Infection in patchy populations: Contrasting pathogen invasion success and dispersal at varying times since host colonization

Dynamics of infection and immunity over 50 years as marine stickleback adapt to freshwater

When a species colonizes a new environment, it may encounter new parasites to which its immune system is poorly adapted. After an initial spike in infection rates in the naïve founder population, the host may subsequently evolve increased immunity thereby reducing infection rates. Here, we present an example of this eco-evolutionary process, in a population of threespine stickleback (Gasterosteus aculeatus) that was founded in Heisholt Quarry, a man-made quarry pond, in 1967. Marine stickleback rarely encounter Schistocephalus solidus tapeworms (which require freshwater to hatch), and so remain highly susceptible to infection. Initially, introduced marine fish were heavily infected by S.solidus. They exhibited low levels of fibrosis, a heritable immune trait which some genotypes activate in response to infection, thereby suppressing tapeworm growth and viability. By the 1990's, the Heisholt Quarry population exhibited high rates of fibrosis, which partly suppressed S.solidus infection. This increased immune response led to reduced infection rates and the tapeworm was apparently extirpated by 2021. Because fibrosis has a strong genetic basis in other stickleback populations, we infer that the newly founded stickleback-parasite interaction exhibit an eco-evolutionary process of increased immunity that effectively reduced infection. The infection and immune dynamics documented here closely match those expected from a simple eco-evo dynamic model presented here.

Does a biological invasion modify host immune responses to parasite infection?

Biological invasions can disrupt the close and longstanding coevolved relationships between host and parasites. At the same time, the shifting selective forces acting on demography during invasion can result in rapid evolution of traits in both host and parasite. Hosts at the invasion front may reduce investment into costly immune defences and redistribute those resources to other fitness-enhancing traits. Parasites at the invasion front may have reduced pathogenicity because traits that negatively impact host dispersal are left behind in the expanding range. The host's immune system is its primary arsenal in the coevolutionary 'arms race' with parasites. To assess the effects of invasion history on immune responses to parasite infection, we conducted a cross-infection experiment which paired common-garden reared cane toads and lungworm parasites originating from various sites in their invaded range across northern Australia. Infected toads had larger spleens and higher concentrations of eosinophils than did uninfected toads. Infected toads also exhibited lower bacteria-killing ability, perhaps reflecting a trade-off of resources towards defences that are more specifically anthelminthic. The impact of infection intensity on multiple immune measures differed among toads and parasites from different parts of the invasion trajectory, supporting the hypothesis that invasion has disrupted patterns of local adaptation.

Can pharmaceutical pollution alter the spread of infectious disease? A case study using fluoxetine

Human activity is changing global environments at an unprecedented rate, imposing new ecological and evolutionary ramifications on wildlife dynamics, including host-parasite interactions. Here we investigate how an emerging concern of modern human activity, pharmaceutical pollution, influences the spread of disease in a population, using the water flea Daphnia magna and the bacterial pathogen Pasteuria ramosa as a model system. We found that exposure to different concentrations of fluoxetine-a widely prescribed psychoactive drug and widespread contaminant of aquatic ecosystems-affected the severity of disease experienced by an individual in a non-monotonic manner. The direction and magnitude of any effect, however, varied with both the infection outcome measured and the genotype of the pathogen. By contrast, the characteristics of unexposed animals, and thus the growth and density of susceptible hosts, were robust to fluoxetine. Using our data to parameterize an epidemiological model, we show that fluoxetine is unlikely to lead to a net increase or decrease in the likelihood of an infectious disease outbreak, as measured by a pathogen's transmission rate or basic reproductive number. Instead, any given pathogen genotype may experience a twofold change in likely fitness, but often in opposing directions. Our study demonstrates that changes in pharmaceutical pollution give rise to complex genotype-by-environment interactions in its influence of disease dynamics, with repercussions on pathogen genetic diversity and evolution. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

Life in the margins: host-parasite relationships in ecological edges

Transitional zones, such as edge habitat, are key landscapes for investigating biodiversity. "Soft edges" are permeable corridors that hosts can cross, while "hard edges" are impermeable borders that hosts cannot pass. Although pathogen transmission in the context of edges is vital to species conservation, drivers of host-parasite relationships in ecological edges remain poorly understood. Thus, we defined a framework for testing hypotheses of host-parasite interactions in hard and soft edges by (1) characterizing hard and soft edges from both the host and parasite perspectives, (2) predicting the types of parasites that would be successful in each type of edge, and (3) applying our framework to species invasion fronts as an example of host-parasite relationships in a soft edge. Generally, we posited that parasites in soft edges are more likely to be negatively affected by habitat fragmentation than their hosts because they occupy higher trophic levels but parasite transmission would benefit from increased host connectivity. Parasites along hard edges, however, are at higher risk of local extinction due to host population perturbations with limited opportunity for parasite recolonization. We then used these characteristics to predict functional traits that would lead to parasite success along soft and hard edges. Finally, we applied our framework to invasive species fronts to highlight predictions regarding host connectivity and parasite traits in soft edges. We anticipate that our work will promote a more complete discussion of habitat connectivity using a common framework and stimulate empirical research into host-parasite relationships within ecological edges and transitional zones.

Parasitism and host dispersal plasticity in an aquatic model system

Dispersal is a central determinant of spatial dynamics in communities and ecosystems, and various ecological factors can shape the evolution of constitutive and plastic dispersal behaviours. One important driver of dispersal plasticity is the biotic environment. Parasites, for example, influence the internal condition of infected hosts and define external patch quality. Thus, state-dependent dispersal may be determined by infection status and context-dependent dispersal by the abundance of infected hosts in the population. A prerequisite for such dispersal plasticity to evolve is a genetic basis on which natural selection can act. Using interconnected microcosms, we investigated dispersal in experimental populations of the freshwater protist Paramecium caudatum in response to the bacterial parasite Holospora undulata. For a collection of 20 natural host strains, we found substantial variation in constitutive dispersal and to a lesser degree in dispersal plasticity. First, infection tended to increase or decrease dispersal relative to uninfected controls, depending on strain identity, indicative of state-dependent dispersal plasticity. Infection additionally decreased host swimming speed compared to the uninfected counterparts. Second, for certain strains, there was a weak negative association between dispersal and infection prevalence, such that uninfected hosts dispersed less when infection was more frequent in the population, indicating context-dependent dispersal plasticity. Future experiments may test whether the observed differences in dispersal plasticity are sufficiently strong to be picked up by natural selection. The evolution of dispersal plasticity as a strategy to mitigate parasite effects spatially may have important implications for epidemiological dynamics.

Evolution under pH stress and high population densities leads to increased density-dependent fitness in the protist Tetrahymena thermophila

Abiotic stress is a major force of selection that organisms are constantly facing. While the evolutionary effects of various stressors have been broadly studied, it is only more recently that the relevance of interactions between evolution and underlying ecological conditions, that is, eco-evolutionary feedbacks, have been highlighted. Here, we experimentally investigated how populations adapt to pH-stress under high population densities. Using the protist species Tetrahymena thermophila, we studied how four different genotypes evolved in response to stressfully low pH conditions and high population densities. We found that genotypes underwent evolutionary changes, some shifting up and others shifting down their intrinsic rates of increase (r₀ ). Overall, evolution at low pH led to the convergence of r₀ and intraspecific competitive ability (α) across the four genotypes. Given the strong correlation between r₀ and α, we argue that this convergence was a consequence of selection for increased density-dependent fitness at low pH under the experienced high density conditions. Increased density-dependent fitness was either attained through increase in r₀ , or decrease of α, depending on the genetic background. In conclusion, we show that demography can influence the direction of evolution under abiotic stress.