Schistocephalus solidus: a molecular test of premature gamete exchange for fertilization in the intermediate host Gasterosteus aculeatus

We tested the hypothesis that the cestode Schistocephalus solidus is capable of premature gamete exchange as a plerocercoid in the last intermediate stickleback host. The existence of such a reproductive mode is suggested by the highly advanced gonadal development in the plerocercoid and the large fitness gain of outcrossing. In addition, eggs from selfing cestodes have a higher hatching rate when the cestode originated from a doubly infected stickleback than when it came from a singly infected fish. We hatched eggs from 10 singly breeding cestodes that originated from doubly infected sticklebacks with the prediction that some should be outcrossed and share alleles with both the breeding cestode and the second cestode in the coinfection if the hypothesis is correct. However, all of the 430 tested larvae matched only the alleles of the breeding cestode. It is therefore very unlikely that S. solidus engaged in gamete exchange in its fish host. We suggest an alternative hypothesis to explain the higher hatching rate of eggs produced by cestodes from doubly infected fish as compared to those from single infections.     

Do heavy burdens of Schistocephalus solidus in juvenile threespine stickleback result in disaster for the parasite?

The diphyllobothriidean cestode Schistocephalus solidus typically infects threespine sticklebacks that are too small to allow the parasite to reach a mature size. As a result, the parasite must allow further growth of its host to reach the size at which it becomes competent to infect and reproduce in the definitive host. At times, however, intensity of infection can be high, leading to crowding among parasites and to heavy burdens causing mortality among hosts. Our data show that, during a previously observed epizootic, large percentages of plerocercoids (average 75% per host, 82% among all parasites pooled) did not grow to become massive enough in 1-yr-old threespine sticklebacks to be capable of establishment and maturation in the definitive host. Massive deaths of 1-yr-old sticklebacks due to infection during the epizootic resulted in the great misfortune of a disaster for a large number of parasites, resulting in dramatically reduced transmission of S. solidus.     

An enzyme study of purine nucleotide biosynthesis in the plerocercoids of cestodes in the fam. Ligulidae]

By means of spectrophotometric method there was determined the activity of three enzymes of biosynthesis of purine nucleotides: amino imidazole ribonucleotide-carboxylase (AIR-carboxylase, EC 4.1.1.21), an enzyme of biosynthesis of purine nucleotides de novo in plerocercoids of Schistocephalus pungitii and Digramma interrupta; inosine monophosphate-dehydrogenase (IMPh-dehydrogenase, EC 1.2.1.14), an enzyme of salvage path, and adenylosuccinate lyase (EC 4.3.2.2), an enzyme taking part both in biosynthesis de novo and salvage in plerocercoids of Schistocephalus pungitii. The activity of AIR-carboxylase was not determined. Specific activities of adenylosuccinate lyase and IMPh-dehydrogenase amount to (1.3 +/- 0.3) x 10(-3) and (1.2 +/- 0.4) x 10(-3) mumole/min.mg protein, respectively. The activity of the three enzymes was determined in the liver of ten-spined stickleback, a host of S. pungitii plerocercoids. The question of metabolic dependence of Ligulidae plerocercoids on hosts to provide for purine bases is discussed.     

Multiple infections: relatedness and time between infections affect the establishment and growth of the cestode Schistocephalus solidus in its stickleback host

We studied experimental double infections of the cestode Schistocephalus solidus in its stickleback host. In particular, we were interested in how two important components of the cestode's transmission success-establishment and growth within the fish host-were affected by the relatedness of the two parasites in a double exposure and by the timing of the two exposures, that is, whether they occurred simultaneously or sequentially. We found that male sticklebacks more often became infected (singly or doubly) if the two cestodes in the exposures were related, whereas female sticklebacks were more easily infected (singly or doubly) when exposed to two unrelated cestodes. Irrespective of the fish's gender, successful infections more often contained both cestodes when they were related. In sequential exposures with related as well as unrelated cestodes, the cestode in the later exposure survived better and also grew larger than the cestode from the first exposure, despite being one week younger. Our results emphasize that within-host dynamics and factors acting at this level can play an important role in determining a parasite's transmission success.     

Distinct lineages of Schistocephalus parasites in threespine and ninespine stickleback hosts revealed by DNA sequence analysis

Parasitic interactions are often part of complex networks of interspecific relationships that have evolved in biological communities. Despite many years of work on the evolution of parasitism, the likelihood that sister taxa of parasites can co-evolve with their hosts to specifically infect two related lineages, even when those hosts occur sympatrically, is still unclear. Furthermore, when these specific interactions occur, the molecular and physiological basis of this specificity is still largely unknown. The presence of these specific parasitic relationships can now be tested using molecular markers such as DNA sequence variation. Here we test for specific parasitic relationships in an emerging host-parasite model, the stickleback-Schistocephalus system. Threespine and ninespine stickleback fish are intermediate hosts for Schistocephalus cestode parasites that are phenotypically very similar and have nearly identical life cycles through plankton, stickleback, and avian hosts. We analyzed over 2000 base pairs of COX1 and NADH1 mitochondrial DNA sequences in 48 Schistocephalus individuals collected from threespine and ninespine stickleback hosts from disparate geographic regions distributed across the Northern Hemisphere. Our data strongly support the presence of two distinct clades of Schistocephalus, each of which exclusively infects either threespine or ninespine stickleback. These clades most likely represent different species that diverged soon after the speciation of their stickleback hosts. In addition, genetic structuring exists among Schistocephalus taken from threespine stickleback hosts from Alaska, Oregon and Wales, although it is much less than the divergence between hosts. Our findings emphasize that biological communities may be even more complex than they first appear, and beg the question of what are the ecological, physiological, and genetic factors that maintain the specificity of the Schistocephalus parasites and their stickleback hosts.     

The effect of Schistocephalus solidus infection on meal size of three‐spined stickleback

The effect of infection with the pseudophyllidean cestode Schistocephalus solidus on the meal size of individually housed three‐spined stickleback Gasterosteus aculeatus was quantified. Infected fish harboured plerocercoid loads that contributed from 1·1 to 33·9% of their total mass. Across this range of infection levels, the presence of S. solidus infection had no significant directional effect on standard length ( L _S) corrected meal size of host three‐spined sticklebacks. Amongst multiply infected fish there was a significant negative relationship between L _S‐corrected meal size and the proportion of host mass contributed by S. solidus parasites. This relationship, however, did not hold for singly‐infected fish. Furthermore, the data suggest that multiply‐infected fish that harbour a combined mass of parasites contributing < c . 15% to host body mass might exhibit meal sizes that exceed those of length‐matched uninfected fish. The results suggest that although heavy infections can significantly reduce the meal size of heavily infected three‐spined sticklebacks, in the early stages of multiple S. solidus infections host food intake may increase. The probable causes of these differential effects on meal size and their consequences for the host‐parasite system are discussed.     

Body condition and reproductive capacity of three‐spined stickleback infected with the cestode Schistocephalus solidus

The relationship between reproductive ability and the residual index of body condition in three‐spined stickleback Gasterosteus aculeatus from a population infected with the cestode macroparasite Schistocephalus solidus in Walby Lake, Alaska, was examined. In general, reproductive activities resulted in significantly lower levels of body condition in three‐spined stickleback during the latter part of the breeding season, and relatively high levels of S. solidus infection intensified the energetic drain. Although female body condition did not differ significantly due just to the presence of S. solidus , increased parasite index did have a significant negative effect on female body condition. Males showed significantly lower levels of body condition in response to S. solidus infection alone and in association with a greater parasite index. Males had greater mean parasite indices than females. Females had significantly lower body condition than males, which may be due to discrepancies in energy expenditure between the sexes during reproduction. Females with greater body condition were significantly more likely to produce a clutch of eggs than those with lower condition, suggesting a threshold effect of body condition on reproductive capacity.     

Who is in control of the stickleback immune system: interactions between Schistocephalus solidus and its specific vertebrate host

The cestode Schistocephalus solidus is a frequent parasite of three-spined sticklebacks and has a large impact on its host's fitness. Selection pressure should therefore be high on stickleback defence mechanisms, like an efficient immune system, and also on parasite strategies to overcome these. Even though there are indications for manipulation of the immune system of its specific second intermediate host by the cestode, nothing is yet known about the chronology of specific interactions of S. solidus with the stickleback immune system. We here expected sticklebacks to first mount an innate immune response directly post-exposure to the parasite to clear the infection at an early stage and after an initial lag phase to upregulate adaptive immunity. Most interestingly, we did not find any upregulation of the specific lymphocyte-mediated immune response. Also, the pattern of activation of the innate immune system did not match our expectations: the proliferation of monocytes followed fluctuating kinetics suggesting that the parasite repeatedly installs a new surface coat not immunogenic to the host. Furthermore, the respiratory burst activity, which has the potential to clear an early S. solidus infection, was upregulated very late during infection, when the parasite was too big to be cleared but ready for transmission to its final host. We here suggest that the late activation of the innate immune system interferes with the neuroendocrine system, which mediates reduced predation avoidance behaviour and so facilitates the transmission to the final host.     

Evolutionary significance of fecundity reduction in threespine stickleback infected by the diphyllobothriidean cestode Schistocephalus solidus

Parasites may cause fecundity reduction in their hosts via life‐history strategies involving simple nutrient theft or manipulation of host energy allocation. Simple theft of nutrients incidentally reduces host energy allocation to reproduction, whereas manipulation is a parasite‐driven diversion of energy away from host reproduction. We aimed to determine whether the diphyllobothriidean cestode parasite Schistocephalus solidus causes loss of fecundity in the threespine stickleback fish (Gasterosteus aculeatus) through simple nutrient theft or the manipulation of host energy allocation. In one stickleback population (Walby Lake, Matanuska‐Susitna Valley, Alaska), there was no difference in the sizes and ages of infected and uninfected reproducing females. Lightly‐ and heavily‐infected females produced clutches of eggs, but increasingly smaller percentages of infected females produced clutches as the parasite‐to‐host biomass ratio (PI) increased. Infected, clutch‐bearing sticklebacks showed reductions in clutch size, egg mass, and clutch mass, which were related to increases in PI and reflected a reduction in reproductive parameters as growth in parasite mass occurs. The findings obtained for this population are consistent with the hypothesis of simple nutrient theft; however, populations of S. solidus in other regions may manipulate host energy allocation. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 835–846.     

Effects of Schistocephalus solidus infection on brain monoaminergic activity in female three-spined sticklebacks Gasterosteus aculeatus.

The three-spined stickleback Gasterosteus aculeatus is an intermediate host of the tapeworm Schistocephalus solidus. Changes in predator avoidance, foraging and shoaling behaviour have been reported in sticklebacks infested with S. solidus, but the mechanisms underlying parasite-induced behavioural changes are not understood. Monoamine neurotransmitters are involved in the control of behaviour and central monoaminergic systems are sensitive to various stressors. Thus, the behavioural effects of S. solidus infestation might be a reflection of changes in brain monoaminergic activity in the stickleback host. The concentrations of 5-hydroxytryptamine (5-HT), dopamine (DA), norepinephrine (NE) and their metabolites 5-hydroxy-indoleacetic acid (5-HIAA), homovanilic acid (HVA) and 3-methoxy-4-hydroxyphenylglycol (MHPG) were measured in the telencephalons, hypothalami and brainstems of parasitized and non-parasitized female sticklebacks held in the laboratory. The ratios of 5-HIAA:5-HT were significantly elevated in both the hypothalami and brainstems of infected sticklebacks. The concentrations of 5-HT and NE were significantly reduced in the telencephalons of infected fish as compared with controls, but there was no elevation of metabolite concentrations. The results are consistent with chronic stress in infected fish, but may also reflect other alterations of neuroendocrine status resulting from parasite infection.     

Consistency of host responses to parasitic infection in the three‐spined stickleback fish infected by the diphyllobothriidean cestode Schistocephalus solidus

Among populations of the three‐spined stickleback fish in Alaska, females appear to show two forms of sterility tolerance to infection by the diphyllobothriidean cestode Schistocephalus solidus. In contrast to sticklebacks in other regions of the northern hemisphere, female fish are capable of producing clutches of eggs despite supporting large parasite burdens. Nonetheless, nutrient loss to the parasite, coupled with the energetic demands of host reproduction, eventually curtails spawning among infected females. Host females in Walby Lake experience ‘fecundity reduction’ resulting from nutrient theft as a side effect of infection. In Scout Lake, infected females show ‘fecundity compensation’, an adaptive, inducible response allowing them to increase current fecundity to compensate for reduction or loss of future reproduction. This multi‐year study of sticklebacks from each lake addresses two empirical questions for a better understanding of the dynamic interplay between host and parasite. First, is there is any annual variation within the two responses to parasitism in each host population; and, if so, is it related to parasite burden? Second, do the two host responses show consistent differences between the populations of sticklebacks despite any yearly variation in them? We found annual, intra‐population variation within the response shown by each population of stickleback which appears to have been influenced by the parasite : host mass ratio and possibly by unknown environmental conditions affecting the reproductive physiology of stickleback females. Moreover, the data support the hypothesis that ovum mass is more sensitive to parasitism (parasite burden) than clutch size in females from Walby Lake which exhibit fecundity reduction. Notwithstanding the intra‐population variation within each host response, the responses to infection occurred consistently within each respective stickleback population and appear to reflect stable, fundamental characteristics of the populations. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 958–968.     

Nesting, courtship and kidney hypertrophy in Schistocephalus-infected male three-spined stickleback from an upland lake

The effect of Schistocephalus solidus infection on reproductive development and behaviour of male three-spined stickleback Gasterosteus aculeatus from an upland lake (Llyn Frongoch, Wales, U.K.) was investigated. Uninfected and infected males were collected on two separate occasions during the 2005 breeding season and encouraged to build nests under favourable laboratory conditions. Male nuptial colouration, courtship levels and nesting activity were recorded daily over two separate 21 day studies. Completed nests were removed, encouraging males to build multiple nests. On termination of each study, males were dissected and the kidney and any S. solidus plerocercoids were removed and weighed. In contrast to uninfected males, which readily built multiple nests, no infected males completed a nest during either study. Infected males also exhibited significantly reduced courtship levels, nesting activity, nuptial colouration, kidney development and body condition, compared with uninfected fish. Among infected three-spined stickleback, courtship and nesting behaviours tended to be more severely impacted in fish harbouring heavier plerocercoid burdens. In contrast to other populations, male three-spined stickleback from Llyn Frongoch appear to be unable to overcome the negative impacts of infections on reproductive development and behaviour when provided with short-term favourable conditions. Discrepancies between populations might have been related to the trophic status at the site of capture, emphasizing the importance of studying the biological effects of infections under different environmental conditions.     

Ecological causes of sex-biased parasitism in threespine stickleback

Males and females can differ in levels of parasitism and such differences may be mediated by the costs of sexual selection or by ecological differences between the genders. In threespine stickleback, Gasterosteus aculeatus , males exhibit paternal care and territorial nest defence and the costs of reproduction may be particularly high for males relative to females. We monitored levels of parasitism for 15 years in a population of stickleback infected by four different parasite species. Consistent with general predictions, overall parasite prevalence (total parasitism) was greater in males than in females. However, this excess did not occur for each species of parasite. Males had higher prevalence of a cestode Cyathocephalus truncatus and a trematode Bunodera sp. relative to females, while females had higher prevalence of a cestode Schistocephalus solidus and nematodes. This suggested ecological sources to differences in parasitism rather than reproductive costs and therefore we examined diet of unparasitized stickleback, predicting that differences in dietary niche would influence relative parasitism. This was partially confirmed and showed that female stomach contents had increased frequency of pelagic items, the major habitat for the primary host of S. solidus whereas males exhibited increased frequency of benthic items, the dominant habitat of C. truncatus and Bunodera. Temporal shifts in the extent and direction of differential parasitism among years between the sexes were associated with temporal shifts in dietary differences. Our results, combined with those in the literature, suggest that ecological differences between genders could be a more important component to patterns of parasitic infection in natural populations than currently appreciated.     

Landscape Genetics of Schistocephalus solidus Parasites in Threespine Stickleback (Gasterosteus aculeatus) from Alaska

The nature of gene flow in parasites with complex life cycles is poorly understood, particularly when intermediate and definitive hosts have contrasting movement potential. We examined whether the fine-scale population genetic structure of the diphyllobothriidean cestode Schistocephalus solidus reflects the habits of intermediate threespine stickleback hosts or those of its definitive hosts, semi-aquatic piscivorous birds, to better understand complex host-parasite interactions. Seventeen lakes in the Cook Inlet region of south-central Alaska were sampled, including ten in the Matanuska-Susitna Valley, five on the Kenai Peninsula, and two in the Bristol Bay drainage. We analyzed sequence variation across a 759 bp region of the mitochondrial DNA (mtDNA) cytochrome oxidase I region for 1,026 S. solidus individuals sampled from 2009-2012. We also analyzed allelic variation at 8 microsatellite loci for 1,243 individuals. Analysis of mtDNA haplotype and microsatellite genotype variation recovered evidence of significant population genetic structure within S. solidus. Host, location, and year were factors in structuring observed genetic variation. Pairwise measures revealed significant differentiation among lakes, including a pattern of isolation-by-distance. Bayesian analysis identified three distinct genotypic clusters in the study region, little admixture within hosts and lakes, and a shift in genotype frequencies over time. Evidence of fine-scale population structure in S. solidus indicates that movement of its vagile, definitive avian hosts has less influence on gene flow than expected based solely on movement potential. Observed patterns of genetic variation may reflect genetic drift, behaviors of definitive hosts that constrain dispersal, life history of intermediate hosts, and adaptive specificity of S. solidus to intermediate host genotype.     

Heterospecific aggression and adaptive divergence in brook stickleback (Culaea inconstans)

Agonistic behavior between heterospecifics, in which individuals of one species attack another, may cause a subordinate species to shift resource or habitat use. Subsequent evolutionary responses to selection may mimic shifts expected under ecological character displacement, but with no role played by exploitative competition. Alternatively, aggressive behavior can evolve when fitness is improved by excluding members of a coexisting species from a defendable resource through interference. We tested whether heterospecific agonistic behavior has evolved in brook stickleback (Culaea inconstans) by comparing replicate allopatric populations to those sympatric with ninespine stickleback (Pungitius pungitius). We also tested for heritable variation in heterospecific aggressive behavior by rearing family groups in a common environment. Allopatric populations of brook stickleback were more aggressive than ninespine stickleback, suggesting that pre-existing aggression in brook stickleback contributed to niche shifts by ninespine stickleback. In addition, sympatric adult brook stickleback were more aggressive toward ninespine stickleback than brook stickleback from allopatric populations. Overt heterospecific aggressive behaviors were heritable, and aggression in juvenile brook stickleback increased with age in sympatric but not in allopatric populations reared in a common environment. Brook stickleback have evolved increased aggression when they coexist with ninespine stickleback. These stickleback communities have been structured by both evolved and pre-existing variation in heterospecific aggressive behavior in brook stickleback.     

Body condition and reproductive status in sticklebacks exposed to a single wave of Schistocephalus solidus infection

This paper describes the results of a study of the effects of Schistocephalus solidus on one of its intermediate hosts, the stickleback Gasterosteus aculeatus , in an annual population in which infection is known to occur in one major wave in autumn. Weight, as a function of length, was lower in infected sticklebacks compared with uninfected fish during autumn and spring; in winter and summer, both categories of fish were in equally poor condition. In early autumn, the hepatosomatic indices of newly infected fish were higher than those of uninfected fish, perhaps due to a pathological response. Thereafter, relative liver size of uninfected and infected sticklebacks was comparable until spring, when it increased sharply in uninfected sticklebacks but remained stable at a low level in infected sticklebacks. Few infected fish reached maturity. The only males to attain maturity whilst sustaining an infection of S. solidus were in particularly good condition. Nuptial coloration, kidney hypertrophy and testes size were unimpaired in these mature infected males, but whether they were capable of successful reproduction remains debatable.     

Molecular epizootiology and evolution of vesicular stomatitis virus New Jersey.

Vesicular stomatitis virus (VSV) has been shown previously to be capable of undergoing rapid mutational change during sequential experimental infections in various tissue culture cell systems (J. Holland, K. Spindler, F. Horodyski, E. Grabau, S. Nichol, and S. Vandepol, Science 215:1577-1585, 1982). The present study was undertaken to determine the degree of genetic diversity and evolution of the virus under natural infection conditions and to gain insight into the epizootiology of the disease. Between 1982 and 1985, numerous outbreaks of VSV of the New Jersey serotype were reported throughout regions of the United States and Mexico. A T1 RNase fingerprint analysis was performed on the RNA genomes of 43 virus isolates from areas of epizootic and enzootic virus activity. This indicates that virus populations were genetically relatively homogeneous within successive U.S. virus epizootics. The data included virus isolates from different epizootic stages, geographical locations, host animals, and host lesion sites. In contrast, only distant genome RNA T1 fingerprint similarities were observed among viruses of the different U.S. epizootics. However, Mexican viruses isolated before or concurrent with U.S. epizootics had very similar RNA genome fingerprints, suggesting that Mexico may have been the possible origin of virus initiating recent U.S. VSV New Jersey outbreaks. Comparison of T1 fingerprints of viruses with enzootic disease areas revealed a greater extent of virus genetic diversity in these areas relative to that observed in epizootic areas. The evolutionary significance of these findings and their relationship to experimental data on VSV evolution are discussed.     

Occurrence of Schistocephalus solidus in anadromous threespine stickleback

Plerocercoids of the cestode Schistocephalus solidus are reported for the first time from the body cavity of anadromous threespine stickleback inhabiting Mud Lake, Alaska. Most infected stickleback harbored a single large plerocerciod (mean weight = 0.447 g, range = 0.228-0.716 g). The overall prevalence of plerocercoids across genders and 2 yr of samples was 1.4%, but prevalence was significantly greater in males than in females. Because of the large size of the plerocercoids, anadromous stickleback were probably infected as juveniles before leaving the lake, suggesting that plerocercoids can live in the body cavity of oceanic stickleback for several years.     

Parasitological serendipity: from Schistocephalus to Echinococcus

Attention is drawn to the situation nowadays, whereby workers are encouraged to undertake research which appears useful or of economic importance, although the History of Science indicates that many major discoveries have been the result of 'serendipity'--'the chance observation falling on the receptive eye'. Some of the more important examples in Medicine and Parasitology are reviewed. The author then relates how he was given a stickleback infected with the plerocercoid of Schistocephalus solidus, an episode which eventually led to the successful in vitro culture of the adult of this species. Attention is also drawn to the largely unrecognized work of the Danish Veterinarian, P. C. Abildgaard, who in 1789 demonstrated that this species completed its life cycle in a bird, thus establishing, for the first time, the transmission of a parasite from one host to another. The in vitro culture of S. solidus led to the development of successful in vitro techniques for Ligula intestinalis and for Echinococcus granulosus and E. multilocularis. The observation that E. granulosus of horse origin failed to grow in vitro led eventually to the concept of physiological 'strains' of E. granulosus, now a subject of much international research.