Schistocephalus infestation improves prey-size selection by three-spined sticklebacks, Gasterosteus aculeatus

Schistocephalus -infected sticklebacks ate slightly less but fed more selectively on larger prey than non-infested fish. It follows that, in the short-term, the net energy gain in foraging is larger for infested fish. This matches the expectation that infested sticklebacks compensate energy losses due to Schistocephalus.     

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

Abstract.— 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, Schis-tocephalus 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 acu-leatus ) 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.     

Effects of Schistocephalus solidus (Cestoda) on the food intake and diet of the three-spined stickleback, Gasterosteus aculeatus

Seasonal differences were detected between uninfected sticklebacks and those infected with Schistocephalus solidus in their stomach fullness and diet composition.     

Costly carotenoids: a trade‐off between predation and infection risk?

Carotenoid reserves in copepods seem costly in terms of predation risk because they make individuals conspicuous. However, carotenoids also seem to play an important role in immune defence as free radical scavengers. To test whether predation risk influences carotenoid levels and whether changes in carotenoid levels are related to changes in immune defence, I examined individual changes in large carotenoid and other lipid droplets upon exposure to predation risk and subsequent exposure to parasites in the copepod Macrocyclops albidus. Copepods reduced carotenoid reserves upon exposure to predators, through which they potentially avoided the costs of being conspicuous under predation risk. Thus, the size of carotenoid reserves is a plastic trait. Such a decrease in carotenoid reserves may also have a negative impact on the copepods' immune system as individuals that decreased their reserves suffered higher parasite prevalence upon exposure to the cestode Schistocephalus solidus. These results suggest that carotenoid reserves may be individually optimized to trade-off each individual's unique costs (predation risk) and benefits (immune defence) of having these reserves.     

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.     

Parasitism, oddity and the mechanism of shoal choice

When choosing between shoals differing in Schistocephalus solidus infection status, uninfected test sticklebacks Gasterosteus aculeatus showed a preference for joining uninfected conspecifics when shoal sizes were equal, but reversed this preference when the relative size of the infected shoal was increased by a factor of 3. When given a choice between a shoal composed of size-matched minnows Phoxinus phoxinus and a shoal composed of the same number of all uninfected or all S. solidus -infected sticklebacks, test fish always preferred the sticklebacks, regardless of their infection status, over the minnow shoal. These observations suggest that species, parasite status and shoal size are all of importance when fish decide which shoal to join.     

Delayed selfing in relation to the availability of a mating partner in the cestode Schistocephalus solidus

Abstract A hermaphroditic individual that prefers to outbreed but that has the potential of selfing faces a dilemma: in the absence of a partner, should it wait for one to arrive or should it produce offspring by selfing? Recent theory on this question suggests that the evolutionary solution is to find an optimal delay of reproduction that balances the potential benefit of outcrossing and the cost of delaying the onset of reproduction. Assuming that resources retained from breeding can be reallocated to future reproduction, isolated individuals, compared with individuals with available mates, are predicted to delay their age at first reproduction to wait for future outcrossing. Here, I present empirical support for this idea with experimental data from the hermaphroditic cestode Schistocephalus solidus. I show that individuals breeding alone delay their reproduction and initially produce their eggs at a slow rate relative to cestodes breeding in pairs. This delay is partly compensated for by a later higher egg production, although singly breeding cestodes still pay a cost of overall lower egg production.     

Variation in stickleback head morphology associated with parasite infection

Parasites can affect host phenotypes, influencing their ecology and evolution. Host morphological changes occurring post-infection might result from pathological by-products of infection, or represent adaptations of hosts or parasites. We investigated the morphology of three-spined sticklebacks, Gasterosteus aculeatus , from a population naturally infected with Schistocephalus solidus , which grows to large sizes in their body cavity. We examined local effects of infection on trunk shape, which are imposed directly by the bulk of the growing parasite, and distant effects on head morphology. We show that trunk shape differed between infection classes, and was affected more severely in fish with heavier total parasite mass. We further show unexpected differences in head morphology. The heads of infected fish were reduced in size and differently shaped to those of non-infected fish, with infected fish having deeper heads. Importantly, both head size and shape were also affected more severely in fish with heavier total parasite mass. This latter result suggests that differences in morphology are caused by post-infection changes. Such changes may be incidental, evolutionarily neutral 'side effects' of infection. However, because head morphology affects foraging ecology, such changes are likely to have fitness consequences for hosts, and may constitute adaptations, either of hosts or of parasites. We discuss our finding in the context of the evolution of phenotypic plasticity, and suggest testable hypotheses examining the proximate mechanisms underlying these morphological effects and their potential evolutionary basis.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 759–768.