Differential energy allocation as an adaptation to different habitats in the parasitic wasp Venturia canescens

A chromosome scale assembly of the parasitoid wasp Venturia canescens provides insight into the process of virus domestication

The parasitoid wasp Venturia canescens is an important biological control agent of stored products moth pests and serves as a model to study the function and evolution of domesticated endogenous viruses (DEVs). The DEVs discovered in V. canescens are known as virus-like particles (VcVLPs), which are produced using nudivirus-derived components and incorporate wasp-derived virulence proteins instead of packaged nucleic acids. Previous studies of virus-derived components in the V. canescens genome identified 53 nudivirus-like genes organized in six gene clusters and several viral pseudogenes, but how VcVLP genes are organized among wasp chromosomes following their integration in the ancestral wasp genome is largely unknown. Here, we present a chromosomal scale genome of V. canescens consisting of 11 chromosomes and 56 unplaced small scaffolds. The genome size is 290.8 Mbp with a N50 scaffold size of 24.99 Mbp. A high-quality gene set including 11,831 protein-coding genes were produced using RNA-Seq data as well as publicly available peptide sequences from related Hymenoptera. A manual annotation of genes of viral origin produced 61 intact and 19 pseudogenized nudivirus-derived genes. The genome assembly revealed that two previously identified clusters were joined into a single cluster and a total of 5 gene clusters comprising of 60 intact nudivirus-derived genes were located in three chromosomes. In contrast, pseudogenes are dispersed among 8 chromosomes with only 4 pseudogenes associated with nudivirus gene clusters. The architecture of genes encoding VcVLP components suggests it originates from a recent virus acquisition and there is a link between the processes of dispersal and pseudogenization. This high-quality genome assembly and annotation represents the first chromosome-scale assembly for parasitoid wasps associated with VLPs, and is publicly available in the National Center for Biotechnology Information Genome and RefSeq databases, providing a valuable resource for future studies of DEVs in parasitoid wasps.

Poor adult nutrition impairs learning and memory in a parasitoid wasp

Animals have evolved cognitive abilities whose impairment can incur dramatic fitness costs. While malnutrition is known to impact brain development and cognitive functions in vertebrates, little is known in insects whose small brain appears particularly vulnerable to environmental stressors. Here, we investigated the influence of diet quality on learning and memory in the parasitoid wasp Venturia canescens. Newly emerged adults were exposed for 24 h to either honey, 20% sucrose solution, 10% sucrose solution, or water, before being conditioned in an olfactory associative learning task in which an odor was associated to a host larvae (reward). Honey fed wasps showed 3.5 times higher learning performances and 1.5 times longer memory retention than wasps fed sucrose solutions or water. Poor diets also reduced longevity and fecundity. Our results demonstrate the importance of early adult nutrition for optimal cognitive function in these parasitoid wasps that must quickly develop long-term olfactory memories for searching suitable hosts for their progeny.

Determining the total energy budget of the tick Ixodes ricinus

Precise and accessible techniques for measuring metabolic responses to environmental stress are essential to allow the likely impacts of climate and climate change on tick distribution, abundance and phenology to be predicted. A more detailed understanding of the metabolic profile of ticks may also help the complex responses to pathogen infection and effects on transmission to be evaluated. Here, a series of biochemical protocols employing spectrophotometric methods are used to determine the entire energy budget of ticks. Protein, carbohydrate, total lipid, neutral lipid and glycogen were measured in individual Ixodes ricinus nymphs and adults. Two key trends were identified: in adults, protein was relatively more abundant than in nymphs, whereas in nymphs, glycogen and carbohydrate were more abundant than in adults, with glycogen alone composing 39% of the mass of metabolites in nymphs compared to 15 and 10% in females and males, respectively. The methods used were able to successfully separate neutral lipids from the polar phospholipids and the importance of distinguishing stored from structural lipid in estimates of lipid reserves is emphasised. The results demonstrate that the spectrophotometric approaches deliver relatively rapid and reliable estimates of the total energetic budget and can be used to quantify the metabolic profiles of individual ticks, demonstrating their suitability for use in ecological and epidemiological studies.

The combined effect of host and food availability on optimized parasitoid life-history traits based on a three-dimensional trade-off surface

The reproductive success of many insects is considered to be limited by two main factors: the availability of mature eggs to lay (termed egg limitation) and the time to locate suitable hosts (termed time limitation). High host density in the environment is likely to enhance oviposition opportunities, thereby selecting for higher investment in egg supply. In contrast, a shortage of food (e.g. sugar sources) is likely to increase the risk of time limitation, thereby selecting for higher allocation to initial energy reserves. To our knowledge, the combined effect of host and food availability on these optimal life-history allocations has never been investigated. We thus modelled their simultaneous effects on a three-dimensional trade-off between initial investment in energy reserves, egg number and egg size, while focusing on insect parasitoids. The model was based on Monte Carlo simulations coupled with genetic algorithms, in order to identify the optimal life-history traits of a single simulated parasitoid female in an environment in which both hosts and food are present in varying densities. Our results reproduced the simple predictions described above. However, some novel predictions were also obtained, especially when specific interactions between the different factors were examined and their effects on the three-dimensional life-history surface were considered. The work sheds light on long-lasting debates regarding the relative importance of time versus egg limitation in determining insect life-history traits and highlights the complexity of life-history evolution, where several environmental factors act simultaneously on multiple traits.

Factors shaping life history traits of two proovigenic parasitoids

What shapes the relative investment in reproduction versus survival of organisms is among the key questions in life history. Proovigenic insects mature all their eggs prior to emergence and are short lived, providing a unique opportunity to quantify their lifetime investments in the different functions. We investigated the initial eggloads and longevity of 2 proovigenic parasitoid wasps: Anagrus erythroneurae and Anagrus daanei, (Hymenoptera: Mymaridae) that develop within leafhopper eggs in both agricultural vineyards and natural riparian habitats in Northern California. We collected Vitis spp. leaves containing developing parasitoids from 3 natural sites (Knight Landing, American River and Putah Creek) and 3 agricultural vineyards (Solano Farm, Davis Campus and Village Homes). We recorded eggloads at parasitoid emergence and female parasitoid longevity with or without honey-feeding. Theory predicts that parasitoids from vineyards (where hosts are abundant) would have higher initial eggloads and lower longevity compared with parasitoids from riparian habitats (where hosts are scarce). Although host density and parasitoid eggloads were, indeed, higher in vineyards than in riparian habitats, parasitoid longevity did not follow the predicted pattern. Longevity without feeding differed among field sites, but it was not affected by habitat type (natural vs agricultural), whereas longevity with feeding was not significantly affected by any of the examined factors. Moreover, longevity was positively, rather than negatively, correlated with eggloads at the individual level, even after correcting for parasitoid body size. The combined results suggest a more complex allocation mechanism than initially predicted, and the possibility of variation in host quality that is independent of size.

Adaptations to different habitats in sexual and asexual populations of parasitoid wasps: a meta-analysis

BACKGROUND

Coexistence of sexual and asexual populations remains a key question in evolutionary ecology. We address the question how an asexual and a sexual form of the parasitoid Venturia canescens can coexist in southern Europe. We test the hypothesis that both forms are adapted to different habitats within their area of distribution. Sexuals inhabit natural environments that are highly unpredictable, and where density of wasps and their hosts is low and patchily distributed. Asexuals instead are common in anthropic environments (e.g., grain stores) where host outbreaks offer periods when egg-load is the main constraint on reproductive output.

METHODS

We present a meta-analysis of known adaptations to these habitats. Differences in behavior, physiology and life-history traits between sexual and asexual wasps were standardized in term of effect size (Cohen's d value; Cohen, 1988).

RESULTS

Seeking consilience from the differences between multiple traits, we found that sexuals invest more in longevity at the expense of egg-load, are more mobile, and display higher plasticity in response to thermal variability than asexual counterparts.

DISCUSSION

Thus, each form has consistent multiple adaptations to the ecological circumstances in the contrasting environments.

Cognitive adaptation in asexual and sexual wasps living in contrasted environments

Differences in learning and memory dynamics between populations are suspected to result from differences in ecological constraints such as resource distribution. The two reproductive modes (strains) of the parasitoid wasp Venturia canescens share the same geographical areas but live in contrasting habitats: arrhenotokous wasps live in the wild (generally orchards), whereas thelytokous ones live mostly in stored-products buildings (e.g. granaries). This species thus represents a relevant biological model for understanding the relationship between the ecological constraints faced by a species and its memory and learning ability. We showed that after having laid eggs in presence of both a synthetic odour and natural olfactory cues of their host, arrhenotokous wasps exhibited a change in their behavioural response towards the synthetic odour that was at least as pronounced as in thelytokous ones even though they were faster in their decision-making process. This is consistent with better learning skills in arrhenotokous wasps. The corresponding memory trace persisted in both strains for at least 51 h. We compare and discuss the learning and memory ablities of both strains as a function of their costs and benefits in their preferential habitats.

The invasive pest Drosophila suzukii uses trans-generational medication to resist parasitoid attack

Animal medication is a behavioral strategy to resist enemies based on the use of substances from the environment. While it has been observed in several animals, whether invasive species can use medication to resist new enemies during its expansion is unknown. Here, we show that the worldwide invasive pest Drosophila suzukii performs trans-generational prophylactic medication by adapting its oviposition behavior in the presence of enemies. We find that flies preferentially lay their eggs on media containing atropine – an entomotoxic alkaloid – in the presence of parasitoids. We further show that flies developing on atropine more efficiently resist parasitization by parasitoids. Finally, we find that developing in hosts reared on atropine strongly impacts the life-history traits of parasitoids. This protective behavior is reported for the first time in a pest and invasive species, and suggests that animal medication may be an important driver of population dynamics during invasions.

Diploid male production correlates with genetic diversity in the parasitoid wasp Venturia canescens: a genetic approach with new microsatellite markers

Sex determination is ruled by haplodiploidy in Hymenoptera, with haploid males arising from unfertilized eggs and diploid females from fertilized eggs. However, diploid males with null fitness are produced under complementary sex determination (CSD), when individuals are homozygous for this locus. Diploid males are expected to be more frequent in genetically eroded populations (such as islands and captive populations), as genetic diversity at the csd locus should be low. However, only a few studies have focused on the relation between population size, genetic diversity, and the proportion of diploid males in the field. Here, we developed new microsatellite markers in order to assess and compare genetic diversity and diploid male proportion (DMP) in populations from three distinct habitat types - mainland, island, or captive -, in the parasitoid wasp Venturia canescens. Eroded genetic diversity and higher DMP were found in island and captive populations, and habitat type had large effect on genetic diversity. Therefore, DMP reflects the decreasing genetic diversity in small and isolated populations. Thus, Hymenopteran populations can be at high extinction risk due to habitat destruction or fragmentation.

Thermal ecology of the fiddler crab Uca panacea: Thermal constraints and organismal responses

Temperature is one of the primary environmental variables limiting organismal performance, fitness, and species distributions. Yet, understanding temperature effects requires thorough exploration of thermal constraints and organismal responses that can translate to fitness and non-lethal long-term consequences under both constant and changing thermal regimes. We examined the thermal ecology of the fiddler crab Uca panacea, including critical thermal limits, thermal sensitivity of locomotion, operative environmental temperatures, preferred body temperatures, and acclimation ability. Operative environmental temperatures frequently reached the critical thermal maximum (41.8±0.8°C, mean ± s.e.m.), especially in unvegetated microhabitats, indicating the need for behavioral thermoregulation to maintain diurnal activity patterns. Preferred body temperatures (21.1-28.6°C) were substantially below the thermal optimum (30-40°C), although further research is needed to determine the driver of this mismatch. Critical thermal limits shifted 2-4°C in response to exposure to low (20°C) or high (35°C) temperatures, with full acclimation occurring in approximately 9d. This capacity for rapid acclimation, combined with the capacity for behavioral thermoregulation, is a strong candidate mechanism that explains the broad habitat use and could help explain the successful pantropical distribution of fiddler crabs.

Sterile males in a parasitoid wasp with complementary sex determination: from fitness costs to population extinction

BACKGROUND

Single-locus complementary sex determination (sl-CSD), which occurs in some insects of the order Hymenoptera, imposes a heavy genetic load that can drive small populations to extinction. The core process in these species is the development of individuals homozygous at the sex-determining locus into unfit diploid males. The risk of extinction of populations with sl-CSD is theoretically much higher if diploid males are viable and capable of mating but sterile, because diploid males then decrease the reproductive output of both their parents and the females with which they mate.

RESULTS

In the parasitoid wasp Venturia canescens (Hymenoptera: Ichneumonidae), diploid males resembled their haploid counterparts in most respects, but their mating success was nevertheless lower than that of haploid males, especially when the two types of males were placed in competition. Furthermore, although diploid males transferred viable sperm during copulation, they sired no daughters: the females with which they mated produced only sons, like virgin females. A simulation model combining behavior, genetics and demography demonstrated that for two alternative hypotheses concerning the fertilization success of diploid sperm, the mating success of diploid males strongly affected population dynamics.

CONCLUSION

The performance of diploid males should be estimated in competitive situations. It is a crucial determinant of the probability of extinction.

Inbreeding depression in a parasitoid wasp with single-locus complementary sex determination

Inbreeding and inbreeding depression are key processes in small or isolated populations and are therefore central concerns for the management of threatened or (re)introduced organisms. Haplodiploid species of the order Hymenoptera have a particular status with regard to inbreeding depression. Although recessive deleterious alleles that are expressed in males should be purged, an alternative form of inbreeding depression exists in species with single-locus complementary sex determination (sl-CSD). Under sl-CSD, genetically-related parents have a high probability of producing sterile sons instead of fertile daughters. In this article, we study inbreeding depression in Venturia canescens (Hymenoptera: Ichneumonidae), a parasitoid wasp with sl-CSD. We used a crossing design to manipulate relatedness according to three levels: within-family, between-family and between-population. For each level, several fitness components were measured on parents and female offspring. We found a 20% reduction in egg load at emergence for inbred crosses. Inbred crosses also yielded a higher proportion of males, as expected in a species with sl-CSD. Mating probability, presence of daughters among offspring, body size, symmetry and longevity were unaffected by inbreeding.

Rising temperature reduces divergence in resource use strategies in coexisting parasitoid species

Coexistence of species sharing the same resources is often possible if species are phylogenetically divergent in resource acquisition and allocation traits, decreasing competition between them. Developmental and life-history traits related to resource use are influenced by environmental conditions such as temperature, but thermal trait responses may differ among species. An increase in ambient temperature may, therefore, affect trait divergence within a community, and potentially species coexistence. Parasitoids are interesting models to test this hypothesis, because multiple species commonly attack the same host, and employ divergent larval and adult host use strategies. In particular, development mode (arrested or continued host growth following parasitism) has been recognized as a major organiser of parasitoid life histories. Here, we used a comparative trait-based approach to determine thermal responses of development time, body mass, egg load, metabolic rate and energy use of the coexisting Drosophila parasitoids Asobara tabida, Leptopilina heterotoma, Trichopria drosophilae and Spalangia erythromera. We compared trait values between species and development modes, and calculated trait divergence in response to temperature, using functional diversity indices. Parasitoids differed in their thermal response for dry mass, metabolic rate and lipid use throughout adult life, but only teneral lipid reserves and egg load were affected by developmental mode. Species-specific trait responses to temperature were probably determined by their adaptations in resource use (e.g. lipogenesis or ectoparasitism). Overall, trait values of parasitoid species converged at the higher temperature. Our results suggest that local effects of warming could affect host resource partitioning by reducing trait diversity in communities.

Does cold tolerance plasticity correlate with the thermal environment and metabolic profiles of a parasitoid wasp?

Tolerance of ectotherm species to cold stress is highly plastic according to thermal conditions experienced prior to cold stress. In this study, we investigated how cold tolerance varies with developmental temperature (at 17, 25 and 30°C) and whether developmental temperature induces different metabolic profiles. Experiments were conducted on the two populations of the parasitoid wasp, Venturia canescens, undergoing contrasting thermal regimes in their respective preferential habitat (thermally variable vs. buffered). We predicted the following: i) development at low temperatures improves the cold tolerance of parasitoid wasps, ii) the shape of the cold tolerance reaction norm differs between the two populations, and iii) these phenotypic variations are correlated with their metabolic profiles. Our results showed that habitat origin and developmental acclimation interact to determine cold tolerance and metabolic profiles of the parasitoid wasps. Cold tolerance was promoted when developmental temperatures declined and population originating from variable habitat presented a higher cold tolerance. Cold tolerance increases through the accumulation of metabolites with an assumed cryoprotective function and the depression of metabolites involved in energy metabolism. Our data provide an original example of how intraspecific cold acclimation variations correlate with metabolic response to developmental temperature.

Omnia tempus habent: habitat-specific differences in olfactory learning and decision making in parasitic wasps

Olfactory learning is generally involved in the host-finding process in parasitic wasps. But the reliability of odour cues for predicting future host-finding success depends on the rate at which host-substrate associations are subject to variation within and between parasitoid generations. Since learning comes at physiological costs, we can expect animals to learn in a way that optimizes costs and benefits. The parasitic wasp Venturia canescens occurs in two reproductive modes that forage in different environments. We tested populations from both habitat types for learning rate, memory duration and speed of decision making and found considerable differences. Thelytokous wasps live in habitats with relatively stable host-substrate associations and might encounter hosts at a high rate. They showed a preference for a new odour after only a single experience. However, the response faded within 24 h, even with spaced learning experiences. Arrhenotokous wasps live in habitats where hosts are scarce and are likely to be found on a variety of substrates. Like the thelytokous ones, arrhenotokous wasps learned a new odour after a single experience, but seemingly took long for information processing: one and four hours after an experience, a speed-accuracy trade-off became visible, while 24 h after the experience, wasps decided quickly and in accordance with what they had learned. In addition, these wasps are likely to have developed an aversion response towards Geraniol in the CleanAir experiment. We conclude that the respective cognitive pattern can be attributed to the ecological circumstances of the wasp's natural habitat.

Maintenance and loss of heterozygosity in a thelytokous lineage of honey bees (Apis mellifera capensis)

An asexual lineage that reproduces by automictic thelytokous parthenogenesis has a problem: rapid loss of heterozygosity resulting in effective inbreeding. Thus, the circumstances under which rare asexual lineages thrive provide insights into the trade-offs that shape the evolution of alternative reproductive strategies across taxa. A socially parasitic lineage of the Cape honey bee, Apis mellifera capensis, provides an example of a thelytokous lineage that has endured for over two decades. It has been proposed that cytological adaptations slow the loss of heterozygosity in this lineage. However, we show that heterozygosity at the complementary sex determining (csd) locus is maintained via selection against homozygous diploid males that arise from recombination. Further, because zygosity is correlated across the genome, it appears that selection against diploid males reduces loss of homozygosity at other loci. Selection against homozygotes at csd results in substantial genetic load, so that if a thelytokous lineage is to endure, unusual ecological circumstances must exist in which asexuality permits such a high degree of fecundity that the genetic load can be tolerated. Without these ecological circumstances, sex will triumph over asexuality. In A. m. capensis, these conditions are provided by the parasitic interaction with its conspecific host, Apis mellifera scutellata.

The ontogeny of host-seeking behaviour in a parasitoid dipteran

The robber fly Mallophora ruficauda is one of the principal apiculture pests in the Pampas region of Argentina. As adults, they prey on honeybees and other insects, while as larvae they are ectoparasitoids of third-instar scarab larvae. Females of M. ruficauda lay eggs in tall grasses. After being dispersed by the wind, larvae drop to the ground, where they dig in search of their hosts. It is known that M. ruficauda larvae exhibit active host-searching behaviour; however, it is unknown which instars are involved in this search. We carried out experiments in the laboratory to determine which larval stages are involved in host location. We report here that the second instar of M. ruficauda orientates specifically toward a source of Cyclocephala signaticollis odour, while first larval instar is indifferent to the host cues. Furthermore, we have determined that second instar larvae are more motivated to initiate exploratory movements than larva of the first stage. So far as we know, this is the first case among parasitoids, where the second instar is responsible for host location. Here we provide relevant information of this parasitoid's host-searching strategy, increasing the available knowledge of this significant apiculture pest.