Consistency of host responses to parasitic infection in the three-spined stickleback fish infected by the diphyllobothriidean cestodeSchistocephalus solidus

How cunning is the puppet-master? Cestode-infected fish appear generally fearless

Trophically transmitted parasites have life cycles that require the infected host to be eaten by the correct type of predator. Such parasites should benefit from an ability to suppress the host’s fear of predators, but if the manipulation is imprecise the consequence may be increased predation by non-hosts, to the detriment of the parasite. Three-spined sticklebacks (Gasterosteus aculeatus) infected by the cestode Schistocephalus solidus express reduced antipredator behaviours, but it is unknown whether this is an example of a highly precise manipulation, a more general manipulation, or if it can even be attributed to mere side effects of disease. In a series of experiments, we investigated several behaviours of infected and uninfected sticklebacks. As expected, they had weak responses to simulated predatory attacks compared to uninfected fish. However, our results suggest that the parasite induced a general fearlessness, rather than a precise manipulation aimed at the correct predators (birds). Infected fish had reduced responses also when attacked from the side and when exposed to odour from a fish predator, which is a “dead-end” for this parasite. We also tested whether the reduced anti-predator behaviours were mere symptoms of a decreased overall vigour, or due to parasite-induced hunger, but we found no support for these ideas. We propose that even imprecise manipulations of anti-predator behaviours may benefit parasites, for example, if other behaviours are altered in a way that increases the exposure to the correct predator.

Specificity of resistance and geographic patterns of virulence in a vertebrate host-parasite system

BACKGROUND

Host genotype - parasite genotype co-evolutionary dynamics are influenced by local biotic and abiotic environmental conditions. This results in spatially heterogeneous selection among host populations. How such heterogeneous selection influences host resistance, parasite infectivity and virulence remains largely unknown. We hypothesized that different co-evolutionary trajectories of a vertebrate host-parasite association result in specific virulence patterns when assessed on a large geographic scale. We used two reference host populations of three-spined sticklebacks and nine strains of their specific cestode parasite Schistocephalus solidus from across the Northern Hemisphere for controlled infection experiments. Host and parasite effects on infection phenotypes including host immune gene expression were determined.

RESULTS

S. solidus strains grew generally larger in hosts coming from a population with high parasite diversity and low S. solidus prevalence (DE hosts). Hosts from a population with low parasite diversity and high S. solidus prevalence (NO hosts) were better able to control the parasite's growth, regardless of the origin of the parasite. Host condition and immunological parameters converged upon infection and parasite growth showed the same geographic pattern in both host types.

CONCLUSION

Our results suggest that NO sticklebacks evolved resistance against a variety of S. solidus strains, whereas DE sticklebacks are less resistant against S. solidus. Our data provide evidence that differences in parasite prevalence can cause immunological heterogeneity and that parasite size, a proxy for virulence and resistance, is, on a geographic scale, determined by main effects of the host and the parasite and less by an interaction of both genotypes.

Are solo infections of the diphyllobothriidean cestode Schistocephalus solidus more virulent than multiple infections?

We performed a long-term natural experiment investigating the impact of the diphyllobotriidean cestode Schistocephalus solidus on the body condition and clutch size (CS) of threespine stickleback Gasterosteus aculeatus, its second intermediate host, and the growth of larval parasites in host fish. We tested the hypothesis that single S. solidus infections were more virulent than multiple infections. We also asked whether the metrics of mean and total parasite mass (proxies for individual and total volume, respectively) were consistent with predictions of the resource constraints or the life history strategy (LHS) hypothesis for the growth of, hence exploitation by, larval helminths in intermediate hosts. The samples were drawn from Walby Lake, Alaska in eight of 11 years. Host body condition and CS (egg number per spawning bout) decreased significantly with intensity after adjustments for host size and parasite index. Thus, infections have an increasingly negative impact on measures of host fitness with greater intensity, in contrast to the hypothesis that single infections are more harmful than multiple infections. We also found that mean parasite mass decreased with intensity while total parasite mass increased with intensity as predicted by the LHS hypothesis.

Drought-associated absence of alien invasive anchorworm, Lernaea cyprinacea (Copepoda: Lernaeidae), is related to changes in fish health

Recently, Mozambique tilapia (Oreochromis mossambicus Peters, 1852) were listed on the IUCN Red List as near-threatened as their populations are at risk due to hybridization. Another factor that potentially contributes to their population decline is that they are regularly infected by the invasive parasitic copepod anchorworm, Lernaea cyprinacea Linnaeus, 1758. Considering anchorworm-infected Mozambique tilapia are common, understanding their condition with respect to infection is difficult as uninfected fish from the same localities have been unavailable for comparison. A severe drought in southern Africa has created hypersaline environments in the Phongolo River floodplain of north-eastern South Africa, such that freshwater parasites cannot survive and uninfected fish are now found. To determine how infection influences host health, infected and uninfected Mozambique tilapia were collected before and during drought conditions, from Nyamiti pan of the Phongolo River floodplain. Anchorworm-infected fish prevalence was recorded, and anchorworms were collected from hosts and identified to the species level using molecular data of the 18S rRNA gene. For each fish, intensity of anchorworm infection, total length, and weights of the gutted body, liver, spleen, and gonads were recorded. Gutted condition factor, hepato-, spleeno-, and gonado-somatic index values per fish, and prevalence of infection per collection were determined. A rapid health assessment was also conducted to determine a health score for each fish. Molecular analyses confirmed the anchorworm studied was L. cyprinacea. Prior to and during drought, prevalence of infection was 100%, and 0%, respectively. Before drought, fish had significantly reduced hepato-, spleeno-, and gonado-somatic index values, and higher health assessment scores, yet significantly higher gutted condition. Anchorworm intensity was indirectly correlated with fish liver and gonad condition. This study demonstrates that host condition and health varies greatly with respect to drought and infection, and provides the necessary data for follow-up studies in post-drought conditions.

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Invasive Lernaea cyprinacea, confirmed by 18S rRNA data, infect Mozambique tilapia.

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Severe drought has induced a natural freshwater ectoparasite removal treatment.

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Prevalence of anchorworm-infected tilapia was 100% before and 0% during drought.

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Anchorworm infection is related to reduced fish health and some host energetics.

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Accordingly infection may alter host fitness and population dynamics.

Iterative development and the scope for plasticity: contrasts among trait categories in an adaptive radiation

Phenotypic plasticity can influence evolutionary change in a lineage, ranging from facilitation of population persistence in a novel environment to directing the patterns of evolutionary change. As the specific nature of plasticity can impact evolutionary consequences, it is essential to consider how plasticity is manifested if we are to understand the contribution of plasticity to phenotypic evolution. Most morphological traits are developmentally plastic, irreversible, and generally considered to be costly, at least when the resultant phenotype is mis-matched to the environment. At the other extreme, behavioral phenotypes are typically activational (modifiable on very short time scales), and not immediately costly as they are produced by constitutive neural networks. Although patterns of morphological and behavioral plasticity are often compared, patterns of plasticity of life history phenotypes are rarely considered. Here we review patterns of plasticity in these trait categories within and among populations, comprising the adaptive radiation of the threespine stickleback fish Gasterosteus aculeatus. We immediately found it necessary to consider the possibility of iterated development, the concept that behavioral and life history trajectories can be repeatedly reset on activational (usually behavior) or developmental (usually life history) time frames, offering fine tuning of the response to environmental context. Morphology in stickleback is primarily reset only in that developmental trajectories can be altered as environments change over the course of development. As anticipated, the boundaries between the trait categories are not clear and are likely to be linked by shared, underlying physiological and genetic systems.

Life-history plasticity in female threespine stickleback

The postglacial adaptive radiation of the threespine stickleback fish (Gasterosteus aculeatus) has been widely used to investigate the roles of both adaptive evolution and plasticity in behavioral and morphological divergence from the ancestral condition represented by present-day oceanic stickleback. These phenotypes tend to exhibit high levels of ecotypic differentiation. Population divergence in life history has also been well studied, but in contrast to behavior and morphology, the extent and importance of plasticity has been much less well studied. In this review, we summarize what is known about life-history plasticity in female threespine stickleback, considering four traits intimately associated with reproductive output: age/size at maturation, level of reproductive effort, egg size and clutch size. We envision life-history plasticity in an iterative, ontogenetic framework, in which females may express plasticity repeatedly across each of several time frames. We contrast the results of laboratory and field studies because, for most traits, these approaches give somewhat different answers. We provide ideas on what the cues might be for observed plasticity in each trait and, when possible, we inquire about the relative costs and benefits to expressed plasticity. We end with an example of how we think plasticity may play out in stickleback life history given what we know of plasticity in the ancestor.