Laser-based killing of a macroparasite inside its live invertebrate host.

Clearing infection is an essential step to address many issues in host-parasite interactions but is challenging when dealing with endoparasites of large size relative to that of their host. Here, we took advantage of the lethality, contactless and versatility of high-energy laser beam to achieve it, using thorny-headed worms (Acanthocephala) and their amphipod intermediate host as a model system. We show that laser-based de-parasitization can be achieved using 450 nm Blue Diode Laser targeting carotenoid pigments in the bird acanthocephalan Polymorphus minutus. Using proboscis evagination failure and DNA degradation to establish parasite death, we found that 80% P. minutus died from within-host exposure to 5 pulses of 50 ms duration, 1.4 W power. Survival of infected gammarids 11 days after laser treatment was 60%. Preliminary tests were also performed with Nanosecond-Green Laser targeting lipids in Pomphorhynchus tereticollis, another acanthocephalan parasite. We discuss the efficiency and side-effect of laser treatment in this host-parasite system and highlight the perspectives that this technology more generally offers in parasitology.

Hooking the scientific community on thorny-headed worms: interesting and exciting facts, knowledge gaps and perspectives for research directions on Acanthocephala.

Although interest in Acanthocephala seems to have reached only a small community of researchers worldwide, we show in this opinion article that this group of parasites is composed of excellent model organisms for studying key questions in parasite molecular biology and cytogenetics, evolutionary ecology, and ecotoxicology. Their shared ancestry with free-living rotifers makes them an ideal group to explore the origins of the parasitic lifestyle and evolutionary drivers of host shifts and environmental transitions. They also provide useful features in the quest to decipher the proximate mechanisms of parasite-induced phenotypic alterations and better understand the evolution of behavioral manipulation. From an applied perspective, acanthocephalans' ability to accumulate contaminants offers useful opportunities to monitor the impacts - and evaluate the possible mitigation - of anthropogenic pollutants on aquatic fauna and develop the environmental parasitology framework. However, exploring these exciting research avenues will require connecting fragmentary knowledge by enlarging the taxonomic coverage of molecular and phenotypic data. In this opinion paper, we highlight the needs and opportunities of research on Acanthocephala in three main directions: (i) integrative taxonomy (including non-molecular tools) and phylogeny-based comparative analysis; (ii) ecology and evolution of life cycles, transmission strategies and host ranges; and (iii) environmental issues related to global changes, including ecotoxicology. In each section, the most promising ideas and developments are presented based on selected case studies, with the goal that the present and future generations of parasitologists further explore and increase knowledge of Acanthocephala.

Title: Accrocher la communauté scientifique à des vers à la tête pleine d'épines : faits intéressants et passionnants, lacunes dans les connaissances et perspectives pour des orientations de recherche sur les Acanthocéphales. Abstract: Bien que l'intérêt pour les acanthocéphales semble n'avoir atteint qu'un petit nombre de chercheurs dans le monde, nous montrons dans cet article que ce groupe de parasites est composé d'excellents organismes modèles pour étudier les questions en suspens en biologie moléculaire et cytogénétique, écologie évolutive et écotoxicologie. Leur ascendance partagée avec les rotifères en fait un groupe idéal pour explorer les origines du mode de vie parasitaire et les moteurs évolutifs des changements d'hôtes et des transitions environnementales. Ils présentent également des caractéristiques intéressantes pour l'étude des mécanismes proximaux sous-tendant les altérations phénotypiques induites par les parasites, et ainsi mieux comprendre l'évolution de la manipulation comportementale. D'un point de vue appliqué, la capacité des acanthocéphales à accumuler les contaminants offre des opportunités utiles pour surveiller les impacts - et évaluer les possibilités d'atténuation - des pollutions anthropiques sur la faune aquatique et développer le domaine de la parasitologie environnementale. Cependant, l'exploration de ces pistes de recherche passionnantes nécessitera de relier des connaissances fragmentaires en élargissant la couverture taxonomique des données moléculaires et phénotypiques. Dans cet article, nous présentons l'état actuel de la recherche sur les acanthocéphales selon trois axes principaux : (i) la taxonomie intégrative (y compris les outils non-moléculaires) et la phylogénie à des fins d'analyse comparative ; (ii) l'écologie et l'évolution des cycles de vie, des stratégies d'exploitation des hôtes et de transmission ; (iii) les questions environnementales liées aux changements globaux, y compris l'écotoxicologie. Dans chaque section, nous soulignons les besoins et les opportunités, en espérant que cela incitera une nouvelle génération de parasitologues à s'intéresser aux acanthocéphales.

Infection with an acanthocephalan helminth reduces anxiety-like behaviour in crustacean host.

Trophically transmitted heteroxenous parasites of diverse clades can decrease or reverse antipredator behaviours in their intermediate hosts, thereby increasing their chances of reaching their final hosts. Such behavioural alterations could result from compromised cognitive abilities affecting fear- or more generally stress-related neurophysiological pathways. We tested this hypothesis in a key model system in the study of parasitic manipulation, the fish acanthocephalan parasite Pomphorhynchus tereticollis and its intermediate crustacean host Gammarus fossarum, using the 'threat of electric shock' paradigm. We exposed uninfected and infected G. fossarum to chronic and/or acute electric shock programs at two different intensities (voltage), and then quantified their sheltering behaviour as a proxy for anxiety-like state. Infected gammarids did not express anxiety-like response to electric shocks, while uninfected gammarids hid more when exposed to acute treatments, and when exposed to the high intensity chronic treatment. Interestingly, the lack of response in infected gammarids depended on parasite developmental stage. Our results support the hypothesis that this acanthocephalan parasite impacts the general anxiety-like circuitry of their intermediate host. Further studies are needed to investigate whether it involves inappropriate processing of information, impaired integration, or altered activation of downstream pathways initiating behavioural action.

Optimization of anesthetic procedure in crustaceans: Evidence for sedative and analgesic-like effect of MS-222 using a semi-automated device for exposure to noxious stimulus.

The implementation of anesthetic procedure in aquatic crustaceans remains mostly limited to studies dealing with sedation and survival from anesthesia, possibly owing to the debated question of pain in invertebrates. However, two important issues are generally overlooked: actual analgesic-like effect, and possible physiological post-anesthesial effects. Here we report on the anesthetic properties and possible after-effects of MS-222 (Tricaine Methanesulfonate or Ethyl 3-aminobenzoate methanesulfonate) and Eugenol in the freshwater amphipod Gammarus pulex. We first optimized the concentration of MS-222, and the induction and recovery time, based on preliminary tests and published studies. We then relied on the nociceptive modulation of sheltering behavior to assess the analgesic-like effect of the two drugs, using a new semi-automated electric shock device. In addition, we monitored the impact of anesthesia with MS-222 on locomotor activity and oxygen consumption and addressed potential adverse effects upon recovery using biomarkers related to metabolism and neurotoxicity. We provide evidence for the sedative and analgesic-like effects of MS-222 at 600 mg.L-1 and, to a lesser extent, of Eugenol at 100 µL.L-1, with no decrease in survival rate at 6 days post anesthesia. Oxygen consumption was reduced -but not eliminated- under full anesthesia with 600 mg.L-1 MS-222. No significant physiological effect of anesthesia was evidenced on the activity of the mitochondrial electron transfer system, or that of acetylcholine esterase, nor on total antioxidant capacity. We therefore conclude to the efficiency of MS-222 as an anesthetic drug in G. pulex. Eugenol should be tested at a higher concentration to reach the same efficiency, providing that increased concentration would not incur side-effects. Furthermore, the new and original semi-automated electric chock device used to induce nociception can be easily adapted to any species of aquatic invertebrates and small-sized fish and tadpoles, offering a standardized and flexible protocol to study nociceptive response and anesthesia in aquatic organisms.

Magnitude and direction of parasite-induced phenotypic alterations: a meta-analysis in acanthocephalans.

Several parasite species have the ability to modify their host's phenotype to their own advantage thereby increasing the probability of transmission from one host to another. This phenomenon of host manipulation is interpreted as the expression of a parasite extended phenotype. Manipulative parasites generally affect multiple phenotypic traits in their hosts, although both the extent and adaptive significance of such multidimensionality in host manipulation is still poorly documented. To review the multidimensionality and magnitude of host manipulation, and to understand the causes of variation in trait value alteration, we performed a phylogenetically corrected meta-analysis, focusing on a model taxon: acanthocephalan parasites. Acanthocephala is a phylum of helminth parasites that use vertebrates as final hosts and invertebrates as intermediate hosts, and is one of the few parasite groups for which manipulation is predicted to be ancestral. We compiled 279 estimates of parasite-induced alterations in phenotypic trait value, from 81 studies and 13 acanthocephalan species, allocating a sign to effect size estimates according to the direction of alteration favouring parasite transmission, and grouped traits by category. Phylogenetic inertia accounted for a low proportion of variation in effect sizes. The overall average alteration of trait value was moderate and positive when considering the expected effect of alterations on trophic transmission success (signed effect sizes, after the onset of parasite infectivity to the final host). Variation in the alteration of trait value was affected by the category of phenotypic trait, with the largest alterations being reversed taxis/phobia and responses to stimuli, and increased vulnerability to predation, changes to reproductive traits (behavioural or physiological castration) and immunosuppression. Parasite transmission would thereby be facilitated mainly by changing mainly the choice of micro-habitat and the anti-predation behaviour of infected hosts, and by promoting energy-saving strategies in the host. In addition, infection with larval stages not yet infective to definitive hosts (acanthella) tends to induce opposite effects of comparable magnitude to infection with the infective stage (cystacanth), although this result should be considered with caution due to the low number of estimates with acanthella. This analysis raises important issues that should be considered in future studies investigating the adaptive significance of host manipulation, not only in acanthocephalans but also in other taxa. Specifically, the contribution of phenotypic traits to parasite transmission and the range of taxonomic diversity covered deserve thorough attention. In addition, the relationship between behaviour and immunity across parasite developmental stages and host-parasite systems (the neuropsychoimmune hypothesis of host manipulation), still awaits experimental evidence. Most of these issues apply more broadly to reported cases of host manipulation by other groups of parasites.

Differential patterns of definitive host use by two fish acanthocephalans occurring in sympatry: Pomphorhynchus laevis and Pomphorhynchus tereticollis.

Parasites with complex life-cycles and trophic transmission are expected to show low specificity towards final hosts. However, testing this hypothesis may be hampered by low taxonomic resolution, particularly in helminths. We investigated this issue using two intestinal fish parasites with similar life-cycles and occurring in sympatry, Pomphorhynchus laevis and Pomphorhynchus tereticollis (Acanthocephala). We used species-specific ITS1 length polymorphism to discriminate parasite species from 910 adult acanthocephalans collected in 174 individual hosts from 12 fish species. Both P. laevis and P. tereticollis exhibited restricted host range within the community of available fish host species, and transmission bias compared to their relative abundance in intermediate hosts. The two parasites also exhibited low niche overlap, primarily due to their contrasting use of bentho-pelagic (P. laevis) and benthic (P. tereticollis) fish. Furthermore, parasite prevalence in intermediate hosts appeared to increase with taxonomic specificity in definitive host use. Comparison of P. laevis and P. tereticollis adult size in the two main definitive hosts, barbel and chub, suggested lower compatibility towards the fish species with the lowest parasite abundance, in particular in P. laevis. The determinants of low niche overlap between these two sympatric acanthocephalan species, and the contribution of definitive host range diversity to parasite transmission success, are discussed.

A minimalist macroparasite diversity in the round goby of the Upper Rhine reduced to an exotic acanthocephalan lineage.

The round goby, Neogobius melanostomus, is a Ponto-Caspian fish considered as an invasive species in a wide range of aquatic ecosystems. To understand the role that parasites may play in its successful invasion across Western Europe, we investigated the parasitic diversity of the round goby along its invasion corridor, from the Danube to the Upper Rhine rivers, using data from literature and a molecular barcoding approach, respectively. Among 1666 parasites extracted from 179 gobies of the Upper Rhine, all of the 248 parasites barcoded on the c oxidase subunit I gene were identified as Pomphorhynchus laevis. This lack of macroparasite diversity was interpreted as a loss of parasites along its invasion corridor without spillback compensation. The genetic diversity of P. laevis was represented by 33 haplotypes corresponding to a haplotype diversity of 0·65 ± 0·032, but a weak nucleotide diversity of 0·0018 ± 0·00015. Eight of these haplotypes were found in 88·4% of the 248 parasites. These haplotypes belong to a single lineage so far restricted to the Danube, Vistula and Volga rivers (Eastern Europe). This result underlines the exotic status of this Ponto-Caspian lineage in the Upper Rhine, putatively disseminated by the round goby along its invasion corridor.

External Influences on Invertebrate Brain Histamine and Related Compounds via an Automated Derivatization Method for Capillary Electrophoresis.

Histamine has been shown to modulate visual system and photic behavior in arthropods. However, few methods are available for the direct quantification of histamine and its precursor and metabolites in arthropod brain. In this work, a method for the separation of histamine, its precursor histidine, and its metabolite N-methyl-histamine from brain extracts of a freshwater crustacean has been developed using capillary electrophoresis with laser-induced fluorescence detection. Molecules were tagged on their primary amine function with naphthalene-2,3-dicarboxaldehyde, but derivatized histamine and N-methyl-histamine exhibited poor stability in contrast to derivatized histidine. To overcome this limitation, an automated derivatization performed within the capillary electrophoresis instrument was optimized and quantitatively validated. The limits of detection were 50, 30, and 60 nmol/L for histidine, histamine, and N-methyl-histamine, respectively. This study reports, for the first time, the amounts of histamine and its related compounds in brain extracts from populations of the freshwater amphipod Gammarus fossarum, and shows that these amounts vary mainly according to population and season, but are not affected by an experimental electrical shock.

Anxiety-like behaviour increases safety from fish predation in an amphipod crustacea.

Anxiety is an emotional state generally expressed as sustained apprehension of the environment and elevated vigilance. It has been widely reported in vertebrates and, more recently, in a few invertebrate species. However, its fitness value remains elusive. We investigated anxiety-like behaviour and its consequences in an amphipod crustacean, using electric shock as aversive stimuli, and pharmacological assays. An anxiety-like state induced by electric shocks in Gammarus fossarum was expressed through increased sheltering behaviour in the absence of predation risk, thereby showing the pervasive nature of such behavioural response. Increasing the number of electric shocks both increased refuge use and delayed behavioural recovery. The behavioural effect of electric shock was mitigated by pre-treatment with LY354740, a metabotropic glutamate receptor group II/III agonist. Importantly, we found that this modulation of decision-making under an anxiety-like state resulted in an increased survival to predation in microcosm experiments. This study confirms the interest in taking an evolutionary view to the study of anxiety and calls for further investigation on the costs counterbalancing the survival benefit of an elevated anxiety level evidenced here.

Multidimensionality in host manipulation mimicked by serotonin injection.

Manipulative parasites often alter the phenotype of their hosts along multiple dimensions. 'Multidimensionality' in host manipulation could consist in the simultaneous alteration of several physiological pathways independently of one another, or proceed from the disruption of some key physiological parameter, followed by a cascade of effects. We compared multidimensionality in 'host manipulation' between two closely related amphipods, Gammarus fossarum and Gammarus pulex, naturally and experimentally infected with Pomphorhynchus laevis (Acanthocephala), respectively. To that end, we calculated in each host-parasite association the effect size of the difference between infected and uninfected individuals for six different traits (activity, phototaxis, geotaxis, attraction to conspecifics, refuge use and metabolic rate). The effects sizes were highly correlated between host-parasite associations, providing evidence for a relatively constant 'infection syndrome'. Using the same methodology, we compared the extent of phenotypic alterations induced by an experimental injection of serotonin (5-HT) in uninfected G. pulex to that induced by experimental or natural infection with P. laevis. We observed a significant correlation between effect sizes across the six traits, indicating that injection with 5-HT can faithfully mimic the 'infection syndrome'. This is, to our knowledge, the first experimental evidence that multidimensionality in host manipulation can proceed, at least partly, from the disruption of some major physiological mechanism.

Cooperation and conflict in host manipulation: interactions among macro-parasites and micro-organisms.

Several parasite species are known to manipulate the phenotype of their hosts in ways that enhance their own transmission. Co-occurrence of manipulative parasites, belonging to the same species or to more than one species, in a single host has been regularly observed. Little is known, however, on interactions between co-occurring manipulative parasites with same or different transmission routes. Several models addressing this problem have provided predictions on how cooperation and conflict between parasites could emerge from multiple infections. Here, we review the empirical evidence in favor of the existence of synergistic or antagonistic interactions between co-occurring parasites, and highlight the neglected role of micro-organisms. We particularly discuss the actual importance of selective forces shaping the evolution of interactions between manipulative parasites in relation to parasite prevalence in natural populations, efficiency in manipulation, and type of transmission (i.e., horizontal versus vertical), and we emphasize the potential for future research.

Parasite-induced alteration of plastic response to predation threat: increased refuge use but lower food intake in Gammarus pulex infected with the acanothocephalan Pomphorhynchus laevis.

Larvae of many trophically-transmitted parasites alter the behaviour of their intermediate host in ways that increase their probability of transmission to the next host in their life cycle. Before reaching a stage that is infective to the next host, parasite larvae may develop through several larval stages in the intermediate host that are not infective to the definitive host. Early predation at these stages results in parasite death, and it has recently been shown that non-infective larvae of some helminths decrease such risk by enhancing the anti-predator defences of the host, including decreased activity and increased sheltering. However, these behavioural changes may divert infected hosts from an optimal balance between survival and foraging (either seeking food or a mate). In this study, this hypothesis was tested using the intermediate host of the acanthocephalan parasite Pomphorhynchus laevis, the freshwater amphipod Gammarus pulex. We compared activity, refuge use, food foraging and food intake of hosts experimentally infected with the non-infective stage (acanthella), with that of uninfected gammarids. Behavioural assays were conducted in four situations varying in predation risk and in food accessibility. Acanthella-infected amphipods showed an increase in refuge use and a general reduction in activity and food intake. There was no effect of parasite intensity on these traits. Uninfected individuals showed plastic responses to water-borne cues from fish by adjusting refuge use, activity and food intake. They also foraged more when the food was placed outside the refuge. At the intra-individual level, refuge use and food intake were positively correlated in infected gammarids only. Overall, our findings suggest that uninfected gammarids exhibit risk-sensitive behaviour including increased food intake under predation risk, whereas gammarids infected with the non-infective larvae of P. laevis exhibit a lower motivation to feed, irrespective of predation risk and food accessibility.

Modulatory effects of the serotonergic and histaminergic systems on reaction to light in the crustacean Gammarus pulex.

Serotonin modulates reaction to light in many animals. In the freshwater amphipod Gammarus pulex, exogenous administration of serotonin induces a transient reversal of photic behaviour from strong photophobia to photophily. We have elucidated further the neuromodulation of photic behaviour in G. pulex by using in vivo pharmacology and behavioural testing. Using several mammalian 5-HT receptor antagonists and agonists, we provide evidence for a role of serotonin receptors in the 5-HT-dependent regulation of G. pulex photic behaviour, possibly involving 5-HTR2 subtype. Serotonin-induced photophily was blocked by the 5-HT receptor antagonists, mianserin, cyproheptadine, and ritanserin, but not by ketanserin and methiothepin. In addition, two agonists at 5-HT receptor, mCPP and α-methylserotonin, partially mimicked 5-HT-induced photophily. The natural photophobia of G. pulex was also decreased by exogenous administration of the histamine receptor antagonist cimetidine, suggesting that photic behaviour may be controlled by more complex interactions between serotonergic and histaminergic systems.

Multidimensionality in parasite-induced phenotypic alterations: ultimate versus proximate aspects.

In most cases, parasites alter more than one dimension in their host phenotype. Although multidimensionality in parasite-induced phenotypic alterations (PIPAs) seems to be the rule, it has started to be addressed only recently. Here, we critically review some of the problems associated with the definition, quantification and interpretation of multidimensionality in PIPAs. In particular, we confront ultimate and proximate accounts, and evaluate their own limitations. We end up by introducing several suggestions for the development of future research, including some practical guidelines for the quantitative analysis of multidimensionality in PIPAs.

Investigating candidate neuromodulatory systems underlying parasitic manipulation: concepts, limitations and prospects.

Studies addressing the functional basis of parasitic manipulation suggest that alteration of the neuromodulatory system is a common feature of manipulated hosts. Screening of the neuromodulatory system has so far been carried out by performing ethopharmacological analysis, biochemical quantification of neurotransmitters and neuromodulators, and/or immunocytochemistry. Here, we review the advantages and limitations of such approaches through the analysis of case studies. We further address whether the analysis of candidate neuromodulatory systems fits the current view of manipulation as being multidimensional. The benefits in combining ethopharmacology with more recent molecular tools to investigate candidate neuromodulatory pathways is also emphasized. We conclude by discussing the value of a multidisciplinary study of parasitic manipulation, combining evolutionary (parasite transmission), behavioural (syndrome of manipulation) and neuroimmunological approaches.

Protection first then facilitation: a manipulative parasite modulates the vulnerability to predation of its intermediate host according to its own developmental stage.

Many trophically transmitted parasites with complex life cycles manipulate their intermediate host behavior in ways facilitating their transmission to final host by predation. This facilitation generally results from lowering host's antipredatory defenses when the parasite is infective to the final host. However, a recent theoretical model predicts that an optimal parasitic strategy would be to protect the intermediate host from predation when noninfective, before switching to facilitation when the infective stage is reached. We tested this hypothesis in the fish acanthocephalan parasite Pomphorhynchus laevis using the amphipod Gammarus pulex as intermediate host. Gammarids parasitized by noninfective stage of P. laevis (acanthella) hid significantly more under refuges than uninfected ones. In addition, acanthella-infected gammarids were less predated upon by trout than uninfected ones. As predicted, a switch toward decreased antipredatory behavior of G. pulex and enhanced vulnerability to predation was found when P. laevis reached the stage infective to its final host. The parasites appear to be able to exploit plasticity in host antipredatory responses, and shift the host optimal response toward their own optimal balance.

Paratenic hosts as regular transmission route in the acanthocephalan Pomphorhynchus laevis: potential implications for food webs.

Although trophically transmitted parasites are recognized to strongly influence food-web dynamics through their ability to manipulate host phenotype, our knowledge of their host spectrum is often imperfect. This is particularly true for the facultative paratenic hosts, which receive little interest. We investigated the occurrence and significance both in terms of ecology and evolution of paratenic hosts in the life cycle of the fish acanthocephalan Pomphorhynchus laevis. This freshwater parasite uses amphipods as intermediate hosts and cyprinids and salmonids as definitive hosts. Within a cohort of parasite larvae, usually reported in amphipod intermediate hosts, more than 90% were actually hosted by small-sized fish. We demonstrated experimentally, using one of these fish, that they get infected through the consumption of parasitized amphipods and contribute to the parasite's transmission to a definitive host, hence confirming their paratenic host status. A better knowledge of paratenic host spectrums could help us to understand the fine tuning of transmission strategies, to better estimate parasite biomass, and could improve our perception of parasite subwebs in terms of host-parasite and predator-parasite links.

Interspecific differences in carotenoid content and sensitivity to UVB radiation in three acanthocephalan parasites exploiting a common intermediate host.

Few endoparasite species are pigmented. Acanthocephalans are an exception however, with several species being characterised by yellow to orange colouration both at the immature (cystacanth) and adult stages. However, the functional and adaptive significance of carotenoid-based colourations in acanthocephalans remains unclear. One possibility is that the carotenoid content of acanthocephalan cystacanths acts as a protective device against ultra-violet radiation (UVR) passing through the translucent cuticle of their crustacean hosts. Indeed, acanthocephalans often bring about behavioural changes in their aquatic intermediate hosts that can increase their exposure to light. Carotenoid composition and damage due to ultra-violet - B (UVB) radiation were investigated in three acanthocephalan parasite species that induce contrasting behavioural alterations in their common intermediate host, the crustacean amphipod Gammarus pulex. The fish acanthocephalans Pomphorhynchus laevis and Pomphorhynchus tereticollis both induce a positive phototaxis in gammarids, such that infected hosts spend more time out of shelters, while remaining benthic. The bird acanthocephalan Polymorphus minutus, on the other hand, induces a negative geotaxis, such that infected hosts typically swim close to the water surface, becoming more exposed to UV radiation. We show that differences in cystacanth colouration between acanthocephalan species directly reflect important differences in carotenoid content. The two fish parasites exhibit a contrasting pattern, with P. tereticollis harbouring a large diversity of carotenoid pigments, whereas P. laevis is characterised by a lower carotenoid content consisting mainly of lutein and astaxanthin. The highest carotenoid content is found in the bright orange P.minutus, with a predominance of esterified forms of astaxanthin. Exposure to UVB radiation revealed a higher susceptibility in P. laevis larvae compared with P. tereticollis and P. minutus, in terms of sublethality (decreased evagination rate) and of damage to DNA (increased cyclobutane pyrimidine dimers production). Although we found important and correlated interspecific differences in carotenoid composition and tolerance to high UVB radiation, our results do not fully support the hypothesis of adaptive carotenoid-based colourations in relation to UV protection. An alternative scenario for the evolution of carotenoid accumulation in acanthocephalan parasites is discussed.

Parasite and host assemblages: embracing the reality will improve our knowledge of parasite transmission and virulence.

Interactions involving several parasite species (multi-parasitized hosts) or several host species (multi-host parasites) are the rule in nature. Only a few studies have investigated these realistic, but complex, situations from an evolutionary perspective. Consequently, their impact on the evolution of parasite virulence and transmission remains poorly understood. The mechanisms by which multiple infections may influence virulence and transmission include the dynamics of intrahost competition, mediation by the host immune system and an increase in parasite genetic recombination. Theoretical investigations have yet to be conducted to determine which of these mechanisms are likely to be key factors in the evolution of virulence and transmission. In contrast, the relationship between multi-host parasites and parasite virulence and transmission has seen some theoretical investigation. The key factors in these models are the trade-off between virulence across different host species, variation in host species quality and patterns of transmission. The empirical studies on multi-host parasites suggest that interspecies transmission plays a central role in the evolution of virulence, but as yet no complete picture of the phenomena involved is available. Ultimately, determining how complex host-parasite interactions impact the evolution of host-parasite relationships will require the development of cross-disciplinary studies linking the ecology of quantitative networks with the evolution of virulence.

Host-manipulation by parasites with complex life cycles: adaptive or not?

The effect of host manipulation by parasites on trophic transmission to final hosts remains unclear. The transmission benefits gained by manipulative parasites are difficult to assess, and evidence for a causal link between manipulation and trophic transmission is missing. In addition, infected intermediate hosts can also be more vulnerable to predation by nonhosts, whereas recent theoretical advances indicate that the evolution of host manipulation does not require increased specificity in trophic transmission. We propose that a deeper consideration of the evolution of complex life cycles in helminth parasites might provide a different perspective on the evolution of host manipulation by parasites.