Gene flow between arrhenotokous and thelytokous populations of Venturia canescens (Hymenoptera)

Microbiome composition is shaped by geography and population structure in the parasitic wasp Asobara japonica, but not in the presence of the endosymbiont Wolbachia

The microbial community composition is crucial for diverse life-history traits in many organisms. However, we still lack a sufficient understanding of how the host microbiome is acquired and maintained, a pressing issue in times of global environmental change. Here we investigated to what extent host genotype, environmental conditions, and the endosymbiont Wolbachia influence the bacterial communities in the parasitic wasp Asobara japonica. We sampled multiple wasp populations across 10 locations in their natural distribution range in Japan and sequenced the host genome (whole genome sequencing) and microbiome (16S rRNA gene). We compared the host population structure and bacterial community composition of wasps that reproduce sexually and are uninfected with Wolbachia with wasps that reproduce asexually and carry Wolbachia. The bacterial communities in asexual wasps were highly similar due to a strong effect of Wolbachia rather than host genomic structure. In contrast, in sexual wasps, bacterial communities appear primarily shaped by a combination of population structure and environmental conditions. Our research highlights that multiple factors shape the bacterial communities of an organism and that the presence of a single endosymbiont can strongly alter their compositions. This information is crucial to understanding how organisms and their associated microbiome will react in the face of environmental change.

A One-Step Multiplex PCR Method to Rapidly Distinguish Two Strains of Diglyphus wani (Hymenoptera: Eulophidae) Against Agromyzid Leafminers

Hymenopteran parasitoids generally show a haplo-diploid sex determination system. Haploid males are produced from unfertilized eggs, whereas diploid females develop from fertilized eggs (arrhenotokous). In some cases, diploid females develop from unfertilized eggs (thelytokous). Diglyphus wani (Hymenoptera: Eulophidae) is a biological control agent for agromyzid leafminers and have arrhenotokous and thelytokous strains. However, the morphological characteristics of two strains of D. wani are so similar that it is difficult to accurately distinguish them based on morphology. Here, a rapid molecular identification method was developed based on the mitochondrial gene cytochrome c oxidase I (COI) and one-step multiplex PCR. Two primer combinations, PC1 (Ar-F1/Th-F1/WR2) and PC2 (Ar-F1/Th-F4/WR2), were designed and repeatedly screened to distinguish two strains simultaneously, of which two special forward primers Th-F1/Th-F4 were used for the thelytokous strain and one special forward primer Ar-F1 was used for the arrhenotokous strain. In addition, a common reverse primer, WR2, was used for both strains. The PC1 and PC2 PCR assays were effective in distinguishing the two strains at different developmental stages and field colonies. This method provides a reliable, highly sensitive, and cost-effective tool for the rapid identification of the two strains of D. wani.

Genomic population structure of sympatric sexual and asexual populations in a parasitic wasp, Meteorus pulchricornis (Hymenoptera: Braconidae), inferred from six hundred single-nucleotide polymorphism loci

In spite of the two-fold reproductive advantage, asexual reproduction is not common in nature, probably due to the associated genetic deterioration or reduced genetic variation. To understand how genetic diversity is maintained in existing asexual populations, we investigated the genetic diversity and population structure of sympatric sexual and asexual populations of a parasitic wasp, Meteorus pulchricornis, using 614 genome-wide single nucleotide polymorphisms. The genetic structures of the apomictic asexual populations were distinct, showing considerable genetic differentiation among them. Most of the asexual populations were highly differentiated from the sympatric sexual population; some asexual individuals could not be distinguished from members of the sexual population. Furthermore, significantly fewer multilocus genotypes were identified in the asexual populations (1-7) compared to the sexual population (42), which is consistent with their apomictic nature. The observed patterns of fixed heterozygous sites suggest that most asexual populations had the same evolutionary origin and have long since evolved individually; the detected gene flow between the sexual population and a few asexual population may indicate independent origins of asexuality. The potential role of occasional males in apomictic wasps is also discussed.

Insects and incest: Sib-mating tolerance in natural populations of a parasitoid wasp

Sib-mating avoidance is a pervasive behaviour that is expected to evolve in species subject to inbreeding depression. Although laboratory studies provide elegant demonstrations, small-scaled bioassays minimize the costs of mate finding and choice, and thus may produce spurious findings. We therefore combined laboratory experiments with field observations to examine the existence of inbreeding avoidance using the parasitoid wasp Venturia canescens. In the laboratory, our approach consisted of mate-choice experiments to assess kin discrimination in population cages with competitive interactions. A higher mating probability after sib rejections suggested that females could discriminate their sibs; however, in contrast to previous findings, sib-mating avoidance was not observed. To compare our laboratory results to field data, we captured 241 individuals from two populations. Females laid eggs in the lab, and 226 daughters were obtained. All individuals were genotyped at 18 microsatellite loci, which allowed inference of the genotype of each female's mate and subsequently the relatedness within each mating pair. We found that the observed rate of sib-mating did not differ from the probability that sibs encountered one another at random in the field, which is consistent with an absence of sib-mating avoidance. In addition, we detected a weak but significant male-biased dispersal, which could reduce encounters between sibs. We also found weak fitness costs associated with sib-mating. As such, the sex-biased dispersal that we found is probably sufficient to mitigate these costs. These results imply that kin discrimination has probably evolved for purposes other than mate choice, such as superparasitism avoidance.

Androgenesis: where males hijack eggs to clone themselves

Androgenesis is a form of quasi-sexual reproduction in which a male is the sole source of the nuclear genetic material in the embryo. Two types of androgenesis occur in nature. Under the first type, females produce eggs without a nucleus and the embryo develops from the male gamete following fertilization. Evolution of this type of androgenesis is poorly understood as the parent responsible for androgenesis (the mother) gains no benefit from it. Ultimate factors driving the evolution of the second type of androgenesis are better understood. In this case, a zygote is formed between a male and a female gamete, but the female genome is eliminated. When rare, androgenesis with genome elimination is favoured because an androgenesis-determining allele has twice the reproductive success of an allele that determines sexual reproduction. Paradoxically, except in hermaphrodites, a successful androgenetic strain can drive such a male-biased sex ratio that the population goes extinct. This likely explains why androgenesis with genome elimination appears to be rarer than androgenesis via non-nucleate eggs, although both forms are either very rare or remain largely undetected in nature. Nonetheless, some highly invasive species including ants and freshwater clams are androgenetic, for reasons that are largely unexplained.This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.

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.

Restricted Gene Flow among Lineages of Thrips tabaci Supports Genetic Divergence Among Cryptic Species Groups

Knowledge of the relative influence of population- versus species-level genetic variation is important to understand patterns of phenotypic variation and ecological relationships that exist among and within morphologically indistinguishable cryptic species and subspecies. In the case of cryptic species groups that are pests, such knowledge is also essential for devising effective population management strategies. The globally important crop pest Thrips tabaci is a taxonomically difficult group of putatively cryptic species. This study examines population genetic structure of T. tabaci and reproductive isolation among lineages of this species complex using microsatellite markers and mitochondrial COI sequences. Overall, genetic structure supports T. tabaci as a cryptic species complex, although limited interbreeding occurs between different clonal groups from the same lineage as well as between individuals from different lineages. These results also provide evidence that thelytoky and arrhenotoky are not fixed phenotypes among members of different T. tabaci lineages that have been generally associated with either reproductive mode. Possible biological and ecological factors contributing to these observations are discussed.

Detection of Gene Flow from Sexual to Asexual Lineages in Thrips tabaci (Thysanoptera: Thripidae)

Populations of Thrips tabaci are known to have two sympatric but genetically isolated reproductive modes, arrhenotoky (sexual reproduction) and thelytoky (asexual reproduction). Herein, we report behavioral, ecological and genetic studies to determine whether there is gene flow between arrhenotokous and thelytokous T. tabaci. We did not detect significant preference by arrhenotokous males to mate with females of a particular reproductive mode, nor did we detect significant behavioral differences between arrhenotokous males mated with arrhenotokous or thelytokous females in their pre-copulation, copulation duration and mating frequency. Productive gene transfer resulting from the mating between the two modes was experimentally confirmed. Gene transfer from arrhenotokous T. tabaci to thelytokous T. tabaci was further validated by confirmation of the passage of the arrhenotokous male-originated nuclear gene (histone H3 gene) allele to the F₂ generation. These behavioral, ecological and genetic studies confirmed gene transfer from the sexual arrhenotokous mode to the asexual thelytokous mode of T. tabaci in the laboratory. These results demonstrate that asexual T. tabaci populations may acquire genetic variability from sexual populations, which could offset the long-term disadvantage of asexual reproduction.

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.

On the fate of sexual traits under asexuality

Environmental shifts and life-history changes may result in formerly adaptive traits becoming non-functional or maladaptive. In the absence of pleiotropy and other constraints, such traits may decay as a consequence of neutral mutation accumulation or selective processes, highlighting the importance of natural selection for adaptations. A suite of traits are expected to lose their adaptive function in asexual organisms derived from sexual ancestors, and the many independent transitions to asexuality allow for comparative studies of parallel trait maintenance versus decay. In addition, because certain traits, notably male-specific traits, are usually not exposed to selection under asexuality, their decay would have to occur as a consequence of drift. Selective processes could drive the decay of traits associated with costs, which may be the case for the majority of sexual traits expressed in females. We review the fate of male and female sexual traits in 93 animal lineages characterized by asexual reproduction, covering a broad taxon range including molluscs, arachnids, diplopods, crustaceans and eleven different hexapod orders. Many asexual lineages are still able occasionally to produce males. These asexually produced males are often largely or even fully functional, revealing that major developmental pathways can remain quiescent and functional over extended time periods. By contrast, for asexual females, there is a parallel and rapid decay of sexual traits, especially of traits related to mate attraction and location, as expected given the considerable costs often associated with the expression of these traits. The level of decay of female sexual traits, in addition to asexual females being unable to fertilize their eggs, would severely impede reversals to sexual reproduction, even in recently derived asexual lineages. More generally, the parallel maintenance versus decay of different trait types across diverse asexual lineages suggests that neutral traits display little or no decay even after extended periods under relaxed selection, while extensive decay for selected traits occurs extremely quickly. These patterns also highlight that adaptations can fix rapidly in natural populations of asexual organisms, in spite of their mode of reproduction.

Niche differentiation and colonization of a novel environment by an asexual parasitic wasp

How do asexual taxa become adapted to a diversity of environments, and how do they persist despite changing environmental conditions? These questions are linked by their mutual focus on the relationship between genetic variation, which is often limited in asexuals, and the ability to respond to environmental variation. Asexual taxa originating from a single ancestor present a unique opportunity to assess rates of phenotypic and genetic change when access to new genetic variation is limited to mutation. Diachasma muliebre is an asexual Hymenopteran wasp that is geographically and genetically isolated from all sexual relatives. D. muliebre attack larvae of the western cherry fruit fly (Rhagoletis indifferens), which in turn feed inside bitter cherry fruit (Prunus emarginata) in August and September. R. indifferens has recently colonized a new host plant with an earlier fruiting phenology (June/July), domesticated sweet cherries (P. avium), and D. muliebre has followed its host into this temporally earlier niche. We tested three hypotheses: 1) that all D. muliebre lineages originate from a single asexual ancestor; 2) that different D. muliebre lineages (as defined by unique mtDNA haplotypes) have differentiated on their ancestral host in an important life-history trait, eclosion timing; and 3) that early-eclosing lineages have preferentially colonized the new sweet cherry niche. We find that mitochondrial COI and microsatellite data provide strong support for a single ancestral origin for all lineages. Furthermore, COI sequencing revealed five mitochondrial haplotypes among D. muliebre, and individual wasps possessing one distinctive mitochondrial haplotype (haplotype II) eclosed as reproductive adults significantly earlier than wasps with all other haplotypes. In addition, this early-eclosing lineage of D. muliebre is one of two lineages that have colonized the P. avium habitat, consistent with the preferential colonization hypothesis. These data suggest that D. muliebre has evolved adaptive phenotypic variation despite limited genetic variation, and that this variation has subsequently allowed an expansion of some wasps into a novel habitat. The D. muliebre system may allow for in-depth study of adaptation and long-term persistence of asexual taxa.

Genome size and ploidy of Thysanoptera

Flow cytometry was used to study the genome sizes and ploidy levels for four thrips species: Franklinothrips orizabensis Johansen (Thysanoptera: Aeolothripidae), Frankliniella occidentalis Pergande, Frankliniella fusca Hinds, and Thrips tabaci Lindeman (Thysanoptera: Thripidae). F. orizabensis males and females had 1C genome sizes of 426 Mb and 422 Mb, respectively. Male and female F. fusca had 1C genome sizes of 392 Mb and 409 Mb, whereas F. occidentalis males and females had smaller 1C genomes that were 345 Mb and 337 Mb, respectively. Male F. orizabensis, F. occidentalis and F. fusca were haploid and females diploid. Five isofemale lines of T. tabaci, initiated from parthenogenetic, thelytokous females and collected from different locations in North Carolina, were included in this study; no males were available. One isofemale line was diploid with a genome size of 1C = 310 Mb, and the other four had a mean genome size of 1C = 482 Mb, which is consistent with evidence from microsatellite data of diploidy and polyploidy, respectively, in these same five thelytokous lines. This is the first study to produce genome size estimates for thysanopteran species, and report polyploidy in T. tabaci populations.

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.

Genetic variability of arrhenotokous and thelytokous Venturia canescens (Hymenoptera)

The ichneumonid wasp Venturia canescens (Hymenoptera) has been studied extensively for foraging behaviour and population dynamics of sexually (arrhenotokous) and parthenogenetically (thelytokous) reproducing individuals. Here we report the development of a set of microsatellite markers for V.canescens and use them to show that arrhenotokous individuals have more genetic variability than thelytokous ones, which are even homozygous for all tested loci. Crosses between arrhenotokous individuals suggested one marker, Vcan071, to be linked with the Complementary Sex Determiner (CSD) locus and one, Vcan109, with the Virus Like Protein (vlp-p40) locus. The genome size of V. canescens was estimated to be 274–279 Mb. We discuss how both reproductive modes can give rise to the observed genetic variability and how the new markers can be used for future genetic studies of V. canescens.

Evolution of reproductive mode variation and host associations in a sexual-asexual complex of aphid parasitoids

Background

The Lysiphlebus fabarum group is a taxonomically poorly resolved complex of aphid parasitoids, presently split into three described species that comprise sexual (arrhenotokous) and asexual (thelytokous) lineages of unknown relationship. Specifically, it is unclear how asexuals evolved from sexuals in this system, to what extent reproductive modes are still connected by genetic exchange, how much the complex is structured by geography or by host-associated differentiation, and whether species designations are valid. Using a combination of population genetic and phylogenetic approaches, we addressed these issues in a comprehensive sample of parasitoid wasps from across Europe.

Results

Asexual reproduction predominated in parasitoids of the L. fabarum group, with asexual populations exhibiting high genotypic diversity. Sexual populations were only common in southern France; elsewhere sexual reproduction was restricted to specific aphid hosts. Although reproductive modes were aggregated on the mitochondrial genealogy and significantly differentiated at nuclear microsatellite loci, there was clear evidence for genetic exchange, especially on hosts attacked by sexual and asexual parasitoids. The microsatellite data further revealed that parasitoids collected from certain host aphids were significantly differentiated, yet the mitochondrial sequence variation across the entire L. fabarum group did not exceed 1.32% and exhibited a very shallow topology. Morphological characters used for delineation of described species were found to be phylogenetically non-conservative.

Conclusions

Our results suggest that the sexual-asexual L. fabarum group represents a young complex of lineages with incomplete isolation between reproductive modes. We propose three mechanisms of genetic exchange that may jointly explain the high genotypic diversity observed in asexual parasitoids: (i) the formation of new asexual lineages via 'contagious parthenogenesis', (ii) introgression from sexual lineages through matings between sexual males and thelytokous females, and (iii) 'cryptic sex' within asexuals, mediated by rare males that thelytokous lines are known to produce spontaneously. The partially strong differentiation among wasps collected from different aphids suggests that host specialization can evolve readily in these parasitoids. Finally, we conclude that in the light of our data, the current taxonomic division of the L. fabarum group into three species cannot be upheld.

Evolutionary origin of Venturia canescens virus-like particles

Insect host-parasitoid interactions provide fascinating examples of evolutionary adaptations in which the parasitoid employs a variety of measures and countermeasures to overcome the immune responses of its host. Maternal factors introduced by the female wasps during egg deposition play an important role in interfering with cellular and humoral components of the host's immune defence. Some of these components actively suppress host immune components and some are believed to confer protection for the developing endoparasitoid by rather passive means. The Venturia canescens/Ephestia kuehniella parasitoid-host system is unique among other systems in that the cellular defence capacity of the host remains virtually intact after parasitization. This system raises some important questions that are discussed in this mini-review: If immune protection of the egg and the emerging larva is achieved by surface properties comprising glycoproteins and virus-like particles (VLPs) produced by the female wasp, why is the prophenoloxidase activating cascade blocked in parasitized caterpillars? Another question is the evolutionary origin of these particles, given that the functional role and structural features of V. canescens VLP proteins are more related to cellular proteins than to viruses.

Molecular methods for assessing insect parasitism

Determining insect parasitism rates is problematic due to the small size and lack of useful distinguishing morphological characters of many parasitoid taxa. To solve this problem, entomologists have employed one of four general methods to detect parasitoid protein or nucleic acid markers: serological assay; random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR); allozyme electrophoresis; or specific PCR. Serological methods, especially with monoclonal antibodies, are unrivalled for specificity, enabling discrimination at the stage as well as species level. However, they have not found favour with many workers, possibly due to complexity and expense. RAPD-PCR has been widely used, but can only be recommended for restricted applications because of its poor reproducibility. Allozyme electrophoresis provides reproducible detection and discrimination of closely related species. Specific-PCR is highly specific and reproducible, and also has the shortest latency for detection, usually 24 h or less after parasitization. The substantial existing literature on allozyme electrophoresis and specific PCR is used to support recommendations on what are apt to be fruitful enzyme systems or genomic regions for detecting and discriminating parasitoids in untried parasitoid-host assemblages.

The influence of temperature and host availability on the host exploitation strategies of sexual and asexual parasitic wasps of the same species

In the hymenopteran parasitoid Venturia canescens, asexual (obligate thelytoky not induced by Wolbachia bacteria) and sexual (arrhenotokous) wasps coexist in field conditions despite the demographic cost incurred due to the production of males by sexual females. Arrhenotoky predominates in field conditions, whereas populations in indoor conditions (mills, granaries) are exclusively thelytokous. These differences in the relative abundance of the two modes of reproduction between environments suggest that the individuals of each reproductive mode may have developed strategies adapted to the conditions prevailing in each kind of habitat. The two environments contrast in temperature variability and in the spatial heterogeneity of host availability. In this study, we considered the combined effect of temperature and host availability on host patch exploitation by thelytokous and arrhenotokous V. canescens. As expected, arrhenotokous females were more sensitive to temperature changes. If the temperature decreased before foraging, they remained longer and exploited patches more thoroughly. This is consistent with the expected behaviour of parasitoids in response to signs of unfavourable conditions that entail increasing risk of time limitation or a reduced probability of attaining further patches. Both arrhenotokous and thelytokous females increased patch exploitation with host availability. However, unexpectedly, we found no difference in the way the two types of wasp responded to differences in host availability. Differences in the strategies adopted under different environmental conditions may indicate divergence of niche-specific life history traits between the two modes of reproduction. Niche displacement may partly account for the coexistence of these two modes of reproduction at a geographical scale.

A single locus determines thelytokous parthenogenesis of laying honeybee workers (Apis mellifera capensis)

The evolution and maintenance of parthenogenetic species are a puzzling issue in evolutionary biology. Although the genetic mechanisms that act to restore diploidy are well studied, the underlying genes that cause the switch from sexual reproduction to parthenogenesis have not been analysed. There are several species that are polymorphic for sexual and parthenogenetic reproduction, which may have a genetic basis. We use the South African honeybee subspecies Apis mellifera capensis to analyse the genetic control of thelytoky (asexual production of female workers). Due to the caste system of honeybees, it is possible to establish classical backcrosses using sexually reproducing queens and drones of both arrhenotokous and thelytokous subspecies, and to score the frequency of parthenogenesis in the resulting workers. We found Mendelian segregation for thelytoky of egg-laying workers, which appears to be controlled by a single major gene (th). The segregation pattern indicates a recessive allele causing thelytoky. We found no evidence for maternal transmission of bacterial endosymbionts controlling parthenogenesis. Thelytokous parthenogenesis of honeybee workers appears to be a classical qualitative trait, because we did not observe mixed parthenogenesis (amphitoky), which might be expected in the case of multi-locus inheritance.

Two coexisting lines of the endoparasitoid Venturia canescens show differences in reproductive success under conspecific superparasitism

In a laboratory colony of the endoparasitic wasp Venturia canescens Grav. (Hymenoptera: Ichneumonidae), two genetically distinct lines (RP, RM) appear to coexist sympatrically. The two lines display pronounced differences in ovarian morphology, parasitism behaviour and number of offspring produced under competing superparasitism. Since V. canescens is a solitary endoparasitoid, larvae inside superparasitised hosts must compete for host possession. We examined the outcome of conspecific superparasitism between the wasp lines with different time intervals between ovipositions. The results showed that the competitive abilities of the two lines were not symmetrical. Further, the RM-line won a significantly higher fraction (around 60%) of the overall contests. Dissection of parasitoid larvae from their hosts indicated that most contests between competing larvae had occurred within the first 24 h of the eggs hatching, suggesting the advantage of the RM-line relates to physical combat. It was previously thought that the coexistence of the two lines was exclusively due to maternal effects. The results of this study indicate for the first time that these differences are based on phenotypic variations in both the larval offspring and the mother.

Differential gene expression in two strains of the endoparasitic wasp Venturia canescens identified by cDNA-amplified fragment length polymorphism analysis

In the endoparasitic wasp Venturia canescens (Hymenoptera: Ichneumonidae) two genetically distinct lines, which differ at the VLP1 gene locus, appear to coexist sympatrically in thelytokous as well as arrhenotokous field and laboratory strains. Both lines display quite distinct morphological, physiological and behavioural variations, such as different oviposition strategies, egg numbers and growth rates during early embryonic development. To examine whether more than one gene is affected in the two lines, we applied cDNA-amplified fragment length polymorphism analysis to examine patterns of gene expression in ovaries of both wasp lines. We show that a number of ovarian transcripts have an altered expression pattern in either line, which was further confirmed by virtual Northern blot analysis. Sequence analysis of the cDNA-amplified fragment length polymorphism fragments revealed that some of the respective genes are expected to be involved in regulation of protein degradation during stress responses and in signal perception/transduction. The full-length sequence of two transcription factors (a homeodomain containing protein and a zinc finger protein) differentially expressed in both lines was obtained by RACE-polymerase chain reaction and their putative role in regulating key developmental processes during embryogenesis is discussed.

Does a deletion in a virus-like particle protein have pleiotropic effects on the reproductive biology of a parasitoid wasp?

Endoparasitoid insects have evolved mechanisms to counteract host immune defences. At oviposition, the endoparasitoid Venturia canescens injects virus-like particles (VLPs) together with the egg that interfere with the immune system of the host. These prevent encapsulation of the parasitoid egg. It has been shown that the gene coding for one of the VLP proteins exists in two variants (called Repeat-Plus (RP) and Repeat-Minus (RM)). Previous observations suggested that these variants induce pleiotropic effects on the reproductive biology of the female resulting, in an impeded transfer of eggs from the ovarioles to the oviducts in RM females, and in alterations in the egg laying sequence. We show that RM females from another geographical locality do not exhibit any phenotypic alteration of the reproductive biology. By showing a lack of association between VLP alleles and the reproductive phenotypic characteristics in a given strain, our results do not support the hypothesis of pleiotropic effects. Conversely, the results suggest that different genes code for the VLPs and for the reproductive biology characteristics. Different phenomena such as linkage, the action of pathogens, etc. could explain the association between characters that is observed in some strains.