence entre l'intensité de lumière qui permet aux guêpes sociales de fourrager, et celle qui produit une réponse des ouvrières sessiles.En général, les basses températures, les vents forts, et les grandes pluies réduisent leur activité, mais ces facteurs ne l'arrêtent pas complètement, à moins qu'ils ne soient très marqués.A l'aube, quand le niveau critique de lumière est atteint, les ouvrières quittent le guêpier, mais, le soir, si elles s'attendent à ce que la lumière vienne à s'abaisser au cours de leur sortie au-dessous du niveau critique, elles ne sortent pas.Les trois espèces de guêpe,Vespula vulgaris, V. rufa, etV. germanica, réagissent au même seuil de lumière, mais le frelon,Vespa crabro, est capable de fourrager au clair de lune par une lumière moins intense. Normalement, les abeilles exigent une lumière plus intense que les guêpes.Dans toutes ces espèces, le seuil de lumière se rapporte à la hauteur des yeux composés, par conséquent les espèces pourvues de grands yeux sont à même de fourrager par une lumière moins intense. De plus, il y a une légère différence entre le seuil de lumière à l'aube, quand les ouvrières quittent le guêpier, et celui du soir lorsqu'elles ont besoin d'une lumière suffisante pour rentrer. Cette différence, quand elle est mesurée à l'échelle logarithmique, comme il est d'usage, est presque constante pour chaque espèce.Enfin, les évaluations du seuil de lumière dans ces expériences dépendent stochastiquement du nombre d'ouvrières en train de fourrager. On a tenu compte de ce fait.

     

Recognition and social dominance inPolistes wasps

It has been known for a long time that relationships among femalePolistes wasps are controlled by dominance through linear hierarchy. A classical model of the development of social hierarchies suggests that individual recognition intervenes to maintain the hierarchy established after the initial confrontation that takes place when two animals meet for the first time. This hypothesis was tested in the present study onPolistes gallicus (L.) foundresses. A tolerance index was calculated from the reactions of a dominant female when already known or unacquainted females were successively encountered. Results show that individualised social recognition is possible and that it takes more than 4 h of inter-individual contact for individual characteristics to be memorised.     

High reproductive skew in tropical hover wasps.

A plethora of recent models examines how genetic and environmental factors might influence partitioning of reproduction ('skew') in animal societies, but empirical data are sparse. We used three microsatellite loci to estimate skew on 13 nests of the Malaysian hover wasp, Liostenogaster flavolineata. Groups are small in L. flavolineata (1-10 females) and all females are capable of mating and laying eggs. Despite considerable variation between nests in parameters expected to influence skew, skew was uniformly high. On 11 of the 13 nests, all female eggs had been laid by a single dominant female. A second female had laid one to two out of 5-10 eggs respectively on the two remaining nests. A likelihood analysis suggested that on average, 90% of the male eggs had also been laid by the dominant. The slightly lower skew among male eggs might reflect the lower average relatedness of subordinates to male versus female offspring of the dominant. We suggest that high skew in L. flavolineata may result from strong ecological constraints and a relatively high probability that a subordinate will eventually inherit the dominant, egg-laying position.     

Ecological volatility and human evolution: A novel perspective on life history and reproductive strategy

Humans are characterized by a suite of traits that seem to differentiate them profoundly from closely related apes such as the gorilla, chimpanzee, and orang‐utan. These traits include longevity, cooperative breeding, stacking of offspring, lengthy maturation, and a complex life‐course profile of adiposity. When, how, and why these traits emerged during our evolutionary history is currently attracting considerable attention. Most approaches to life history emphasize dietary energy availability and the risk of mortality as the two key stresses shaping life‐history variability between and within species. The high energy costs of the large Homo brain are also seen as the central axis around which other life‐history traits were reorganized. I propose that ecological volatility may have been a key stress, selecting in favor of the suite of traits in order to tolerate periods of energy scarcity, and increase reproductive output during periods of good conditions. Theses life‐history adaptations may have preceded and enabled the trend toward encephalization. © 2012 Wiley Periodicals, Inc.     

Drifting behaviour as an alternative reproductive strategy for social insect workers

Restricted reproduction is traditionally posited as the defining feature of eusocial insect workers. The discovery of worker reproduction in foreign colonies challenges this view and suggests that workers’ potential to pursue selfish interests may be higher than previously believed. However, whether such reproductive behaviour truly relies on a reproductive decision is still unknown. Workers’ reproductive decisions thus need to be investigated to assess the extent of workers’ reproductive options. Here, we show in the bumblebee Bombus terrestris that drifting is a distinct strategy by which fertile workers circumvent competition in their nest and reproduce in foreign colonies. By monitoring workers’ movements between colonies, we show that drifting is a remarkably dynamic behaviour, widely expressed by both fertile and infertile workers. We demonstrate that a high fertility is, however, central in determining the propensity of workers to enter foreign colonies as well as their subsequent reproduction in host colonies. Moreover, our study shows that the drifting of fertile workers reflects complex decision-making processes associated with in-nest reproductive competition. This novel finding therefore adds to our modern conception of cooperation by showing the previously overlooked importance of alternative strategies which enable workers to assert their reproductive interests.     

Climate change impact on neotropical social wasps

Establishing a direct link between climate change and fluctuations in animal populations through long-term monitoring is difficult given the paucity of baseline data. We hypothesized that social wasps are sensitive to climatic variations, and thus studied the impact of ENSO events on social wasp populations in French Guiana. We noted that during the 2000 La Niña year there was a 77.1% decrease in their nest abundance along ca. 5 km of forest edges, and that 70.5% of the species were no longer present. Two simultaneous 13-year surveys (1997–2009) confirmed the decrease in social wasps during La Niña years (2000 and 2006), while an increase occurred during the 2009 El Niño year. A 30-year weather survey showed that these phenomena corresponded to particularly high levels of rainfall, and that temperature, humidity and global solar radiation were correlated with rainfall. Using the Self-Organizing Map algorithm, we show that heavy rainfall during an entire rainy season has a negative impact on social wasps. Strong contrasts in rainfall between the dry season and the short rainy season exacerbate this effect. Social wasp populations never recovered to their pre-2000 levels. This is probably because these conditions occurred over four years; heavy rainfall during the major rainy seasons during four other years also had a detrimental effect. On the contrary, low levels of rainfall during the major rainy season in 2009 spurred an increase in social wasp populations. We conclude that recent climatic changes have likely resulted in fewer social wasp colonies because they have lowered the wasps'' resistance to parasitoids and pathogens. These results imply that Neotropical social wasps can be regarded as bio-indicators because they highlight the impact of climatic changes not yet perceptible in plants and other animals.     

The ecology and evolution of male reproductive behaviour in the bees and wasps

Males of aculeate Hymenoptera differ in the behavioural adaptations employed to locate and secure mates. The ecological and evolutionary bases of these differences are explored in this paper. Male bees and wasps search for females by patrolling widely within emergence-nesting areas or within patches of flowers attractive to conspecific females, or by waiting at landmarks, at specific emergence sites, or at nests. Nest dispersion, flower distribution, the type of female mating system and the nature of male-male competition appear to be key factors in determining the mate-locating behaviour of males. Of special interest in multiple-mating by females, which may be an evolutionary response to the costs of attempting to resist copulation in certain situations. When polyandry occurs, males are under selection pressure to be the last male to copulate with a female prior to oviposition if sperm precedence occurs. In species in which females mate just once, a selective premium is placed on being the first male to reach a virgin female. In either case, because receptive females are a limited resource, there is intense competition among males for access to the resource. The density of competitor males may play an important role in determining whether holding a relatively restricted territory is preferable to the strategy of patrolling widely at various sites which may have females. Territoriality is practiced by males of several species of aculeate Hymenoptera when the number of male competitors is relatively few in number and the distribution of emergence sites or foraging areas of females is clumped in space.     

Resource choice of social wasps: influence of presence, size and species of resident wasps

Summary: The role of visual cues provided by resident wasps on resource choice by yellowjacket and paper wasp foragers was investigated. Large spring queen yellowjackets and small early season yellowjacket foragers (Vespula germanica, Vespula maculifrons, and Vespula vidua) were extracted in hexane to remove odors and posed as though feeding at petri dish feeders bearing daisy-like flower models, equipped with microcapillary feeding tubes, and containing 1:3 honey:water solution. An array of five feeders was presented to foragers at a suburban and a woodland site in Saratoga Springs, New York. The visual cues provided by resident wasps influenced resource choice by approaching social wasp foragers. Vespula germanica, an introduced yellowjacket species that tends to dominate at rich resources, was the only wasp visiting the suburban feeders. Foragers of this species preferentially fed on feeders and flowers with posed wasps and fed most often next to large wasps. Polistes fuscatus foragers at the woodland site similarly preferred to feed on occupied feeders and flowers. Vespula maculifrons and V. consobrina preferentially visited unoccupied feeders. Individual V. maculifrons, V. consobrina and V. vidua foragers that landed on occupied feeders all preferentially visited unoccupied flowers on those feeders. Vespula vidua and V. flavopilosa foragers did not demonstrate a feeder preference based on the presence/absence of posed wasps. Vespula consobrina foragers that visited occupied feeders preferred those occupied by extracted V. maculifrons queens and workers; no other wasps showed species based landing preferences.     

Larval sex identification of in vivo and long-term ethanol-preserved social wasps

Larval genital discs are sexually dimorphic in many social wasps (Hymenoptera: Vespidae). These sexual traits are visible in vivo and in long-term ethanol-preserved samples. Genital disc morphology provides a cheap, quick, and accurate way to distinguish larvae of females and males.     

Comparison of reproductive strategies in two externally ovipositing non-pollinating fig wasps

The reproductive strategies of Walkerella sp.1 associated with Ficus curtipes and Walkerella sp.2 associated with Ficus benjamina were investigated. Both species oviposited from outside the fig wall. Walkerella sp.1 was the first non-pollinating fig to oviposit on Ficus curtipes and began to do this ten days after figs syconia began to develop. The larvae of Walkerella sp.1 were only found in the most external ovary layer of the fig. Walkerella sp.2 starts ovipositing after several other non-pollinating fig wasps have already laid their eggs in F. benjamina. The progeny of Walkerella sp.2 are distributed in the external ovary layer, the middle ovary layer, and/or the inner ovary layer of the figs. However, more than a quarter of the offspring were found in the most external layer and only a few in the inner layer. Experimental studies proved that the two Walkerella species are gall formers. In both manipulated figs and in natural figs, the sex ratios of Walkerella sp.1 and Walkerella sp.2 were female-biased. In Walkerella sp.2, the overall sex ratio increased with the proportion of figs parasitized in a crop, but this was not the case for Walkerella sp.1. Females of both Walkerella species appear not to have information about the patches on which they oviposit because sex ratios of both species decreased as brood sizes within individual figs increased and foundresses of both species were able to lay clutches containing a single male egg and several female eggs.     

Gene expression and the evolution of phenotypic diversity in social wasps

Background  

Organisms are capable of developing different phenotypes by altering the genes they express. This phenotypic plasticity provides a means for species to respond effectively to environmental conditions. One of the most dramatic examples of phenotypic plasticity occurs in the highly social hymenopteran insects (ants, social bees, and social wasps), where distinct castes and sexes all arise from the same genes. To elucidate how variation in patterns of gene expression affects phenotypic variation, we conducted a study to simultaneously address the influence of developmental stage, sex, and caste on patterns of gene expression in Vespula wasps. Furthermore, we compared the patterns found in this species to those found in other taxa in order to investigate how variation in gene expression leads to phenotypic evolution.     

Fitness in invasive social wasps: the role of variation in viral load,immune response and paternity in predicting nest size and reproductive output

Within any one habitat, the relative fitness of organisms in a population can vary substantially. Social insects like the common wasp are among the most successful invasive animals, but show enormous variation in nest size and other fitness‐related traits. Some of this variation may be caused by pathogens such as viruses that can have serious consequences in social insects, which range from reduced productivity to colony death. Both individual immune responses and colony‐level traits such as genetic diversity are likely to influence effects of pathogen infections on colony fitness. Here we investigate how infections with Kashmir Bee Virus (KBV), immune response and intracolony genetic diversity (due to queen polyandry) affect nest size in the invasive common wasp Vespula vulgaris. We show that KBV is highly prevalent in wasps and expression of antiviral immune genes is significantly increased with higher viral loads across individuals. Patriline membership within a nest did not influence KBV susceptibility or immune response. A permutational MANCOVA revealed that polyandry, viral load and expression of the immune gene Dicer were significant predictors of variation in nest size. High intracolony genetic diversity due to polyandry has previously been hypothesized to improve colony‐level resistance to parasites and pathogens. Consistent with this hypothesis, we observed genetically diverse colonies to be significantly larger and to produce more queens, although this effect was not driven by the pathogen we investigated. Invasive wasps clearly suffer from pathogens and expend resources, as indicated here by elevated immune gene expression, toward reducing pathogen‐impact on colony fitness.     

The evolution of host use and unusual reproductive strategies in Achrysocharoides parasitoid wasps

We studied host selection and exploitation, two crucial aspects of parasite ecology, in Achrysocharoides parasitoid wasps, which show remarkable host specificity and unusual offspring sex allocation. We estimated a molecular phylogeny of 15 Achrysocharoides species and compared this with host (plant and insect) phylogenies. This tri-trophic phylogenetic comparison provides no evidence for cospeciation, but parasitoids do show phylogenetic conservation of the use of plant genera. Patterns of sequence divergence also suggest that the parasitoids radiated more recently (or evolved much faster) than their insect hosts. Three main categories of brood production occur in parasitoids: (1) solitary offspring, (2) mixed sex broods and (3) separate (split) sex broods. Split sex broods are very rare and virtually restricted to Achrysocharoides, while the other types occur very widely. Our phylogeny suggests that split sex broods have evolved twice and provides evidence for a transition from solitary to mixed sex broods, via split sex broods, as predicted by theory.     

Coevolution of reproductive characteristics in three dioecious fig species and their pollinator wasps

This study investigates dioecious fig species using a pollinator introduction experiment. Our aims were to determine: (1) whether there was a significant difference in foundress distribution between sexes per fig species; (2) whether fig size and foundress number affect reproductive success of dioecious figs; and (3) who is the ‘controlling partner’ in the fig/pollinator mutualism. Three dioecious fig species: Ficus semicordata, Ficus hispida and Ficus tinctoria from Xishuangbanna, China, were selected for this experiment. We found that there was no significant difference of the foundress number in female and male figs of F. semicordata, F. hispida and F. tinctoria. Also, the foundress number did not depend on the fig diameter. The numbers and the proportions of fig seeds and female wasp offspring significantly increased with more foundresses; and fig seed number was significantly higher than female wasp offspring in F. semicordata and F. hispida, but not in F. tinctoria. Our results indicate that figs are generally the ‘controlling partner’ in fig-wasp mutualisms in species with large figs, but not with small figs. Compared with published studies of reproductive success in monoecious figs, the dioecious figs seem to be more efficient in producing both seeds and wasp offspring when there is a high number of foundress.     

Evolutionary trends in the chromosome numbers of swarm-founding social wasps

Chromosome number and morphology are relevant aspects of genomic organization of eukaryotes and are considered key components to comprehend evolutionary mechanisms and karyotypic differentiation. Social wasps belonging to the tribe Epiponini show complex social characteristics that make these insects interesting models for genetic and evolutionary studies. However, there is a paucity of genetic information in social wasps as a whole. Aiming to investigate the process of chromosomal evolution of the social Epiponini wasps, chromosomes of ten species of this group were analyzed using Giemsa and fluorochrome staining techniques and fluorescence in situ hybridization in two species. In this study a high variation in the chromosome number and morphology was found and the previous range of chromosome number of Epiponini was broadened from n = 8–32 to n = 5–33. We also suggest that chromosomal segments with high GC content must have had a key role in the karyotype diversification of these wasps. Moreover, based on a phylogenetic background we find evidence of a main role of chromosomal fusion in the occurrence of gradual decrease of chromosome number in Epiponini during its chromosomal evolutionary history.