Aphid symbionts and endogenous resistance traits mediate competition between rival parasitoids

Competitive interactions in insect parasitoids: effects of microbial symbionts across tritrophic levels

Competition for hosts is a common ecological interaction in insect parasitoids. In the recent years, it has become increasingly evident that microorganisms can act as 'hidden players' in parasitoid ecology. In this review, we propose that parasitoid competition should take into consideration the microbial influence. In particular, we take a tritrophic perspective and discuss how parasitoid competition can be modulated by microorganisms associated with the parasitoids, their herbivore hosts, or the plants attacked by the herbivores. Although research is still in its infancy, recent studies have shown that microbial symbionts can modulate the contest outcome. The emerging pattern is that microorganisms not only affect the competitive traits of parasitoids but also the fighting arena (i.e. the herbivore host and its food plant), in which competition takes place. We have also identified important gaps in the literature that should be addressed in future studies to advance our understanding about parasitoid competition.

Biological control needs evolutionary perspectives of ecological interactions

While ecological interactions have been identified as determinant for biological control efficiency, the role of evolution remains largely underestimated in biological control programs. With the restrictions on the use of both pesticides and exotic biological control agents (BCAs), the evolutionary optimization of local BCAs becomes central for improving the efficiency and the resilience of biological control. In particular, we need to better account for the natural processes of evolution to fully understand the interactions of pests and BCAs, including in biocontrol strategies integrating human manipulations of evolution (i.e., artificial selection and genetic engineering). In agroecosystems, the evolution of BCAs traits and performance depends on heritable phenotypic variation, trait genetic architecture, selection strength, stochastic processes, and other selective forces. Humans can manipulate these natural processes to increase the likelihood of evolutionary trait improvement, by artificially increasing heritable phenotypic variation, strengthening selection, controlling stochastic processes, or overpassing evolution through genetic engineering. We highlight these facets by reviewing recent studies addressing the importance of natural processes of evolution and human manipulations of these processes in biological control. We then discuss the interactions between the natural processes of evolution occurring in agroecosystems and affecting the artificially improved BCAs after their release. We emphasize that biological control cannot be summarized by interactions between species pairs because pests and biological control agents are entangled in diverse communities and are exposed to a multitude of deterministic and stochastic selective forces that can change rapidly in direction and intensity. We conclude that the combination of different evolutionary approaches can help optimize BCAs to remain efficient under changing environmental conditions and, ultimately, favor agroecosystem sustainability.

Symbiont-conferred immunity interacts with effects of parasitoid genotype and intraguild predation to affect aphid immunity in a clone-specific fashion

Background

Host-parasite interactions represent complex co-evolving systems in which genetic and associated phenotypic variation within a species can significantly affect selective pressures on traits, such as host immunity, in the other. While often modelled as a two-species interaction between host and parasite, some systems are more complex due to effects of host enemies, intraguild predation, and endosymbionts, all of which affect host immunity. However, it remains unclear how these factors, combined with genetic variation in the host and the parasitoid, affect host immunity. We address this question in an important agricultural pest system, the pea aphid Acyrthosiphon pisum, which shows significant intraspecific variability in immunity to the parasitoid wasp Aphidius ervi. In a complex experiment, we use a quantitative genetic design in the parasitoid, two ecologically different aphid lineages and the aphid lion Chrysoperla carnea as an intraguild predator to unravel the complex interdependencies.

Results

We demonstrate that aphid immunity as a key trait of this complex host-parasite system is affected by intraspecific genetic variation in the parasitoid and the aphid, the interaction of intraspecific genetic variation with intraguild predation, and differences in defensive endosymbionts between aphid lineages. Further, aphid lineages differ in their altruistic behaviour whereby infested aphids move away from the clonal colony to facilitate predation.

Conclusions

Our findings provide new insights into the influence of endosymbiosis and genetic variability in an important host-parasitoid system which is influenced by natural enemies of the parasitoid and the aphid, including its endosymbiont communities. We show that endosymbiosis can mediate or influence the evolutionary arms race between aphids and their natural enemies. The outcome of these complex interactions between species has significant implications for understanding the evolution of multitrophic systems, including eco-agricultural settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-01991-1.

Defensive Symbionts and the Evolution of Parasitoid Host Specialization

Insect host-parasitoid interactions abound in nature and are characterized by a high degree of host specialization. In addition to their behavioral and immune defenses, many host species rely on heritable bacterial endosymbionts for defense against parasitoids. Studies on aphids and flies show that resistance conferred by symbionts can be very strong and highly specific, possibly as a result of variation in symbiont-produced toxins. I argue that defensive symbionts are therefore an important source of diversifying selection, promoting the evolution of host specialization by parasitoids. This is likely to affect the structure of host-parasitoid food webs. I consider potential changes in terms of food web complexity, although the nature of these effects will also be influenced by whether maternally transmitted symbionts have some capacity for lateral transfer. This is discussed in the light of available evidence for horizontal transmission routes. Finally, I propose that defensive mutualisms other than microbial endosymbionts may also exert diversifying selection on insect parasitoids.

Does getting defensive get you anywhere?-Seasonal balancing selection, temperature, and parasitoids shape real-world, protective endosymbiont dynamics in the pea aphid

Facultative, heritable endosymbionts are found at intermediate prevalence within most insect species, playing frequent roles in their hosts' defence against environmental pressures. Focusing on Hamiltonella defensa, a common bacterial endosymbiont of aphids, we tested the hypothesis that such pressures impose seasonal balancing selection, shaping a widespread infection polymorphism. In our studied pea aphid (Acyrthosiphon pisum) population, Hamiltonella frequencies ranged from 23.2% to 68.1% across a six-month longitudinal survey. Rapid spikes and declines were often consistent across fields, and we estimated that selection coefficients for Hamiltonella-infected aphids changed sign within this field season. Prior laboratory research suggested antiparasitoid defence as the major Hamiltonella benefit, and costs under parasitoid absence. While a prior field study suggested these forces can sometimes act as counter-weights in a regime of seasonal balancing selection, our present survey showed no significant relationship between parasitoid wasps and Hamiltonella prevalence. Field cage experiments provided some explanation: parasitoids drove modest ~10% boosts to Hamiltonella frequencies that would be hard to detect under less controlled conditions. They also showed that Hamiltonella was not always costly under parasitoid exclusion, contradicting another prediction. Instead, our longitudinal survey - and two overwintering studies - showed temperature to be the strongest predictor of Hamiltonella prevalence. Matching some prior lab discoveries, this suggested that thermally sensitive costs and benefits, unrelated to parasitism, can shape Hamiltonella dynamics. These results add to a growing body of evidence for rapid, seasonal adaptation in multivoltine organisms, suggesting that such adaptation can be mediated through the diverse impacts of heritable bacterial endosymbionts.

Timing alters how a heat shock affects a host-parasitoid interaction

Abiotic factors' effects on species are now well-studied, yet they are still often difficult to predict, especially for strongly interacting species. If these altered abiotic factors and species interactions occur as discrete events in time, such complications may occur because of the events' relative timing. One such discrete abiotic factor is the short-duration, large magnitude increase in temperature called a heat shock. This study investigates how the timing of heat shocks affects the successful attack and reproduction of a parasitoid wasp (Aphidius ervi) attacking its host, the pea aphid (Acyrthosiphon pisum). We tested three relative timings: 1) heat shock before the wasp attacks hosts, 2) heat shock while the wasp is foraging, and 3) heat shock after the wasp has attacked hosts. In each scenario we compared wasp mummy production (pupal stage) with and without a heat shock. Our results showed that a heat shock had the largest effect when it occurred while wasps actively foraged, with fewer mummies produced when exposed to a heat shock compared to the no heat shock control. Follow-up behavioral tests suggest this was caused by wasps becoming inactive during heat shocks. In contrast, when heat shocks were applied three days before or after foraging, we found no difference in mummy production between the heat shock treatment and no heat shock control. These results show the potential importance of timing when considering the ramifications of an altered abiotic factor, especially with relatively discrete abiotic events and interactions.

The preference-performance relationship as a means of classifying parasitoids according to their specialization degree

Host range in parasitoids could be described by the preference-performance hypothesis (PPH) where preference is defined as host acceptance and performance is defined as the sum of all species on which parasitoid offspring can complete their life cycle. The PPH predicts that highly suitable hosts will be preferred by ovipositing females. However, generalist parasitoids may not conform to this hypothesis if they attack a large range of hosts of varying suitability. Under laboratory conditions, we tested the PPH relationship of three aphid parasitoids currently considered as generalist species (Aphelinus abdominalis, Aphidius ervi, Diaeretiella rapae). As expected, the three parasitoids species showed low selectivity, i.e., females stung all aphid species encountered (at least in some extent). However, depending on the parasitoid species, only 42%-58% of aphid species enabled producing parasitoid offspring. We did not find a correlation between the extent of preference and the performance of three generalist aphid parasitoids. For A. ervi, host phylogeny is also important as females showed higher attack and developmental rates on hosts closely related to the most suitable one. In addition, traits such as (a) the presence of protective secondary endosymbionts, for example, Hamiltonella defensa detected in Aphis fabae and Metopolophium dirhodum and (b) the sequestration of plant toxins as defense mechanism against parasitism, for example, in Aphis nerii and Brevicoryne brassicae, were likely at play to some extent in narrowing parasitoid host range. The lack of PPH relationship involved a low selectivity leading to a high adaptability, as well as selection pressure; the combination of which enabled the production of offspring in a new host species or a new environment. Testing for PPH relationships in parasitoids may provide useful cues to classify parasitoids in terms of specialization degree.

Effect of heritable symbionts on maternally-derived embryo transcripts

Maternally-transmitted endosymbiotic bacteria are ubiquitous in insects. Among other influential phenotypes, many heritable symbionts of arthropods are notorious for manipulating host reproduction through one of four reproductive syndromes, which are generally exerted during early developmental stages of the host: male feminization; parthenogenesis induction; male killing; and cytoplasmic incompatibility (CI). Major advances have been achieved in understanding mechanisms and identifying symbiont factors involved in reproductive manipulation, particularly male killing and cytoplasmic incompatibility. Nonetheless, whether cytoplasmically-transmitted bacteria influence the maternally-loaded components of the egg or early embryo has not been examined. In the present study, we investigated whether heritable endosymbionts that cause different reproductive phenotypes in Drosophila melanogaster influence the mRNA transcriptome of early embryos. We used mRNA-seq to evaluate differential expression in Drosophila embryos lacking endosymbionts (control) to those harbouring the male-killing Spiroplasma poulsonii strain MSRO-Br, the CI-inducing Wolbachia strain wMel, or Spiroplasma poulsonii strain Hyd1; a strain that lacks a reproductive phenotype and is naturally associated with Drosophila hydei. We found no consistent evidence of influence of symbiont on mRNA composition of early embryos, suggesting that the reproductive manipulation mechanism does not involve alteration of maternally-loaded transcripts. In addition, we capitalized on several available mRNA-seq datasets derived from Spiroplasma-infected Drosophila melanogaster embryos, to search for signals of depurination of rRNA, consistent with the activity of Ribosome Inactivating Proteins (RIPs) encoded by Spiroplasma poulsonii. We found small but statistically significant signals of depurination of Drosophila rRNA in the Spiroplasma treatments (both strains), but not in the symbiont-free control or Wolbachia treatment, consistent with the action of RIPs. The depurination signal was slightly stronger in the treatment with the male-killing strain. This result supports a recent report that RIP-induced damage contributes to male embryo death.

Diversity begets diversity: do parasites promote variation in protective symbionts?

Insects commonly possess heritable microbial symbionts that increase their resistance to particular parasites. A diverse community of defensive symbionts may thus provide hosts with effective and specific protection against multiple parasites, although costs might constrain the accumulation of many symbionts. In parallel to the allelic diversity in the MHC complex of the vertebrate immune system, parasite diversity could be the driving force behind symbiont diversity. There is indeed evidence that parasites have the ability to drive frequencies of defensive symbionts in their hosts, and that these symbionts influence parasite communities, but direct evidence that parasite diversity can promote symbiont diversity is still lacking. We provide suggestions to investigate this potential link.

Impact of host endosymbionts on parasitoid host range - from mechanisms to communities

In insects, bacterial endosymbionts are known to influence the ecology of their hosts by modifying interactions with natural enemies such as parasitoids. Symbionts can modulate both parasitoid behavioral and/or physiological traits as well as host behaviors and life-history traits. Together these suggest that endosymbionts may impact the host range of parasitoids. For example, endosymbionts may narrow parasitoid host range through first, reducing parasitoid ability to locate hosts and/or larval survival, second, affecting fitness traits of the emerging adult parasitoid and/or third, modulating the outcome of interference and exploitative competition between parasitoid species. From both a fundamental and applied point of view, these symbiotic effects would influence the ecology and evolution of parasitoids and associated population-level processes and ecosystem services (e.g. biocontrol).

Variations on a protective theme: Hamiltonella defensa infections in aphids variably impact parasitoid success

Protective mutualisms are common in nature and include insect infections with cryptic symbionts that defend against pathogens and parasites. An archetypal defensive symbiont, Hamiltonella defensa protects aphids against parasitoids by disabling wasp development. Successful defense requires H. defensa infection with bacteriophages (APSEs), which play other key roles in mutualism maintenance. Genomes of H. defensa strains are highly similar in gene inventories, varying primarily in mobile element content. Protective phenotypes are highly variable across aphid models depending on H. defensa/APSE, aphid and wasp genotypes. Infection frequencies of H. defensa are highly dynamic in field populations, influenced by a variety of selective and non-selective factors confounding biological control implications. Overall, H. defensa infections likely represent a global aphid protection network with effects radiating outward from focal interactions.

Cascading effects of defensive endosymbionts

Defensive endosymbionts are now understood to be widespread among insects, targeting many different threats, including predators, parasites and disease. The effects on natural enemies can be significant, resulting in dramatic changes in the outcome of interactions between insects and their attackers. Evidence is now emerging from laboratory and field work that defensive symbionts can have important effects on the surrounding insect community, as well as on vulnerable enemy species; for example, by reducing prey available for the trophic level above the enemy. However, there is a need for more experimental work across a greater taxonomic range of species in order to understand the different ways in which defensive symbionts influence insect communities.

Heritable variation in prey defence provides refuge for subdominant predators

Generalist predators with broadly overlapping niches commonly coexist on seemingly identical sets of prey. Here, we provide empirical demonstration that predators can differentially exploit fine-grained niches generated by variable, heritable and selective defences within a single prey species. Some, but not all, clones of the aphid Aphis craccivora are toxic towards the dominant invasive predatory ladybeetle, Harmonia axyridis However, other less competitive ladybeetle species are not affected by the aphid's toxic trait. In laboratory and open field experiments, we show: (i) that subdominant ladybeetle species were able to exploit the toxic aphids, benefitting from the suppression of the dominant predator; and (ii) that this narrow-spectrum toxicity can function as an anti-predator defence for the aphid, but depends on enemy community context. Our results demonstrate that niche differentiation among generalist predators may hinge upon previously underappreciated heritable variation in prey defence, which, in turn, may promote diversity and stability of enemy communities invaded by a dominant predator.

Culture-Facilitated Comparative Genomics of the Facultative Symbiont Hamiltonella defensa

Many insects host facultative, bacterial symbionts that confer conditional fitness benefits to their hosts. Hamiltonella defensa is a common facultative symbiont of aphids that provides protection against parasitoid wasps. Protection levels vary among strains of H. defensa that are also differentially infected by bacteriophages named APSEs. However, little is known about trait variation among strains because only one isolate has been fully sequenced. Generating complete genomes for facultative symbionts is hindered by relatively large genome sizes but low abundances in hosts like aphids that are very small. Here, we took advantage of methods for culturing H. defensa outside of aphids to generate complete genomes and transcriptome data for four strains of H. defensa from the pea aphid Acyrthosiphon pisum. Chosen strains also spanned the breadth of the H. defensa phylogeny and differed in strength of protection conferred against parasitoids. Results indicated that strains shared most genes with roles in nutrient acquisition, metabolism, and essential housekeeping functions. In contrast, the inventory of mobile genetic elements varied substantially, which generated strain specific differences in gene content and genome architecture. In some cases, specific traits correlated with differences in protection against parasitoids, but in others high variation between strains obscured identification of traits with likely roles in defense. Transcriptome data generated continuous distributions to genome assemblies with some genes that were highly expressed and others that were not. Single molecule real-time sequencing further identified differences in DNA methylation patterns and restriction modification systems that provide defense against phage infection.