Does resource availability affect host manipulation? – an experimental test with Schistocephalus solidus

Parasite infection impairs the shoaling behaviour of uninfected shoal members under predator attack

Abstract

A key benefit of sociality is a reduction in predation risk. Cohesive group behaviour and rapid collective decision making are essential for reducing predation risk in groups. Parasite infection might reduce an individuals’ grouping behaviours and thereby change the behaviour of the group as a whole. To investigate the relationship between parasite infection and grouping behaviours, we studied groups of three-spined sticklebacks, Gasterosteus aculeatus, varying the number of individuals experimentally infected with the cestode Schistocephalus solidus. We studied groups of six sticklebacks containing 0, 2, 3, 4 or 6 infected individuals before and after a simulated bird attack. We predicted that infected individuals would have reduced shoaling and swimming speed and that the presence of infected individuals within a group would reduce group cohesion and speed. Uninfected fish increased shoaling and reduced swimming speed more than infected fish after the bird attack. In groups containing both infected and uninfected fish, the group behaviours were dominated by the more frequent character (uninfected versus infected). Interestingly, groups with equal numbers of uninfected and infected fish showed the least shoaling and had the lowest swimming speeds, suggesting that these groups failed to generate a majority and therefore displayed signs of indecisiveness by reducing their swimming speed the most. Our results provide evidence for a negative effect of infection on a group’s shoaling behaviour, thereby potentially deteriorating collective decision making. The presence of infected individuals might thus have far-reaching consequences in natural populations under predation risk.

Significance statement

Parasite-infected individuals often show deviating group behaviours. This might reduce the anti-predator benefits of group living. However, it is unknown whether such deviations in group behaviour might influence the shoaling behaviour of uninfected group members and thereby the behaviour of the group as a whole. By experimentally infecting sticklebacks and investigating groups varying in infection rates, we show that infected sticklebacks differ in their shoaling behaviours from uninfected sticklebacks. Additionally, the presence of infected sticklebacks within the group affected the behaviour of uninfected shoal members. We show that shoals of infected fish are less cohesive and move slower compared to shoals of uninfected fish. Furthermore, we show that the infection rate of the shoal is crucial for how the group behaves.

Tapeworm manipulation of copepod behaviour: parasite genotype has a larger effect than host genotype

Compared with uninfected individuals, infected animals can exhibit altered phenotypes. The changes often appear beneficial to parasites, leading to the notion that modified host phenotypes are extended parasite phenotypes, shaped by parasite genes. However, the phenotype of a parasitized individual may reflect parasitic manipulation, host responses to infection or both, and disentangling the contribution of parasite genes versus host genes to these altered phenotypes is challenging. Using a tapeworm (Schistocephalus solidus) infecting its copepod first intermediate host, I performed a full-factorial, cross-infection experiment with five host and five parasite genotypes. I found that a behavioural trait modified by infection, copepod activity, was affected by both host and parasite genotype. There was no clear evidence for host genotype by parasite genotype interactions. Several observations indicated that host behaviour was chiefly determined by parasite genes: (i) all infected copepods, regardless of host or parasite genotype, exhibited behavioural changes, (ii) parasitism reduced the differences among copepod genotypes, and (iii) within infected copepods, parasite genotype had twice as large an effect on behaviour as host genotype. I conclude that the altered behaviour of infected copepods primarily represents an extended parasite phenotype, and I discuss how genetic variation in parasitic host manipulation could be maintained.

Behavior out of control: Experimental evolution of resistance to host manipulation

Many parasites alter their host's phenotype in a manner that enhances their own fitness beyond the benefits they would gain from normal exploitation. Such host manipulation is rarely consistent with the host's best interests resulting in suboptimal and often fatal behavior from the host's perspective. In this case, hosts should evolve resistance to host manipulation. The cestode Schistocephalus solidus manipulates the behavior of its first intermediate copepod host to reduce its predation susceptibility and avoid fatal premature predation before the parasite is ready for transmission to its subsequent host. Thereafter, S. solidus increases host activity to facilitate transmission. If successful, this host manipulation is necessarily fatal for the host. I selected the copepod Macrocyclops albidus, a first intermediate host of S. solidus, for resistance or susceptibility to host manipulation to investigate their evolvability. Selection on the host indeed increased host manipulation in susceptible and reduced host manipulation in resistant selection lines. Interestingly, this seemed to be at least partly due to changes in the baseline levels of the modified trait (activity) rather than actual changes in resistance or susceptibility to host manipulation. Hence, hosts seem restricted in how rapidly and efficiently they can evolve resistance to host manipulation.

Experimental evolution of parasitic host manipulation

Host manipulation is a parasite-induced alteration of a host's phenotype that increases parasite fitness. However, if genetically encoded in the parasite, it should be under selection in the parasite. Such host manipulation has often been assumed to be energetically costly, which should restrict its evolution. Evidence of such costs, however, remains elusive. The trophically transmitted cestode Schistocephalus solidus manipulates the activity of its first intermediate copepod host to reduce its predation susceptibility before the parasite is ready for transmission. Thereafter, S. solidus increases host activity to facilitate transmission to its subsequent fish host. I selected S. solidus for or against host manipulation over three generations to investigate the evolvability of manipulation and identify potential trade-offs. Host manipulation responded to selection, confirming that this trait is heritable in the parasite and hence can present an extended phenotype. Changes in host manipulation were not restrained by any obvious costs.

Crowding in the first intermediate host does not affect infection probability in the second host in two helminths

When many worms co-infect the same host, their average size is often reduced. This negative density-dependent growth is called the crowding effect. Crowding has been reported many times for worms in their intermediate hosts, but rarely have the fitness consequences of crowding been examined. This study tested whether larval crowding reduces establishment success in the next host for two parasites with complex life cycles, the nematode Camallanus lacustris and the cestode Schistocephalus solidus. Infected copepods, the first host, were fed to sticklebacks, the second host. Fish received a constant dose, but the infection intensity in copepods was varied (e.g. giving two singly infected copepods or one doubly infected copepod). Worms from higher-intensity infections did not have significantly reduced infection success in fish. However, crowded treatments had a disproportionate number of low and high infection rates, and although this trend was not significant, it hints at the possibility that multiple worms within a copepod are more likely to either all infect or all die when transmitted to the next host. These results indicate that a smaller larval size due to crowding need not reduce the establishment probability of a worm in the next host.

Differences between populations in host manipulation by the tapeworm Schistocephalus solidus - is there local adaptation?

Host manipulation whereby a parasite increases its transmission to a subsequent host by altering the behaviour of its current host is very far spread. It also occurs in host-parasite systems that are widely distributed. This offers the potential for local adaptation. The tapeworm Schistocephalus solidus modifies its first intermediate copepod host's predation susceptibility to suit its own needs by reducing its activity before it becomes infective and increasing it thereafter. To investigate potential differences in host manipulation between different populations and test for potential local adaptation with regard to host manipulation, I experimentally infected hosts from two distinct populations with parasites from either population in a fully crossed design. Host manipulation differed between populations mostly once the parasite had reached infectivity. These differences in infective parasites were mostly due to differences between different parasite populations. In not yet infective parasites, however, host population also had a significant effect on host manipulation. There was no evidence of local adaptation; parasites were able to manipulate foreign and local hosts equally well. Likewise, hosts were equally poor at resisting host manipulation by local and foreign parasites.

Positive density-dependent growth supports costs sharing hypothesis and population density sensing in a manipulative parasite

Parasites manipulate their hosts' phenotype to increase their own fitness. Like any evolutionary adaptation, parasitic manipulations should be costly. Though it is difficult to measure costs of the manipulation directly, they can be evaluated using an indirect approach. For instance, theory suggests that as the parasite infrapopulation grows, the investment of individual parasites in host manipulation decreases, because of cost sharing. Another assumption is that in environments where manipulation does not pay off for the parasite, it can decrease its investment in the manipulation to save resources. We experimentally infected rainbow trout Oncorhynchus mykiss with the immature larvae of the trematode Diplostomum pseudospathaceum, to test these assumptions. Immature D. pseudospathaceum metacercariae are known for their ability to manipulate the behaviour of their host enhancing its anti-predator defenses to avoid concomitant predation. We found that the growth rate of individual parasites in rainbow trout increased with the infrapopulation size (positive density-dependence) suggesting cost sharing. Moreover, parasites adjusted their growth to the intensity of infection within the eye lens where they were localized suggesting population density sensing. Results of this study support the hypothesis that macroparasites can adjust their growth rate and manipulation investment according to cost sharing level and infrapopulation size.

Autonomy and integration in complex parasite life cycles

Complex life cycles are common in free-living and parasitic organisms alike. The adaptive decoupling hypothesis postulates that separate life cycle stages have a degree of developmental and genetic autonomy, allowing them to be independently optimized for dissimilar, competing tasks. That is, complex life cycles evolved to facilitate functional specialization. Here, I review the connections between the different stages in parasite life cycles. I first examine evolutionary connections between life stages, such as the genetic coupling of parasite performance in consecutive hosts, the interspecific correlations between traits expressed in different hosts, and the developmental and functional obstacles to stage loss. Then, I evaluate how environmental factors link life stages through carryover effects, where stressful larval conditions impact parasites even after transmission to a new host. There is evidence for both autonomy and integration across stages, so the relevant question becomes how integrated are parasite life cycles and through what mechanisms? By highlighting how genetics, development, selection and the environment can lead to interdependencies among successive life stages, I wish to promote a holistic approach to studying complex life cycle parasites and emphasize that what happens in one stage is potentially highly relevant for later stages.

Inter- and intraspecific conflicts between parasites over host manipulation

Host manipulation is a common strategy by which parasites alter the behaviour of their host to enhance their own fitness. In nature, hosts are usually infected by multiple parasites. This can result in a conflict over host manipulation. Studies of such a conflict in experimentally infected hosts are rare. The cestode Schistocephalus solidus (S) and the nematode Camallanus lacustris (C) use copepods as their first intermediate host. They need to grow for some time inside this host before they are infective and ready to be trophically transmitted to their subsequent fish host. Accordingly, not yet infective parasites manipulate to suppress predation. Infective ones manipulate to enhance predation. We experimentally infected laboratory-bred copepods in a manner that resulted in copepods harbouring (i) an infective C plus a not yet infective C or S, or (ii) an infective S plus a not yet infective C. An infective C completely sabotaged host manipulation by any not yet infective parasite. An infective S partially reduced host manipulation by a not yet infective C. We hence show experimentally that a parasite can reduce or even sabotage host manipulation exerted by a parasite from a different species.

Can the behaviour of threespine stickleback parasitized with Schistocephalus solidus be replicated by manipulating host physiology?

Sticklebacks infected by the parasitic flatworm Schistocephalus solidus show dramatic changes in phenotype, including a loss of species-typical behavioural responses to predators. The timing of host behaviour change coincides with the development of infectivity of the parasite to the final host (a piscivorous bird), making it an ideal model for studying the mechanisms of infection-induced behavioural modification. However, whether the loss of host anti-predator behaviour results from direct manipulation by the parasite, or is a by-product (e.g. host immune response) or side-effect of infection (e.g. energetic loss), remains controversial. To understand the physiological mechanisms that generate these behavioural changes, we quantified the behavioural profiles of experimentally infected fish and attempted to replicate these in non-parasitized fish by exposing them to treatments including immunity activation and fasting, or by pharmacologically inhibiting the stress axis. All fish were screened for the following behaviours: activity, water depth preference, sociability, phototaxis, anti-predator response and latency to feed. We were able to change individual behaviours with certain treatments. Our results suggest that the impact of S. solidus on the stickleback might be of a multifactorial nature. The behaviour changes observed in infected fish may be due to the combined effects of modifying the serotonergic axis, the lack of energy, and the activation of the immune system.