At this point, increasing population size has a strong influence in colonization success see the curve slope in Fig 6. As the population increases, the gain in colonization success becomes less and less dependent on population size but still dependent on IS. Thus, success in host switching is not driven solely by propagule pressure i.
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This makes intuitive sense since, as discussed above, successful responses to ecological novelties require both opportunity and compatibility. Ecological fitting via resource tracking can be accomplished by any given parasite population, whenever the new and the ancestral hosts represent similar resources and selective pressures ecophysiologically equivalent hosts, see [ 27 — 29 ]. On the other hand, ecological fitting via sloppy fitness space is directly related to the IS of the parasite population, so that host-switching success increases with increasing IS.
Ahlroth et al. They suggested that propagule pressure is associated with the amount of genetic variation in the introduced population, improving the chances that the invasive population will be able to adapt successfully to novel selection pressures in the recipient location. Fourth, the relationship between host-switching success and amplitude of IS varies with the functional i. Although host-switching success generally increases with increasing IS, when IS is small host-switching success between distant resources decreases with IS Fig 5a due to the interaction between increasing variability and selection.
Our model recognizes that the colonizing founder population associated with a new host not only may originate, but may also persist far from the resource optimum. The farther from the resource optimum, the stronger the selective pressure for the colonizing population to evolve toward the optimum point. The changes in population phenotypic distribution occurring during this phase refer to co-accommodation [ 15 ], which includes possible co-adaptive responses.
Our model indicates that co-accommodation may occur more rapidly than the increase in variation if the parasite population is far from the fitness optimum of the new host Fig 4. This means that immediate subsequent generations of the colonizing population might have reduced ability to switch to distant resources Figs 4b and 5a.
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Such a loss might be temporary; as the colonizing population evolves towards the optimum, the rate of co-accommodation should slow while variation continues to increase, as non-optimal phenotypes accumulate. This implies that opportunity and compatibility are more important than conflict resolution in establishing new host-parasite associations. Fifth, the model shows how populations can explore the host fitness space, increasing the ability to switch to hosts that are not similar to the original host.
This can be accomplished by small populations with sub-optimal fit and by large populations with large IS. Entire parasite populations with sub-optimal fit to the host were able to persist for a large number of generations Fig 2. While the survival of individual parasites with sub-optimal fit is assumed in the model, the survival of an entire population comprising such individuals for several generations is an outcome of the dynamics, previously predicted by Agosta and Klemens [ 22 ] and Agosta et al. Small populations with low fitness may be able to switch between distant resources precisely because they are not optimally adapted to the ancestral host.
Survival of a population at a sub-optimal fit in relation to the available resource has been empirically demonstrated recently. Li et al. The giant panda "did not evolve any enzymes for bamboo digestion, and it still retains all necessary enzyme homologs for a carnivorous digestive system" [ 35 ]. Furthermore, Xue et al. Sixth, our model shows that parasites may colonize hosts by a "stepping-stone" process [ 37 ], accessing hosts that are quite divergent from each other in fitness space by host switching events involving hosts that are in relative proximity see Fig 2.
Such processes could result from host switches among sympatric species, sequential geographic overlap among hosts colonized during the biotic expansion phase of a taxon pulse, or from changes in trophic structure in conjunction with, or independent of, geographic expansion. Stepping-stone dynamics have been implicated for emerging disease dynamics, on historical and contemporaneous time scales.
For example, Gorman et al. Subsequently, swine acquired the virus, which differentiated into a swine form. Human Influenza A is apparently a derivative of the swine form. Swine remain suitable hosts for all three forms of the virus. Swine infected with avian and human, or swine and human Influenza A provide the possibility of genetic exchange producing strains highly pathogenic to humans. Braga et al. Our model attempts to understand host switching at the lowest level of complexity possible. Host colonization that does not require correlated genetic change provides formal recognition of ecological fitting and its significance relative to issues of host range dynamics and host switching by ecological specialists [ 5 , 6 , 20 ].
Such a minimalist model, however, does not consider complications such as implicit host variability, other components of FS as proposed by Agosta and Klemens [ 22 ] , continuous dispersal and genetic exchange of parasite populations on distinct host species and host mortality, among others.
Christe Group - Evolution of Host-parasite Interactions
Other components of the Stockholm Paradigm the Oscillation Hypothesis, the Geographic Mosaic Theory of Coevolution and the Taxon Pulse hypothesis remain to be evaluated by future studies. According to the model, large populations can explore host fitness space extensively by accumulating non-optimally adapted phenotypes, which increase the capability of the population to use a larger variety of hosts Fig 5.
A fundamental Darwinian view is that variants well-suited to existing local conditions dominate numerically, but variants with low non-zero fitness are more likely to survive changes in those conditions [ 5 , 41 ]. Extinction of an optimal host thus will not necessarily result in extinction of a parasite lineage, even if the parasite is highly specialized on that host true specialist of Brooks and McLennan [ 5 ] or has had no opportunity to interact with other hosts prior to the episode of environmental perturbation faux specialist of Brooks and McLennan [ 5 ].
The discovery of parasite lineages that are much older than current host lineages and in which persistence and diversification have resulted from episodes of host colonization indicates this is a non-trivial aspect of parasite evolution [ 2 ]. Documented cases of helminths inhabiting vertebrates in terrestrial and marine systems comprise episodes of retro-colonization, lineage persistence, and novel diversification initiated by host switching across ecologically equivalent groups, before or during episodes of global-scale extinction and ecological perturbation [ 42 — 47 ].
These episodes of host colonization have been the drivers for persistence of major parasite lineages on evolutionary and ecological time scales [ 48 — 50 ]. This wealth of empirical findings, in conjunction with the simulation results presented herein, suggest novel insights into the complex phenomena associated with emerging infectious diseases. For example, we must be careful about assuming that newly-documented strains of emerging pathogens are the result of new mutations associated with, or caused by, host switches.
Our model suggests that such changes are unnecessary for host switches to occur, and that host switches into sub-optimal hosts may involve rare but pre-existing phenotypes, difficult to sample in native populations of the parasite. The working assumption that any parasite, even a virus, is genetically monomorphic until it mutates into a form allowing it to switch to a new host may seriously under-estimate the amount of naturally occurring variation [ 51 , 52 ].
The Stockholm Paradigm suggests that host switching and host range expansion are more often the consequence of taking advantage of opportunities offered by a dynamic host landscape than of the evolution of novel host-use capabilities. Therefore, host switches are likely more relevant than previously considered to the evolutionary history of parasite lineages.
Given opportunity, ecological fitting allows even highly specialized parasites to explore the available resources, increasing host-ranges. Phenotypic variation accumulated during periods of environmental stability enhances the ability to host-switch by ecological fitting during periods of environmental change [ 2 ].
Taken together, our simulations support empirical findings and theoretical frameworks suggesting that emerging diseases are "evolutionary accidents waiting to happen" [ 53 ] that can be anticipated, rather than rare events occurring randomly [ 3 , 23 , 54 — 56 ]. That a parasite population may, as suggested by our simulations, survive for many generations in sub-optimal hosts, is especially significant in planning strategies to avoid introduction of new parasitic diseases into new geographical areas. Many "reservoir" species may not be promptly recognized due to low density of the parasite population associated with its sub-optimal fit in that host.
The expansion of human population and its correlates—agriculture and urbanization—combined with global climate change make this a daunting task [ 56 ]. Thus, for pathogens such as Ebola, one of the most critical elements is finding the natural reservoirs and assessing the range of potential non-human reservoirs both optimal and sub-optimal host species.
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Any time a human infection is reported in a new geographic location thus, evidencing opportunity , searches for infections in non-human reservoirs must occur simultaneously with public health efforts to treat the infected human host and prevent human-to-human transmission. Once established in non-human reservoirs, the pathogen may persist as a never-ending source of evolutionary experiments in host switches.
And the more geographic ranges expand and trophic structures are altered—i. Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field. Abstract Despite the fact that parasites are highly specialized with respect to their hosts, empirical evidence demonstrates that host switching rather than co-speciation is the dominant factor influencing the diversification of host-parasite associations. The work is made available under the Creative Commons CC0 public domain dedication Data Availability: All relevant data are within the paper.
Introduction Co-speciation has been considered the most important process governing the distribution of symbionts, especially host-parasite systems. Materials and Methods The model was initially based on the biology and life cycle of the Gyrodactylidae Platyhelminthes, Monogenoidea. Download: PPT. Fig 2. Temporal evolution of the information space of the parasite population. Reproduction At each time step of the cycle, the population can reproduce with a birth rate of b offspring per individual.
Success in host-switching A successful host-switch is a colonization event that does not lead to population extinction; conversely, if all the parasites in the new resource die regardless of the generation in which it occurs , it is a failed host-switch attempt. Table 1. Short description of model parameters with the values imposed in all figures and variables. Fig 3. Relation between the size of IS and the number of generations in a specific host just before a successful colonization. Fig 4. Midpoint of IS a and its maximum amplitude b in relation to the utilized resource as function of IS.
Fig 6. Effect of information space amplitude and population size on migration success probability. Discussion Our model is based on two properties critical to the evolutionary process: acquisition of evolutionary novelties e. References 1. Roquist F.
Parsimony analysis of coevolving species associations. In: Page RDM, editor. Tangled Trees: Phylogeny, Cospeciation, and Coevolution.
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Understanding Host-Switching by Ecological Fitting
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Bats and ectoparasites
Evolution N Y. View Article Google Scholar Emerging pathogens: the epidemiology and evolution of species jumps. Trends Ecol Evol. This information gives us some clues about how to prevent and alleviate current and future disease threats to biodiversity. His research interests span the fields of evolutionary biology, biogeography, behavioural ecology and conservation biology. He has always been captivated by bird migration, an interest which led him to look at how diversity of migration patterns affects the interactions between birds and their symbionts commensals, mutualists or parasites.