Cross-continental experimental infections reveal distinct defence mechanisms in populations of the three-spined stickleback Gasterosteus aculeatus

Epidemiological traits of host-parasite associations depend on the effects of the host, the parasite and their interaction. Parasites evolve mechanisms to infect and exploit their hosts, whereas hosts evolve mechanisms to prevent infection and limit detrimental effects. The reasons why and how these traits differ across populations still remain unclear. Using experimental cross-infection of three-spined stickleback Gasterosteus aculeatus and their species-specific cestode parasites Schistocephalus solidus from Alaskan and European populations, we disentangled host, parasite and interaction effects on epidemiological traits at different geographical scales. We hypothesized that host and parasite main effects would dominate both within and across continents, although interaction effects would show geographical variation of natural selection within and across continents. We found that mechanisms preventing infection (qualitative resistance) occurred only in a combination of hosts and parasites from different continents, while mechanisms limiting parasite burden (quantitative resistance) and reducing detrimental effects of infection (tolerance) were host-population specific. We conclude that evolution favours distinct defence mechanisms on different geographical scales and that it is important to distinguish concepts of qualitative resistance, quantitative resistance and tolerance in studies of macroparasite infections.

Early stages of infection of three-spined stickleback (Gasterosteus aculeatus) with the cestode Schistocephalus solidus

Parasitic helminths have evolved strategies to evade their host's immune systems. Particularly, the early time of interactions between helminths and their hosts might be decisive for their infection success. We used the cestode Schistocephalus solidus, and its highly specific second intermediate host, the three-spined stickleback (Gasterosteus aculeatus) to investigate parasite infection and host cellular immune responses starting 1 day postexposure (dpe). We recovered live parasites from stickleback body cavities already 24 hr after exposure. Infection rates increased up to 50% and did not change from 4 dpe onwards. Thus, not all parasites had reached the body cavity at the early time points and clearance of the parasite at later time points did not occur. Stickleback head kidney leucocytes (HKLs) did not show distinct signs of activation and lymphocyte proliferation, granulocyte-to-lymphocyte ratios and respiratory burst activity of infected sticklebacks did not deviate from controls significantly. The immune system was activated only late, as indicated by an increase in the total count of HKL relative to stickleback weight (HKL per mg fish), which was significantly elevated in infected fish 32 dpe. S. solidus seems to evade leucocyte activity early during infection facilitating its establishment in the hosts' body cavity.

Aquatic Parasite Cultures and Their Applications

In this era of unprecedented growth in aquaculture and trade, aquatic parasite cultures are essential to better understand emerging diseases and their implications for human and animal health. Yet culturing parasites presents multiple challenges, arising from their complex, often multihost life cycles, multiple developmental stages, variable generation times and reproductive modes. Furthermore, the essential environmental requirements of most parasites remain enigmatic. Despite these inherent difficulties, in vivo and in vitro cultures are being developed for a small but growing number of aquatic pathogens. Expanding this resource will facilitate diagnostic capabilities and treatment trials, thus supporting the growth of sustainable aquatic commodities and communities.

An experimental approach to the immuno-modulatory basis of host-parasite local adaptation in tapeworm-infected sticklebacks

The evolutionary arms race of hosts and parasites often results in adaptations, which may differ between populations. Investigation of such local adaptation becomes increasingly important to understand dynamics of host-parasite interactions and co-evolution. To this end we performed an infection experiment involving pairs of three-spined sticklebacks and their tapeworm parasite Schistocephalus solidus from three geographically separated origins (Germany, Spain and Iceland) in a fully-crossed design for sympatric and allopatric host/parasite combinations. We hypothesized that local adaptation of the hosts results in differences in parasite resistance with variation in parasite infection rates and leukocyte activation, whereas parasites from different origins might differ in virulence reflected in host exploitation rates (parasite indices) and S. solidus excretory-secretory products (SsESP) involved in immune manipulation. In our experimental infections, sticklebacks from Iceland were more resistant to S. solidus infection compared to Spanish and German sticklebacks. Higher resistance of Icelandic sticklebacks seemed to depend on adaptive immunity, whereas sticklebacks of German origin, which were more heavily afflicted by S. solidus, showed elevated activity of innate immune traits. German S. solidus were less successful in infecting and exploiting allopatric hosts compared to their Icelandic and Spanish conspecifics. Nevertheless, exclusively SsESP from German S. solidus triggered significant in vitro responses of leukocytes from naïve sticklebacks. Interestingly, parasite indices were almost identical across the sympatric combinations. Differences in host resistance and parasite virulence between the origins were most evident in allopatric combinations and were consistent within origin; i.e. Icelandic sticklebacks were more resistant and their S. solidus were more virulent in all allopatric combinations, whereas German sticklebacks were less resistant and their parasites less virulent. Despite such differences between origins, the degree of host exploitation was almost identical in the sympatric host-parasite combinations, suggesting that the local evolutionary arms race of hosts and parasites resulted in an optimal virulence, maximising parasite fitness while avoiding host overexploitation.

The role of prezygotic isolation mechanisms in the divergence of two parasite species

BACKGROUND

The formation of reproductive barriers in diverging lineages is a prerequisite to complete speciation according to the biological species concept. In parasites with complex life cycles, speciation may be driven by adaptation to different intermediate hosts, yet diverging lineages can still share the same definitive host where reproduction takes place. In these cases, prezygotic isolation mechanisms should evolve very early and be particularly strong, preventing costly unfavourable matings. In this study, we investigated the importance of prezygotic barriers to reproduction in two cestode species that diverged 20-25mya and show an extraordinary degree of specificity to different intermediate hosts. Both species share the same definitive hosts and hybridize in the laboratory. Yet, natural hybrids have so far not been detected.

METHODS

We used a combination of different experiments to investigate the role of prezygotic barriers to reproduction in the speciation of these parasites. First, we investigated whether hybridization is possible under natural conditions by exposing lab-reared herring gulls (Larus argentatus, the definitive hosts) to both parasites of either sympatric or allopatric combinations. In a second experiment, we tested whether the parasites prefer conspecifics over parasites from a different species in dichotomous mate choice trials.

RESULTS

Our results show that the two species hybridize under natural conditions with parasites originating either from sympatric or allopatric populations producing hybrid offspring. Surprisingly, the mate choice experiment indicated that both parasite species prefer mates of the different species to conspecifics.

CONCLUSIONS

Neither fundamental constraints against hybridization in a natural host nor assortative mate choice sufficiently explain the persistent segregation of the two tapeworm species in nature. Hence, postzygotic ecological selection against hybrids is presumably the more important driving force limiting gene flow between the two parasite sister species.

Copulation order, density cues and variance in fertilization success in a cestode

Simultaneous hermaphrodites maximize their fitness by optimizing their investment into male or female functions. Allocation of resources to male function (tissues, traits, and/or behaviours increasing paternity) is predicted to increase as density, and the associated level of sperm competition, increases. We tested whether the simultaneous hermaphroditic cestode Schistocephalus solidus uses cues of potential partner densities in its fish intermediate host to improve its male reproductive success in the final host. We had two worms, one originating from a multiple infection in the fish intermediate host and one from a single infection, sequentially compete to fertilize the eggs of a third worm. The fertilization rates of the two competitors nearly always differed from the 50-50 null expectation, sometimes considerably, implying there was a 'winner' in each experimental competition. However, we did not find a significant effect of density in the fish host (single vs multiple) or mating order on paternity. Additional work will be needed to identify the traits and environmental conditions that explain the high variance in male reproductive success observed in this experiment.

Sex allocation and investment into pre- and post-copulatory traits in simultaneous hermaphrodites: the role of polyandry and local sperm competition

Sex allocation theory predicts the optimal allocation to male and female reproduction in sexual organisms. In animals, most work on sex allocation has focused on species with separate sexes and our understanding of simultaneous hermaphrodites is patchier. Recent theory predicts that sex allocation in simultaneous hermaphrodites should strongly be affected by post-copulatory sexual selection, while the role of pre-copulatory sexual selection is much less clear. Here, we review sex allocation and sexual selection theory for simultaneous hermaphrodites, and identify several strong and potentially unwarranted assumptions. We then present a model that treats allocation to sexually selected traits as components of sex allocation and explore patterns of allocation when some of these assumptions are relaxed. For example, when investment into a male sexually selected trait leads to skews in sperm competition, causing local sperm competition, this is expected to lead to a reduced allocation to sperm production. We conclude that understanding the evolution of sex allocation in simultaneous hermaphrodites requires detailed knowledge of the different sexual selection processes and their relative importance. However, little is currently known quantitatively about sexual selection in simultaneous hermaphrodites, about what the underlying traits are, and about what drives and constrains their evolution. Future work should therefore aim at quantifying sexual selection and identifying the underlying traits along the pre- to post-copulatory axis.

The three-spined stickleback-Schistocephalus solidussystem: an experimental model for investigating host-parasite interactions in fish

Plerocercoids of the pseudophyllidean cestodeSchistocephalus solidusinfect the three-spined sticklebackGasterosteus aculeatus, with important consequences for the biology of host fish. Techniques for culturing the parasitein vitroand generating infective stages that can be used to infect sticklebacks experimentally have been developed, and the system is increasingly used as a laboratory model for investigating aspects of host-parasite interactions. Recent experimental laboratory studies have focused on the immune responses of hosts to infection, the consequences of infection for the growth and reproductive development of host fish and the effects of infection on host behaviour. Here we introduce the host and the parasite, review the major findings of these recent experimental infection studies and identify further aspects of host parasite interactions that might be investigated using the system.

Hermaphroditism: What's not to like?

Hermaphroditism is rare and phylogenically in decline among animal species. The evolutionary basis for this development is not well understood. This paper focusses on self-incompatible simultaneous hermaphroditism in animals. It proposes that such hermaphroditism is not stable in sufficiently heterogeneous populations, suggesting a possible reason for why hermaphroditism is rare among evolved animal species. The argument turns on the Bateman principle, namely that male reproductive success (RS) is limited by partner availability, while female RS is not. We show that: low-quality individuals do better if female; secondary sexual differentiation may be important for understanding the existence of males; and that hermaphroditic mating is reciprocal. Reciprocity may be key to understanding promiscuity and attendant phenomena such as cryptic female choice, sperm competition and love darts-common features of hermaphroditic mating. We also argue that hermaphrodites are especially vulnerable to male violence, suggesting a reason for the rarity of trioecy. Finally, we propose that external fertilization, and the scope for streaking, may be one reason fish are the only simultaneously hermaphroditic vertebrates.

INCESTUOUS MATE PREFERENCE BY A SIMULTANEOUS HERMAPHRODITE WITH STRONG INBREEDING DEPRESSION

Inbreeding depression and its consequences for mate choice have been extensively studied in free-living animals. However, very little is known about its significance for parasites, although it is well recognized that the mating systems of parasites can have important implications for their epidemiology and evolution. In this article, we show that the cestode Schistocephalus solidus shows incestuous mate preference despite evidence for very strong inbreeding depression. When given the simultaneous choice between mating with a sibling and an unrelated partner, on average, the cestode preferred its sibling. To explain this surprising result, we present three hypotheses that suggest different benefits to fitness of incestuous mating, which could, alone or in concert, outweigh the cost of inbreeding depression.

Sex, parasites and resistance--an evolutionary approach

Immune systems are among the most diverse biological systems. An evolutionary arms race between hosts and rapidly evolving pathogens is supposed to be a reason for this diversity, and might explain why most eukaryotic hosts and parasites reproduce sexually. In this review, I will focus on possible benefits of sexual reproduction in hosts and parasites, using a model system consisting of a tapeworm and its two intermediate hosts, copepods and sticklebacks. We found that the hermaphroditic tapeworms can increase their infection success by reproducing sexually with a partner (outcrossing), instead of reproducing alone. The defence system of the copepods provides highly specific discrimination of antigenic characteristics of the tapeworms. This supports the finding that tapeworms benefit from outcrossing, but contradicts the conventional notion that the immune system of invertebrates, in contrast to vertebrates, is not able to react with specificity. Finally, sticklebacks seem to benefit from optimal diversity in their specific immune system. Previous studies showed that female sticklebacks prefer mates, which sire offspring with an optimal diversity in the MHC (genes involved in antigen presentation). We now found that these individuals suffer less from tapeworm infection. Furthermore, they are able to reduce the expression of an unspecific immune trait, thereby possibly avoiding harmful side effects of a highly activated, unspecific immune system.

Parasitism and growth in the earthworm Lumbricus terrestris: fitness costs of the gregarine parasite Monocystis sp

Parasites inflict fitness costs on their hosts, but often the exact reduction in fitness is not well understood. We investigated the influence of infection by the gregarine genus Monocystis sp. on growth and female investment (cocoon production) of its earthworm host, Lumbricus terrestris. Earthworms (n = 81) were observed in a laboratory setting for 8 months, after which parasite load was determined. The results revealed a significant negative relationship between parasite load and growth, yet no association to cocoon production was found. Although the exact nature, strength, and evolutionary consequence of reduced growth are still unclear, the results are the first indication for a clear, albeit weak effect of Monocystis on host fitness.

Simultaneous hermaphrodites reproducing in pairs self-fertilize some of their eggs: an experimental test of predictions of mixed-mating and Hermaphrodite's Dilemma theory

Theory predicts (1) that mixed-mating systems (i.e. reproduction through both selfing and outcrossing) should usually not evolve and (2) that reproducing simultaneous hermaphrodites should be in a conflict over the preferred sexual role (The Hermaphrodite's Dilemma). In an in vitro system with the endoparasitic cestode Schistocephalus solidus, a simultaneous hermaphrodite, we tested predictions of both the mixed-mating and the Hermaphrodite's Dilemma theory. Using microsatellite markers, we measured the proportion of selfed offspring and the total reproductive output of each worm within pairs varying in mean weight and weight difference. Worms produced more outbred offspring not only with increasing total weight of the pair, but also with decreasing weight difference between the two paired worms. These results suggest: (1) that this parasite species reproduces by mixed-mating, which may be maintained by stochastic density fluctuations in the definitive host and hence unpredictability of self reproduction and (2) reproductive conflict may prevent worm pairs from achieving an optimal intermediate selfing rate.

Outcrossing increases infection success and competitive ability: experimental evidence from a hermaphrodite parasite

The maintenance of two genetically distinct reproductive modes such as outcrossing and selfing within a population of animals or plants is still a matter of considerable debate. Hermaphroditic parasites often reproduce either alone by selfing or in pairs by outcrossing. They can be used as a model to study potential benefits of outcrossing. Any advantage from outcrossing may be important, especially in host-parasite coevolution, but has not, to our knowledge, been studied yet in any parasite species. We studied the potential effect of outcrossing in a tapeworm, Schistocephalus solidus, on both infection success and growth in its first intermediate host, the copepod Macrocyclops albidus. Tapeworms that had been obtained from natural populations of three-spined sticklebacks (Gasterosteus aculeatus) were allowed to reproduce either alone or in pairs, in an in vitro system that replaced the final host's gut. This resulted in either selfed or outcrossed offspring, respectively. In one part of the experiment, copepods were exposed to either selfed or outcrossed parasites, in a second part to both types simultaneously, in order to study the effect of competition between them. To discriminate parasites of either origin within the same host, a novel method for fluorescent vital labeling was used. We show here for the first time that outcrossed parasites had a higher infection success and faster development in the host. This advantage of outcrossing became apparent only in the competitive situation, in which superior abilities of parasites to extract limiting resources from the host become crucial.