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1. An experimental test of parasite adaptation to common versus rare host genotypes

   https://doi.org/10.1098/rsbl.2020.0210  

   Gibson, Amanda K. ; White, P. Signe ; Penley, McKenna J. ; de Roode, Jacobus C. ; Morran, Levi T. ( July 2020 , Biology Letters)

   null (Ed.)

   A core hypothesis in coevolutionary theory proposes that parasites adapt to specifically infect common host genotypes. Under this hypothesis, parasites function as agents of negative frequency-dependent selection, favouring rare host genotypes. This parasite-mediated advantage of rarity is key to the idea that parasites maintain genetic variation and select for outcrossing in host populations. Here, we report the results of an experimental test of parasite adaptation to common versus rare host genotypes. We selected the bacterial parasite Serratia marcescens to kill Caenorhabdiis elegans hosts in uneven mixtures of host genotypes. To examine the effect of commonness itself, independent of host identity, each of four host genotypes was represented as common or rare in experimental host mixtures. After experimental selection, we evaluated a parasite line's change in virulence—the selected fitness trait—on its rare and common host genotypes. Our results were consistent with a slight advantage for rare host genotypes: on average, parasites lost virulence against rare genotypes but not against common genotypes. The response varied substantially, however, with distinct patterns across host genotype mixtures. These findings support the potential for parasites to impose negative frequency-dependent selection, while emphasizing that the cost of being common may vary with host genotype. 

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2. The evolution of parasite host range in heterogeneous host populations

   https://doi.org/10.1111/jeb.13608  

   Gibson, Amanda K. ; Baffoe‐Bonnie, Helena ; Penley, McKenna J. ; Lin, Julie ; Owens, Raythe ; Khalid, Arooj ; Morran, Levi T. ( March 2020 , Journal of Evolutionary Biology)

   Theory on the evolution of niche width argues that resource heterogeneity selects for niche breadth. For parasites, this theory predicts that parasite populations will evolve, or maintain, broader host ranges when selected in genetically diverse host populations relative to homogeneous host populations. To test this prediction, we selected the bacterial parasiteSerratia marcescensto killCaenorhabditis elegansin populations that were genetically heterogeneous (50% mix of two experimental genotypes) or homogeneous (100% of either genotype). After 20 rounds of selection, we compared the host range of selected parasites by measuring parasite fitness (i.e. virulence, the selected fitness trait) on the two focal host genotypes and on a novel host genotype. As predicted, heterogeneous host populations selected for parasites with a broader host range: these parasite populations gained or maintained virulence on all host genotypes. This result contrasted with selection in homogeneous populations of one host genotype. Here, host range contracted, with parasite populations gaining virulence on the focal host genotype and losing virulence on the novel host genotype. This pattern was not, however, repeated with selection in homogeneous populations of the second host genotype: these parasite populations did not gain virulence on the focal host genotype, nor did they lose virulence on the novel host genotype. Our results indicate that host heterogeneity can maintain broader host ranges in parasite populations. Individual host genotypes, however, vary in the degree to which they select for specialization in parasite populations.

    

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3. Multiple transmission routes sustain high prevalence of a virulent parasite in a butterfly host

   https://doi.org/10.1098/rspb.2019.1630  

   Majewska, Ania A. ; Sims, Stuart ; Schneider, Anna ; Altizer, Sonia ; Hall, Richard J. ( September 2019 , Proceedings of the Royal Society B: Biological Sciences)

   Understanding factors that allow highly virulent parasites to reach high infection prevalence in host populations is important for managing infection risks to human and wildlife health. Multiple transmission routes have been proposed as one mechanism by which virulent pathogens can achieve high prevalence, underscoring the need to investigate this hypothesis through an integrated modelling-empirical framework. Here, we examine a harmful specialist protozoan infecting monarch butterflies that commonly reaches high prevalence (50–100%) in resident populations. We integrate field and modelling work to show that a combination of three empirically-supported transmission routes (vertical, adult transfer and environmental transmission) can produce and sustain high infection prevalence in this system. Although horizontal transmission is necessary for parasite invasion, most new infections post-establishment arise from vertical transmission. Our study predicts that multiple transmission routes, coupled with high parasite virulence, can reduce resident host abundance by up to 50%, suggesting that the protozoan could contribute to declines of North American monarchs. 

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4. Antagonistic Coevolution Limits the Range of Host Defense in C. elegans Populations

   https://doi.org/10.3389/fcimb.2022.758745  

   Lewis, Jordan A. ; Penley, McKenna J. ; Sylla, Hannan ; Ahumada, Sebastián Durán ; Morran, Levi T. ( February 2022 , Frontiers in Cellular and Infection Microbiology)

   Host populations often evolve defenses against parasites due to the significant fitness costs imposed by infection. However, adaptation to a specific parasite may alter the effectiveness of the host’s defenses in general. Consequently, the specificity of host defense may be influenced by a host population’s evolutionary history with parasites. Further, the degree of reciprocal change within an interaction may profoundly alter the range of host defense, given that antagonistic coevolutionary interactions are predicted to favor defense against specific parasite genotypes. Here, we examined the effect of host evolutionary history on host defense range by assessing the mortality rates of Caenorhabditis elegans host populations exposed to an array of Serratia marcescens bacterial parasite strains. Importantly, each of the host populations were derived from the same genetic background but have different experimental evolution histories with parasites. Each of these histories (exposure to either heat-killed, fixed genotype, or coevolving parasites) carries a different level of evolutionary reciprocity. Overall, we observed an effect of host evolutionary history in that previously coevolved host populations were generally the most susceptible to novel parasite strains. This data demonstrates that host evolutionary history can have a significant impact on host defense, and that host-parasite coevolution can increase host susceptibility to novel parasites. 

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5. Shoaling guppies evade predation but have deadlier parasites

   https://doi.org/10.1038/s41559-022-01772-5  

   Walsman, Jason C. ; Janecka, Mary J. ; Clark, David R. ; Kramp, Rachael D. ; Rovenolt, Faith ; Patrick, Regina ; Mohammed, Ryan S. ; Konczal, Mateusz ; Cressler, Clayton E. ; Stephenson, Jessica F. ( July 2022 , Nature Ecology & Evolution)

   Parasites exploit hosts to replicate and transmit, but overexploitation kills both host and parasite. Predators may shift this cost–benefit balance by consuming infected hosts or changing host behaviour, but the strength of these effects remains unclear. Here we use field and lab data on Trinidadian guppies and their Gyrodactylus spp. parasites to show how differential predation pressure influences parasite virulence and transmission. We use an experimentally demonstrated virulence–transmission trade-off to parametrize a mathematical model in which host shoaling (as a means of anti-predator defence), increases contact rates and selects for higher virulence. Then we validate model predictions by collecting parasites from wild, Trinidadian populations; parasites from high-predation populations were more virulent in common gardens than those from low-predation populations. Broadly, our results indicate that reduced social contact selects against parasite virulence. 

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