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1. Parasitism and host behavior in the context of a changing environment: The Holocene record of the commercially important bivalve Chamelea gallina, northern Italy

   https://doi.org/10.1371/journal.pone.0247790  

   Huntley, John Warren; Scarponi, Daniele (April 2021, PLOS ONE)

   Vermeij, Geerat J. (Ed.)

   Rapid warming and sea-level rise are predicted to be major driving forces in shaping coastal ecosystems and their services in the next century. Though forecasts of the multiple and complex effects of temperature and sea-level rise on ecological interactions suggest negative impacts on parasite diversity, the effect of long term climate change on parasite dynamics is complex and unresolved. Digenean trematodes are complex life cycle parasites that can induce characteristic traces on their bivalve hosts and hold potential to infer parasite host-dynamics through time and space. Previous work has demonstrated a consistent association between sea level rise and increasing prevalence of trematode traces, but a number of fundamental questions remain unanswered about this paleoecological proxy. Here we examine the relationships of host size, shape, and functional morphology with parasite prevalence and abundance, how parasites are distributed across hosts, and how all of these relationships vary through time, using the bivalve Chamelea gallina from a Holocene shallow marine succession in the Po coastal plain. Trematode prevalence increased and decreased in association with the transition from a wave-influenced estuarine system to a wave-dominated deltaic setting. Prevalence and abundance of trematode pits are associated with large host body size, reflecting ontogenetic accumulation of parasites, but temporal trends in median host size do not explain prevalence trends. Ongoing work will test the roles of temperature, salinity, and nutrient availability on trematode parasitism. Parasitized bivalves in one sample were shallower burrowers than their non-parasitized counterparts, suggesting that hosts of trematodes can be more susceptible to their predators, though the effect is ephemeral. Like in living parasite-host systems, trematode-induced malformations are strongly aggregated among hosts, wherein most host individuals harbor very few parasites while a few hosts have many. We interpret trace aggregation to support the assumption that traces are a reliable proxy for trematode parasitism in the fossil record. 

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2. Tracking the assembly of nested parasite communities: using β-diversity to understand variation in parasite richness and composition over time and scale

   Moss*, W. E. (January 2020, Journal of animal ecology)

   Community composition is driven by a few key assembly processes: ecological selection, drift and dispersal. Nested parasite communities represent a powerful study system for understanding the relative importance of these processes and their relationship with biological scale. Quantifying β‐diversity across scales and over time additionally offers mechanistic insights into the ecological processes shaping the distributions of parasites and therefore infectious disease. To examine factors driving parasite community composition, we quantified the parasite communities of 959 amphibian hosts representing two species (the Pacific chorus frog, Pseudacris regilla and the California newt, Taricha torosa) sampled over 3 months from 10 ponds in California. Using additive partitioning, we estimated how much of regional parasite richness (γ‐diversity) was composed of within‐host parasite richness (α‐diversity) and turnover (β‐diversity) at three biological scales: across host individuals, across species and across habitat patches (ponds). We also examined how β‐diversity varied across time at each biological scale. Differences among ponds comprised the majority (40%) of regional parasite diversity, followed by differences among host species (23%) and among host individuals (12%). Host species supported parasite communities that were less similar than expected by null models, consistent with ecological selection, although these differences lessened through time, likely due to high dispersal rates of infectious stages. Host individuals within the same population supported more similar parasite communities than expected, suggesting that host heterogeneity did not strongly impact parasite community composition and that dispersal was high at the individual host-level. Despite the small population sizes of within‐host parasite communities, drift appeared to play a minimal role in structuring community composition. Dispersal and ecological selection appear to jointly drive parasite community assembly, particularly at larger biological scales. The dispersal ability of aquatic parasites with complex life cycles differs strongly across scales, meaning that parasite communities may predictably converge at small scales where dispersal is high, but may be more stochastic and unpredictable at larger scales. Insights into assembly mechanisms within multi‐host, multi‐parasite systems provide opportunities for understanding how to mitigate the spread of infectious diseases within human and wildlife hosts. 

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3. How does host social behavior drive parasite non-selective evolution from the within-host to the landscape-scale?

   https://doi.org/10.1007/s00265-021-03089-y  

   Janecka, Mary J.; Rovenolt, Faith; Stephenson, Jessica F. (November 2021, Behavioral Ecology and Sociobiology)

   Social interactions with conspecifics are key to the fitness of most animals and, through the transmission opportunities they provide, are also key to the fitness of their parasites. As a result, research to date has largely focused on the role of host social behavior in imposing selection on parasites, particularly their virulence and transmission phenotypes. However, host social behavior also influences the distribution of parasites among hosts, with implications for their evolution through non-random mating, gene flow, and genetic drift, and thus ability to respond to that selection. Here, we review the paucity of empirical studies on parasites, and draw from empirical studies of free-living organisms and population genetic theory to propose several mechanisms by which host social behavior potentially drives parasite evolution through these less-well studied mechanisms. We focus on the guppy host and Gyrodactylus (Monogenea) ectoparasitic flatworm system and follow a spatially hierarchical outline to highlight that social behavior varies between individuals, and between host populations across the landscape, generating a mosaic of ecological and evolutionary outcomes for their infecting parasites. We argue that the guppy-Gyrodactylus system presents a unique opportunity to address this fundamental knowledge gap in our understanding of the connection between host social behavior and parasite evolution. Individual differences in host social behavior generates fine-scale changes in the spatial distribution of parasite genotypes, shape the size, and diversity of their infecting parasite populations and may generate non-random mating on, and non-random transmission between hosts. While at population and metapopulation level, variation in host social behavior interacts with landscape structure to affect parasite gene flow, effective population size, and genetic drift to alter the coevolutionary potential of local adaptation. 

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4. Evaluating host diet effects on microparasites by measuring the stoichiometry of infrapopulations one cell at a time

   https://doi.org/10.1002/ece3.11645  

   Narr, Charlotte_F; Binger, Scott; Sedlacek, Erin; Anderson, Bianca; Shoemaker, Grace; Stanley, Adrienne; Stokoski, Madison; Hall, Ed (July 2024, Ecology and Evolution)

   Abstract Progress in the field of ecological stoichiometry has demonstrated that the outcome of ecological interactions can often be predicted a priori based on the nutrient ratios (e.g., carbon: nitrogen: phosphorus, C:N:P) of interacting organisms. However, the challenges of accurately measuring the nutrient content of active parasites within hosts has limited our ability to rigorously apply ecological stoichiometry to host–parasite systems. Traditional nutrient analyses require high parasite biomasses, often preventing individual‐level analyses. This prevents researchers from estimating variation in the nutrient content of individual parasites within a single host infrapopulation, a critical factor that could define how the ecology of the parasite affects the host–parasite interaction. Here, we explain how energy dispersive technology, a technique currently used to measure the elemental content of free‐living microbes, can be adapted for parasitic microbial infrapopulations. We demonstrate the power of accurately quantifying the biomass stoichiometry of individual microbial parasites sampled directly from individual hosts. Using this approach, we show that the stoichiometric composition of two microbial parasites capable of infecting the same host are stoichiometrically distinct and respond to host diet quality differently. We also demonstrate that characteristics of the stoichiometric trait distributions of these infrapopulations were important predictors of host fecundity, a proxy for virulence in this system, and better predictors of parasite load than the mean parasite stoichiometry or our parasite and diet treatments alone. EDS provides a rigorous tool for applying ecological stoichiometry to host–parasite systems and enables researchers to explore the nutritional physiology of host–parasite interactions at a scale that is more relevant to the ecology and evolution of the system than traditional nutrient analyses. Here we demonstrate that this level of resolution provides useful insights into the diet‐dependent physiology of microbial parasites and their hosts. We anticipate that this improved level of resolution has the potential to elucidate a range of eco–evo interactions in host–parasite systems that were previously unobservable. 

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5. Symbioses between fungi and bacteria: from mechanisms to impacts on biodiversity

   https://doi.org/10.1016/j.mib.2024.102496  

   Pawlowska, Teresa E (August 2024, Current Opinion in Microbiology)

   Symbiotic interactions between fungi and bacteria range from positive to negative. They are ubiquitous in free-living as well as host-associated microbial communities worldwide. Yet, the impact of fungal-bacterial symbioses on the organization and dynamics of microbial communities is uncertain. There are two reasons for this uncertainty: (1) knowledge gaps in the understanding of the genetic mechanisms underpinning fungal-bacterial symbioses and (2) prevailing interpretations of ecological theory that favor antagonistic interactions as drivers stabilizing biological communities despite the existence of models emphasizing contributions of positive interactions. This review synthesizes information on fungal-bacterial symbioses common in the free-living microbial communities of the soil as well as in host-associated polymicrobial biofilms. The interdomain partnerships are considered in the context of the relevant community ecology models, which are discussed critically. 

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