Multiple host use and the dynamics of host switching in host–parasite systems

1. The link between multi-host use and host switching in host–parasite interactions is a continuing area of debate. Lycaenid butterflies in the genus Maculinea, for example, exploit societies of different Myrmica ant species across their ranges, but there is only rare evidence that they simultaneously utilise multiple hosts at a local site, even where alternative hosts are present. 2. We present a simple population-genetic model accounting for the proportion of two alternative hosts and the fitness of parasite genotypes on each host. In agreement with standard models, we conclude that simultaneous host use is possible whenever fitness of heterozygotes on alternative hosts is not too low. 3. We specifically focus on host-shifting dynamics when the frequency of hosts changes. We find that (i) host shifting may proceed so rapidly that multiple host use is unlikely to be observed, (ii) back and forth transition in host use can exhibit a hysteresis loop, (iii) the parasites’ host use may not be proportional to local host frequencies and be restricted to the rarer host under some conditions, and (iv) that a substantial decline in parasite abundance may typically precede a shift in host use. 4. We conclude that focusing not just on possible equilibrium conditions but also considering the dynamics of host shifting in non-equilibrium situations may provide added insights into host–parasite systems

Red Queen Coevolution on Fitness Landscapes

Species do not merely evolve, they also coevolve with other organisms. Coevolution is a major force driving interacting species to continuously evolve ex- ploring their fitness landscapes. Coevolution involves the coupling of species fit- ness landscapes, linking species genetic changes with their inter-specific ecological interactions. Here we first introduce the Red Queen hypothesis of evolution com- menting on some theoretical aspects and empirical evidences. As an introduction to the fitness landscape concept, we review key issues on evolution on simple and rugged fitness landscapes. Then we present key modeling examples of coevolution on different fitness landscapes at different scales, from RNA viruses to complex ecosystems and macroevolution.Comment: 40 pages, 12 figures. To appear in "Recent Advances in the Theory and Application of Fitness Landscapes" (H. Richter and A. Engelbrecht, eds.). Springer Series in Emergence, Complexity, and Computation, 201

Low migration decreases interference competition among parasites and increases virulence

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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