Conflict between parasites with different transmission strategies infecting an amphipod host

Competition between parasites within a host can influence the evolution of parasite virulence and host resistance, but few studies examine the effects of unrelated parasites with conflicting transmission strategies infecting the same host. Vertically transmitted (VT) parasites, transmitted from mother to offspring, are in conflict with virulent, horizontally transmitted (HT) parasites, because healthy hosts are necessary to maximize VT parasite fitness. Resolution of the conflict between these parasites should lead to the evolution of one of two strategies: avoidance, or sabotage of HT parasite virulence by the VT parasite. We investigated two co-infecting parasites in the amphipod host, Gammarus roeseli: VT microsporidia have little effect on host fitness, but acanthocephala modify host behaviour, increasing the probability that the amphipod is predated by the acanthocephalan's definitive host. We found evidence for sabotage: the behavioural manipulation induced by the Acanthocephala Polymorphus minutus was weaker in hosts also infected by the microsporidia Dictyocoela sp. (roeselum) compared to hosts infected by P. minutus alone. Such conflicts may explain a significant portion of the variation generally observed in behavioural measures, and since VT parasites are ubiquitous in invertebrates, often passing undetected, conflict via transmission may be of great importance in the study of host-parasite relationships.     

Coexistence of three microsporidia parasites in populations of the freshwater amphipod Gammarus roeseli: evidence for vertical transmission and positive effect on reproduction

We investigated the prevalence, transmission mode and fitness effects of infections by obligatory intracellular, microsporidian parasites in the freshwater amphipod Gammarus roeseli. We found three different microsporidia species in this host, all using transovarial (vertical) transmission. All three coexist at different prevalences in two host populations, but bi-infected individuals were rarely found, suggesting no (or very little) horizontal transmission. It is predicted that vertically-transmitted parasites may exhibit sex-specific virulence in their hosts, or they may have either positive or neutral effects on host fitness. All three species differed in their transmission efficiency and infection intensity and our data suggest that these microsporidia exert sex-specific virulence by feminising male hosts. The patterns of infection we found exhibit convergent evolution with those of another amphipod host, Gammarus duebeni. Interestingly, we found that infected females breed earlier in the reproductive season than uninfected females. This is the first study, to our knowledge, to report a positive effect of microsporidian infection on female host reproduction.     

Symbiont-mediated protection

Despite the fact that all vertically transmitted symbionts sequester resources from their hosts and are therefore costly to maintain, there is an extraordinary diversity of them in invertebrates. Some spread through host populations by providing their hosts with fitness benefits or by manipulating host sex ratio, but some do not: their maintenance in host lineages remains an enigma. In this review, I explore the evolutionary ecology of vertically transmitted symbionts and their impact on host resistance, and provide an overview of the evidence for the three-way interactions between these symbionts, natural enemies and invertebrate hosts. A number of recent empirical and theoretical studies suggest that vertically transmitted symbionts may protect their hosts from pathogens. If this 'symbiont-mediated protection' is widespread, it is likely that vertically transmitted symbionts contribute significantly to variation in measures of invertebrate resistance to natural enemies.     

Liposomal encapsulation of glucocorticoids alters their mode of action in the treatment of experimental autoimmune encephalomyelitis

Glucocorticoids (GCs) are widely used to treat acute relapses of multiple sclerosis (MS). In this study, we demonstrate that liposomal encapsulation augments the therapeutic potency of GCs as they ameliorate experimental autoimmune encephalomyelitis (EAE) to the same extent as free GC, but at strongly reduced dosage and application frequency. Importantly, this is accompanied by an altered mode of action. Unlike free GCs, which mainly target T lymphocytes during EAE therapy, liposomal GCs only marginally affect T cell apoptosis and function. In contrast, liposomal GCs efficiently repress proinflammatory macrophage functions and upregulate anti-inflammatory genes associated with the alternatively activated M2 phenotype. The GC receptor (GR) per se is indispensable for the therapeutic efficacy of liposomal GC. In contrast to free GCs, however, the individual deletion of the GR either in T cells or myeloid cells has little effect on the efficacy of liposomal GCs in the treatment of EAE. Only the combined deletion of the GR in both cellular compartments markedly compromises the therapeutic effect of liposomal GCs on disease progression. In conclusion, encapsulation of GC does not only enhance their efficacy in the treatment of EAE but also alters their target cell specificity and their mode of action compared with free GCs.     

No genetic bottleneck or associated microparasite loss in invasive populations of a freshwater amphipod

Understanding what factors determine the success of an invasive species in its adopted range is crucial from an evolutionary ecology point of view, because it can provide insights into which biological characteristics are required for survival in varied environmental conditions. Successful establishment may depend on both maintaining genetic diversity, which will allow the species to evolve and/or adapt to new environments, and the presence or absence of natural enemies such as parasites. We tested these two hypotheses by studying populations of the amphipod crustacean Dikerogammarus villosus . This Ponto-Caspian invader has rapidly and successfully invaded western Europe and threatens macroinvertebrate biodiversity in its adopted ranges. It is a unique system to study since both its colonisation history and its geographic origins are well-known. Using samples from the whole geographic range of the invasion route, and using four molecular markers, we found no evidence for genetic bottlenecks during the invasion of D. villosus in western Europe, despite slight variations in allelic proportions according to spatio-temporal subdivisions of our dataset. In addition, we analysed the prevalence and diversity of parasites across its native and adopted range. We found no macro-parasites, and no significant parasite loss of microsporidian parasites during the invasive process. Our data suggest that D. villosus invasion was either massive, or recurrent, or both, allowing a parasitic cortege to follow the host. The maintenance of genetic diversity may have contributed to its success, including the variation in resistance in the face of the natural enemies.     

Directional orientation of pomacentrid larvae to ambient reef sound

The mechanisms by which reef fish larvae locate settlement habitat at the end of their pelagic phase are unclear. We used an in situ binary choice chamber and an artificial source of reef sound to determine whether pomacentrid larvae can use ambient sound to locate reefs. Larvae were caught in light traps and then placed in a submerged binary choice chamber with an artificial source of reef sound ~80 m from one end of the chamber. At night, larvae moved towards the sound source; during the day, larvae showed no preference. These results suggest that pomacentrid larvae can detect reef sound and are capable of directional hearing. While other studies have shown that reef fish larvae respond to reef sound, and that the adults of some species can localize underwater sound sources, the localization of underwater sound by fish larvae has not been demonstrated previously.Communicated by Ecological Editor P.F. Sale