Water-seeking behavior in worm-infected crickets and reversibility of parasitic manipulation

Massive horizontal gene transfer and the evolution of nematomorph-driven behavioral manipulation of mantids

To complete their life cycle, a wide range of parasites must manipulate the behavior of their hosts.1 This manipulation is a well-known example of the "extended phenotype,2" where genes in one organism have phenotypic effects on another organism. Recent studies have explored the parasite genes responsible for such manipulation of host behavior, including the potential molecular mechanisms.3,4 However, little is known about how parasites have acquired the genes involved in manipulating phylogenetically distinct hosts.4 In a fascinating example of the extended phenotype, nematomorph parasites have evolved the ability to induce their terrestrial insect hosts to enter bodies of water, where the parasite then reproduces. Here, we comprehensively analyzed nematomorphs and their mantid hosts, focusing on the transcriptomic changes associated with host manipulations and sequence similarity between host and parasite genes to test molecular mimicry. The nematomorph's transcriptome changed during host manipulation, whereas no distinct changes were found in mantids. We then discovered numerous possible host-derived genes in nematomorphs, and these genes were frequently up-regulated during host manipulation. Our findings suggest a possible general role of horizontal gene transfer (HGT) in the molecular mechanisms of host manipulation, as well as in the genome evolution of manipulative parasites. The evidence of HGT between multicellular eukaryotes remains scarce but is increasing and, therefore, elucidating its mechanisms will advance our understanding of the enduring influence of HGT on the evolution of the web of life.

Rampant loss of universal metazoan genes revealed by a chromosome-level genome assembly of the parasitic Nematomorpha

Parasites may manipulate host behavior to increase the odds of transmission or to reach the proper environment to complete their life cycle.1,2 Members of the phylum Nematomorpha (known as horsehair worms, hairworms, or Gordian worms) are large endoparasites that affect the behavior of their arthropod hosts. In terrestrial hosts, they cause erratic movements toward bodies of water,3,4,5,6 where the adult worm emerges from the host to find mates for reproduction. We present a chromosome-level genome assembly for the freshwater Acutogordius australiensis and a draft assembly for one of the few known marine species, Nectonema munidae. The assemblies span 201 Mbp and 213 Mbp in length (N50: 38 Mbp and 716 Kbp), respectively, and reveal four chromosomes in Acutogordius, which are largely rearranged compared to the inferred ancestral condition in animals. Both nematomorph genomes have a relatively low number of genes (11,114 and 8,717, respectively) and lack a high proportion (∼30%) of universal single-copy metazoan orthologs (BUSCO genes7). We demonstrate that missing genes are not an artifact of the assembly process, with the majority of missing orthologs being shared by the two independent assemblies. Missing BUSCOs are enriched for Gene Ontology (GO) terms associated with the organization of cilia and cell projections in other animals. We show that most cilium-related genes conserved across eukaryotes have been lost in Nematomorpha, providing a molecular basis for the suspected absence of ciliary structures in these animals.

Host manipulation by parasites through the lens of Niche Construction Theory

The effect of parasites on host behaviour is generally considered an example of the extended phenotype, implying that parasite genes alter host behaviour to benefit the parasite. While the extended phenotype is a valid perspective supported by empirical examples, this approach was proposed from an evolutionary perspective and it does not fully explain all processes that occur at ecological time scales. For instance, the roles of the ontogenetic environment, memory and learning in forming the host phenotype are not explicitly mentioned. Furthermore, the cumulative effect of diverse populations or communities of parasites on host phenotype cannot be attributed to a particular genotype, much less to a particular gene. Building on the idea that the behaviour of a host is the result of a complex process, which certainly goes beyond a specific parasite gene, we use Niche Construction Theory to describe certain systems that are not generally the main focus in the extended phenotype (EP) model. We introduce three niche construction models with corresponding empirical examples that capture the diversity and complexity of host-parasite interactions, providing predictions that simpler models cannot generate. We hope that this novel perspective will inspire further research on the topic, given the impact of ecological factors on both short-, and long-term effects of parasitism.

Infection patterns and new definitive host records for New Zealand gordiid hairworms (phylum Nematomorpha)

Some parasites modify the phenotype of their host in order to increase transmission to another host or to an environment suitable for reproduction. This phenomenon, known as host manipulation, is found across many parasite taxa. Freshwater hairworms are known for the behavioural changes they cause in their terrestrial arthropod hosts, increasing their likelihood of entering water to exit the host and reproduce. Understanding how infected arthropods move around in the natural environment could help uncover alterations in spatial distribution or movement induced by hairworms in their terrestrial definitive hosts. Moreover, few hairworm-host records exist for New Zealand, so any additional record could help elucidate their true host specificity. Here, we investigated whether infected terrestrial arthropods were more likely to approach streams in two subalpine communities of invertebrates, using a spatial grid of specialised pitfall traps. Although hairworm infection could not explain the movements of arthropod hosts near streams, we found several new host records for hairworms, including the first records for the recently described Gordionus maori. We also found some new host-parasite associations for mermithid nematodes. These records show that the host specificity of hairworms is quite low, suggesting that their diversity and distribution may be greater than what is currently known for New Zealand.

Baculoviruses hijack the visual perception of their caterpillar hosts to induce climbing behavior, thus promoting virus dispersal

Baculoviruses can induce climbing behavior in their caterpillar hosts to ensure they die at elevated positions to enhance virus transmission, providing an excellent model to study parasitic manipulation of host behavior. Here, we demonstrate that climbing behavior occurred mostly during daylight hours, and that the height at death of Helicoverpa armigera single nucleopolyhedrovirus (HearNPV)-infected larvae increases with the height of the light source. Phototaxic and electroretinogram (ERG) responses were enhanced after HearNPV-infection in host larvae, and ablation of stemmata in infected larvae prevented both phototaxis and climbing behavior. Through transcriptome and quantitative PCR, we confirmed that two opsin genes (a blue light-sensitive gene, HaBL; and a long wave-sensitive gene, HaLW) as well as the TRPL (transient receptor potential-like channel protein) gene, all integral to the host's visual perception pathway, were significantly up-regulated after HearNPV infection. Knockout of HaBL, HaLW, or TRPL genes using the CRISPR/Cas9 system resulted in significantly reduced ERG responses, phototaxis, and climbing behavior in HearNPV-infected larvae. These results reveal that HearNPV alters the expression of specific genes to hijack host visual perception at fundamental levels - photoreception and phototransduction - in order to induce climbing behavior in host larvae.

Hijacking time: How Ophiocordyceps fungi could be using ant host clocks to manipulate behavior

Ophiocordyceps fungi manipulate ant behaviour as a transmission strategy. Conspicuous changes in the daily timing of disease phenotypes suggest that Ophiocordyceps and other manipulators could be hijacking the host clock. We discuss the available data that support the notion that Ophiocordyceps fungi could be hijacking ant host clocks and consider how altering daily behavioural rhythms could benefit the fungal infection cycle. By reviewing time‐course transcriptomics data for the parasite and the host, we argue that Ophiocordyceps has a light‐entrainable clock that might drive daily expression of candidate manipulation genes. Moreover, ant rhythms are seemingly highly plastic and involved in behavioural division of labour, which could make them susceptible to parasite hijacking. To provisionally test whether the expression of ant behavioural plasticity and rhythmicity genes could be affected by fungal manipulation, we performed a gene co‐expression network analysis on ant time‐course data and linked it to available behavioural manipulation data. We found that behavioural plasticity genes reside in the same modules as those affected during fungal manipulation. These modules showed significant connectivity with rhythmic gene modules, suggesting that Ophiocordyceps could be indirectly affecting the expression of those genes as well.

The PI3K/AKT Pathway and PTEN Gene Are Involved in “Tree-Top Disease” of Lymantria dispar

Nucleopolyhedrovirus (NPV) can alter its host behaviour such that infected larvae hang at the top of trees before their death. This phenomenon was firstly described by Hofmann in 1891 and named as “tree-top disease”. Subsequent studies have described effects during the infection proceedings as NPVs manipulate the host to avoid the immune response, cross defensive barriers and regulate hormones. In this study, we demonstrate that the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) pathway is involved in host manipulation by Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV). Particularly at the late stage of infection, a multifunctional dephosphorylase in the PI3K/AKT signaling pathway is dynamically upregulated, namely, the phosphatidylinositol-3, 4, 5-trisphosphate 3-phosphatase and dual-specificity protein phosphatase (PTEN) gene. The biological assays of PTEN gene knockdown showed that an increase in PTEN gene expression was necessary for the infected Lymantria dispar larvae’s terminal climbing behavior, death postponement and virion production. The results imply that the PI3K/AKT signaling pathway and PTEN gene might play an essential role in “tree-top disease” induced by LdMNPV.

The return to land: association between hairworm infection and aquatic insect development

Host manipulation by parasites can shape host behaviour, community structure, and the flow of energy through food webs. A well-known example of host manipulation comes from hairworms (phylum Nematomorpha), which somehow cause their terrestrial insect definitive hosts to enter water, a phenomenon that has received lots of attention in recent years. However, little focus has been directed towards the interactions between hairworms and their aquatic insect hosts and the return of dormant hairworms from water to land. Here, we ask whether hairworm cyst infections impact, either directly or indirectly, the life history of their aquatic transport hosts. By observing the development of last-instar Olinga jeanae (Trichoptera: Conoesucidae) caddisfly larvae naturally infected with Gordius-type hairworm cysts under controlled conditions, we found that higher numbers of cysts per infected caddisfly correlated with a decrease in time to pupation. These new observations suggest that, apart from the striking host manipulation that brings the parasite from land to water, the presence of dormant hairworms is associated with changes in the development of their aquatic hosts, either through direct or indirect mechanisms, which may accelerate their transition from water to land.

Enhanced polarotaxis can explain water-entry behaviour of mantids infected with nematomorph parasites

A wide range of parasites manipulate the behaviours of their hosts in order to complete their life cycle¹. Alteration of phototaxis is thought to be involved in host manipulation in many cases^2,3. However, very little is known about what features of the light (intensity, spectrum, polarization) alter behaviour. Here we report that arboreal mantids (Hierodula patellifera) infected by nematomorph parasites (Chordodes sp.) are attracted to horizontally polarized light, which could induce the mantids to enter water, where the parasites can then emerge and reproduce. In a two-choice test, infected mantids were attracted to horizontally but not vertically polarized light. Uninfected mantids were not attracted to either. In a field experiment, 14 infected mantids entered a deep pool, where the water surface strongly reflected horizontally polarized light. By contrast, only two mantids entered a shallow pool, where the surface reflection had higher light intensity but weaker polarization. To our knowledge, this is the first study demonstrating that a manipulative parasite can take advantage of its hosts' ability to perceive polarized light stimuli to alter host behaviour. VIDEO ABSTRACT.

The Adaptiveness of Host Behavioural Manipulation Assessed Using Tinbergen's Four Questions

Host organisms show altered phenotypic reactions when parasitised, some of which result from adaptive host manipulation, a phenomenon that has long been debated. Here, we provide an overview and discuss the rationale in distinguishing adaptive versus nonadaptive host behavioural manipulation. We discuss Poulin's criteria of adaptive host behavioural manipulation within the context of Tinbergen's four questions of ethology, while highlighting the importance of both the proximate and evolutionary explanations of such traits. We also provide guidelines for future studies exploring the adaptiveness of host behavioural manipulation. Through this article, we seek to encourage researchers to consider both the proximate and ultimate causes of host behavioural manipulation to infer on the adaptiveness of such traits.

When fiction becomes fact: exaggerating host manipulation by parasites

In an era where some find fake news around every corner, the use of sensationalism has inevitably found its way into the scientific literature. This is especially the case for host manipulation by parasites, a phenomenon in which a parasite causes remarkable change in the appearance or behaviour of its host. This concept, which has deservedly garnered popular interest throughout the world in recent years, is nearly 50 years old. In the past two decades, the use of scientific metaphors, including anthropomorphisms and science fiction, to describe host manipulation has become more and more prevalent. It is possible that the repeated use of such catchy, yet misleading words in both the popular media and the scientific literature could unintentionally hamper our understanding of the complexity and extent of host manipulation, ultimately shaping its narrative in part or in full. In this commentary, the impacts of exaggerating host manipulation are brought to light by examining trends in the use of embellishing words. By looking at key examples of exaggerated claims from widely reported host-parasite systems found in the recent scientific literature, it would appear that some of the fiction surrounding host manipulation has since become fact.

A new millipede-parasitizing horsehair worm, Gordius chiashanus sp. nov., at medium altitudes in Taiwan (Nematomorpha, Gordiida)

Gordiuschiashanussp. nov., a newly described horsehair worm that parasitizes the Spirobolus millipede, is one of the three described horsehair worm species in Taiwan. It is morphologically similar to G.helveticus Schmidt-Rhaesa, 2010 because of the progressively broadening distribution of bristles concentrated on the male tail lobes, but it is distinguishable from G.helveticus because of the stout bristles on the mid-body. In addition, a vertical white stripe on the anterior ventral side and areoles on the inside wall of the cloacal opening are rarely mentioned in other Gordius species. Free-living adults emerged and mated on wet soil under the forest canopy in the winter (late November to early February) at medium altitudes (1100–1700 m). Mucus-like structure covering on the body surface, which creates a rainbow-like reflection, might endow the worm with high tolerance to dehydration. Although Gordiuschiashanussp. nov. seems to be more adaptive to the terrestrial environment than other horsehair worm species, cysts putatively identified as belonging to this hairworm species found in the aquatic paratenic host, Ephemeraorientalis McLachlan, 1875, suggest the life cycle of Gordiuschiashanussp. nov. could involve water and land. The free-living adults emerged from the definitive hosts might reproduce in the terrestrial environment or enter an aquatic habitat by moving or being washed away by heavy rain instead of manipulating the behavior of their terrestrial definitive hosts.

The evolutionary ecology of circadian rhythms in infection

Biological rhythms coordinate organisms' activities with daily rhythms in the environment. For parasites, this includes rhythms in both the external abiotic environment and the within-host biotic environment. Hosts exhibit rhythms in behaviours and physiologies, including immune responses, and parasites exhibit rhythms in traits underpinning virulence and transmission. Yet, the evolutionary and ecological drivers of rhythms in traits underpinning host defence and parasite offence are largely unknown. Here, we explore how hosts use rhythms to defend against infection, why parasites have rhythms and whether parasites can manipulate host clocks to their own ends. Harnessing host rhythms or disrupting parasite rhythms could be exploited for clinical benefit; we propose an interdisciplinary effort to drive this emerging field forward.

Ectosymbionts alter spontaneous responses to the Earth's magnetic field in a crustacean

Magnetic sensing is used to structure every-day, non-migratory behaviours in many animals. We show that crayfish exhibit robust spontaneous magnetic alignment responses. These magnetic behaviours are altered by interactions with Branchiobdellidan worms, which are obligate ectosymbionts. Branchiobdellidan worms have previously been shown to have positive effects on host growth when present at moderate densities, and negative effects at relatively high densities. Here we show that crayfish with moderate densities of symbionts aligned bimodally along the magnetic northeast-southwest axis, similar to passive magnetic alignment responses observed across a range of stationary vertebrates. In contrast, crayfish with high symbiont densities failed to exhibit consistent alignment relative to the magnetic field. Crayfish without symbionts shifted exhibited quadramodal magnetic alignment and were more active. These behavioural changes suggest a change in the organization of spatial behaviour with increasing ectosymbiont densities. We propose that the increased activity and a switch to quadramodal magnetic alignment may be associated with the use of systematic search strategies. Such a strategy could increase contact-rates with conspecifics in order to replenish the beneficial ectosymbionts that only disperse between hosts during direct contact. Our results demonstrate that crayfish perceive and respond to magnetic fields, and that symbionts influence magnetically structured spatial behaviour of their hosts.

Phototransduction and circadian entrainment are the key pathways in the signaling mechanism for the baculovirus induced tree-top disease in the lepidopteran larvae

The tree-top disease is an altered behavioral state, displayed by baculovirus-infected lepidopteran larvae, and characterized by climbing to an elevated position before death. The detailed molecular mechanism underlying this phenomenal behavior change has not been reported yet. Our study focused on the transcriptomic changes in the host larvae due to baculovirus infection from pre-symptomatic to tree-top disease stage. Enrichment map visualization of the gene sets grouped based on the functional annotation similarity revealed 34 enriched pathways in signaling mechanism cluster during LdMNPV induced tree-top disease in third instar Lymantria dispar asiatica larvae. Directed light bioassay demonstrated the positively phototactic larvae during tree-top disease and the gene expression analysis showed altered rhythmicity of the host’s core circadian genes (per and tim) during the course of infection emphasizing the role of Circadian entrainment and Phototransduction pathways in the process, which also showed maximum interactions (>50% shared genes with 24 and 23 pathways respectively) among other signaling pathways in the enrichment map. Our study provided valuable insights into different pathways and genes, their coordinated response and molecular regulation during baculovirus infection and also improved our understanding regarding signaling mechanisms in LdMNPV induced tree-top disease.

Parasite infection and host personality: Glugea-infected three-spined sticklebacks are more social

Abstract

The existence of animal personality is now well-documented, although the causes and consequences of this phenomenon are still largely unclear. Parasite infection can have pervasive effects on hosts, including altering host behaviour, and may thus contribute to differences in host personality. We investigated the relationship between the three-spined stickleback and its common parasite Glugea anomala, with focus on differences in host personality. Naturally infected and uninfected individuals were assayed for the five personality traits activity, exploration, boldness, sociability, and aggression. If infected fish behaved differently from uninfected, to benefit this parasite with horizontal transmission, we predicted behaviour increasing interactions with other sticklebacks to increase. Infection status explained differences in host personality. Specifically, Glugea-infected individuals were more social than uninfected fish. This confirms a link between parasite infection and host behaviour, and a relationship which may improve the horizontal transmission of Glugea. However, future studies need to establish the consequences of this for the parasite, and the causality of the parasite-host personality relationship.

Significance statement

Parasite infection that alters host behaviour could be a possible avenue of research into the causes of animal personality. We studied the link between infection and personality using the three-spined stickleback and its parasite Glugea anomala. We predicted that infected individuals would be more prone to interact with other sticklebacks, since this would improve transmission of this parasite. The personality of uninfected and naturally infected fish was measured and we observed that Glugea-infected sticklebacks were more social. Our results confirm a link between parasitism and variation in host personality.

Electronic supplementary material

The online version of this article (10.1007/s00265-018-2586-3) contains supplementary material, which is available to authorized users.

Mind Control: How Parasites Manipulate Cognitive Functions in Their Insect Hosts

Neuro-parasitology is an emerging branch of science that deals with parasites that can control the nervous system of the host. It offers the possibility of discovering how one species (the parasite) modifies a particular neural network, and thus particular behaviors, of another species (the host). Such parasite-host interactions, developed over millions of years of evolution, provide unique tools by which one can determine how neuromodulation up-or-down regulates specific behaviors. In some of the most fascinating manipulations, the parasite taps into the host brain neuronal circuities to manipulate hosts cognitive functions. To name just a few examples, some worms induce crickets and other terrestrial insects to commit suicide in water, enabling the exit of the parasite into an aquatic environment favorable to its reproduction. In another example of behavioral manipulation, ants that consumed the secretions of a caterpillar containing dopamine are less likely to move away from the caterpillar and more likely to be aggressive. This benefits the caterpillar for without its ant bodyguards, it is more likely to be predated upon or attacked by parasitic insects that would lay eggs inside its body. Another example is the parasitic wasp, which induces a guarding behavior in its ladybug host in collaboration with a viral mutualist. To exert long-term behavioral manipulation of the host, parasite must secrete compounds that act through secondary messengers and/or directly on genes often modifying gene expression to produce long-lasting effects.

Reduction and recovery of keystone predation pressure after disease-related mass mortality

Disturbances such as disease can reshape communities through interruption of ecological interactions. Changes to population demographics alter how effectively a species performs its ecological role. While a population may recover in density, this may not translate to recovery of ecological function. In 2013, a sea star wasting syndrome outbreak caused mass mortality of the keystone predator Pisaster ochraceus on the North American Pacific coast. We analyzed sea star counts, biomass, size distributions, and recruitment from long‐term intertidal monitoring sites from San Diego to Alaska to assess regional trends in sea star recovery following the outbreak. Recruitment, an indicator of population recovery, has been spatially patchy and varied within and among regions of the coast. Despite sea star counts approaching predisease numbers, sea star biomass, a measure of predation potential on the mussel Mytilus californianus, has remained low. This indicates that post‐outbreak populations have not regained their full predation pressure. The regional variability in percent of recovering sites suggested differences in factors promoting sea star recovery between regions but did not show consistent patterns in postoutbreak recruitment on a coast‐wide scale. These results shape predictions of where changes in community composition are likely to occur in years following the disease outbreak and provide insight into how populations of keystone species resume their ecological roles following mortality‐inducing disturbances.

Timely trigger of caterpillar zombie behaviour: temporal requirements for light in baculovirus-induced tree-top disease

Host behavioural manipulation is a common strategy used by parasites to enhance their survival and/or transmission. Baculoviruses induce hyperactivity and tree-top disease (pre-death climbing behaviour) in their caterpillar hosts. However, little is known about the underlying mechanisms of this behavioural manipulation. A previous study showed that the baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) induced tree-top disease at 3 days post infection in third instar S. exigua larvae and that light plays a key role in triggering this behaviour. Here we investigated the temporal requirements for the presence of light to trigger this behaviour and found that light from above was needed between 43 and 50 h post infection to induce tree-top disease. Infected larvae that were not exposed to light from above in this period finally died at low positions. Exposure to light prior to this period did not affect the final positions where larvae died. Overall we conclude that light in a particular time frame is needed to trigger SeMNPV-induced tree-top disease in S. exigua larvae.

Why do nematomorphs leave their hosts?

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Nematomorphs induce their arthropod host to jump into water.

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The drowning host is often eaten by aquatic predators.

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This creates an opportunity for nematomorphs to increase life-cycle complexity.

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I speculate on why nematomorphs have always dropped this opportunity.

Baculovirus infection triggers a positive phototactic response in caterpillars to induce 'tree-top' disease

Many parasites manipulate host behaviour to enhance parasite transmission and survival. A fascinating example is baculoviruses, which often induce death in caterpillar hosts at elevated positions ('tree-top' disease). To date, little is known about the underlying processes leading to this adaptive host manipulation. Here, we show that the baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) triggers a positive phototactic response in S. exigua larvae prior to death and causes the caterpillars to die at elevated positions. This light-dependent climbing behaviour is specific for infected larvae, as movement of uninfected caterpillars during larval development was light-independent. We hypothesize that upon infection, SeMNPV captures a host pathway involved in phototaxis and/or light perception to induce this remarkable behavioural change.

Plastic behaviors in hosts promote the emergence of retaliatory parasites

Mafia like behavior, where individuals cooperate under the threat of punishment, occurs not only in humans, but is also observed in several animal species. Observations suggest that avian hosts tend to accept a certain degree of parasitism in order to avoid retaliating punishment from the brood parasite. To understand under which conditions it will be beneficial for a host to cooperate, we model the interaction between hosts and parasites as an evolutionary game. In our model, the host's behavior is plastic, and thus, its response depends on the previous interactions with the parasite. We find that such learned behavior in turn is crucial for the evolution of retaliating parasites. The abundance of this kind of mafia behavior oscillates in time and does not settle to an equilibrium. Our results suggest that retaliation is a mechanism for the parasite to evade specialization and to induce acceptance by the host.

Localizing viruses in their insect vectors

The mechanisms and impacts of the transmission of plant viruses by insect vectors have been studied for more than a century. The virus route within the insect vector is amply documented in many cases, but the identity, the biochemical properties, and the structure of the actual molecules (or molecule domains) ensuring compatibility between them remain obscure. Increased efforts are required both to identify receptors of plant viruses at various sites in the vector body and to design competing compounds capable of hindering transmission. Recent trends in the field are opening questions on the diversity and sophistication of viral adaptations that optimize transmission, from the manipulation of plants and vectors ultimately increasing the chances of acquisition and inoculation, to specific "sensing" of the vector by the virus while still in the host plant and the subsequent transition to a transmission-enhanced state.

Host–parasite molecular cross-talk during the manipulative process of a host by its parasite

Many parasite taxa are able to alter a wide range of phenotypic traits of their hosts in ways that seem to improve the parasite’s chance of completing its life cycle. Host behavioural alterations are classically seen as compelling illustrations of the ‘extended phenotype’ concept, which suggests that parasite genes have phenotype effects on the host. The molecular mechanisms and the host–parasite cross-talk involved during the manipulative process of a host by its parasite are still poorly understood. In this Review, the current knowledge on proximate mechanisms related to the ‘parasite manipulation hypothesis’ is presented. Parasite genome sequences do not themselves provide a full explanation of parasite biology nor of the molecular cross-talk involved in host–parasite associations. Recently, first-generation proteomics tools have been employed to unravel some aspects of the parasite manipulation process (i.e. proximate mechanisms and evolutionary convergence) using certain model arthropod-host–parasite associations. The pioneer proteomics results obtained on the manipulative process are here highlighted, along with the many gaps in our knowledge. Candidate genes and biochemical pathways potentially involved in the parasite manipulation are presented. Finally, taking into account the environmental factors, we suggest new avenues and approaches to further explore and understand the proximate mechanisms used by parasite species to alter phenotypic traits of their hosts.

The cost of a bodyguard

Host manipulation by parasites not only captures the imagination but has important epidemiological implications. The conventional view is that parasites face a trade-off between the benefits of host manipulation and their costs to fitness-related traits, such as longevity and fecundity. However, this trade-off hypothesis remains to be tested. Dinocampus coccinellae is a common parasitic wasp of the spotted lady beetle Coleomegilla maculata. Females deposit a single egg in the haemocoel of the host, and during larval development the parasitoid feeds on host tissues. At the prepupal stage, the parasitoid egresses from its host by forcing its way through the coccinellid's abdominal segments and begins spinning a cocoon between the ladybird's legs. Remarkably, D. coccinellae does not kill its host during its development, an atypical feature for parasitoids. We first showed under laboratory conditions that parasitoid cocoons that were attended by a living and manipulated ladybird suffered less predation than did cocoons alone or cocoons under dead ladybirds. We then demonstrated that the length of the manipulation period is negatively correlated with parasitoid fecundity but not with longevity. In addition to documenting an original case of bodyguard manipulation, our study provides the first evidence of a cost required for manipulating host behaviour.