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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. Mistletoe distributions and abundance in the forest canopy are vulnerable to climate change.

   Trang, Kenneth; Wilson, Carol A. (July 2022, Botany 2022)

   Mistletoes are branch parasites of trees and shrubs, and keystone species found world-wide that have diverse biotic interactions with seed dispersers, pollinators, and animals that use mistletoes for shelter. They also restructure ecological communities, increasing productivity and biotic diversity. Given their important roles within their communities, a better understanding of their correlations with contemporary and predicted future climates will facilitate our understanding of the challenging aerial landscapes they inhabit. Currently mistletoe occurrence is largely attributed to host preference and availability, even though most mistletoes have many host trees and generally host tree ranges are greater than those of their mistletoes. Using Australian occurrence records and climate data with statistical analyses and modeling, we investigated correlations between climatic variables and mistletoe species distribution, richness, and predicted contemporary and future habitat suitability. Distributions of Australian mistletoe revealed differences among haustorial type and in some cases also genera and showed that ancestral haustorial types were associated with mesic ancestral habitats while derived types were generally associated with drier habitats that are considered derived within Australia. We found significant correlations with a suite of climatic variables, especially but not exclusively precipitation and temperature variables. We conclude that mistletoe distributional patterns, especially when haustorium type is considered, are correlated with climate, similar to other angiosperms. Mistletoes are vulnerable to the warmer, drier climates predicted for Australia and are expected to lose suitable habitat, primarily in interior arid regions of Australia. Ranges of species currently in northeastern tropical and subtropical regions will contract further north while those in semi-arid and arid regions are predicted to mostly shift south and southwest into temperate, montane, and Mediterranean habitat types. 

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3. Continental‐scale interactions of Australian showy mistletoes and their hosts

   https://doi.org/10.1002/ajb2.16443  

   Santiago‐Rosario, Luis Y; Book, Jordan; Mathews, Sarah (December 2024, American Journal of Botany)

   Abstract PremiseShowy mistletoes are obligate hemiparasites of woody plants. Host specificity is therefore a fundamental determinant of mistletoe diversity, persistence, geographic distribution, and abundance. Investigations of host specificity in Australian Loranthaceae have focused mostly on host range (taxon counts), but additional insights into specificity are gained by quantifying mistletoe prevalence on taxa in their host range and by exploring specificity in a phylogenetic context. MethodsWe estimated measures of host specificity to characterize mistletoe–host interactions at a continental scale by using occurrence records in the Atlas of Living Australia. We calculated host taxon richness, mistletoe prevalence, and phylogenetic diversity, and used rarefaction curves to evaluate sampling coverage. ResultsMany mistletoe taxa were represented by few records that listed the host, which often was identified to genus only. Mistletoe genera were recorded on 29 orders and 80 families, and no association was observed between host richness and number of records per genus. Rarefaction curves suggested that additional host orders and families remain to be discovered forAmylotheca,Decaisnina,Dendrophthoe, andMuellerina. Four mistletoe genera were most prevalent on Myrtales, one on Fabales, and one on Laurales. Rosids were most often the recorded hosts (84.3% of all records). We found evidence of significant phylogenetic clustering in host use byAmyema,Amylotheca, andDecasinina. ConclusionsOur results, particularly the high prevalence on rosids, suggest that relationships of mistletoes with rainforest lineages may have been established early in the history of Australian Loranthaceae and that some lineages co‐diversified with their hosts in arid regions. 

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4. Reproductive ecology of a parasitic plant differs by host species: vector interactions and the maintenance of host races

   https://doi.org/https://doi.org/10.1007/s00442-017-4038-6  

   Yule, Kelsey M; Bronstein, Judith L (February 2018, Oecologia)

   Parasitic plants often attack multiple host species with unique defenses, physiology, and ecology. Reproductive phenology and vectors of parasitic plant genes (pollinators and dispersers) can contribute to or erode reproductive isolation of populations infecting different host species. We asked whether desert mistletoe, Phoradendron californicum (Santalaceae tribe Visceae syn. Viscaceae), differs ecologically across its dominant leguminous hosts in ways affecting reproductive isolation. Parasite flowering phenology on one host species (velvet mesquite, Prosopis velutina) differed significantly from that on four others, and phenology was not predicted by host species phenology or host individual. Comparing mistletoe populations on mesquite and another common host species (catclaw acacia, Senegalia greggii) for which genetically distinct host races are known, we tested for differences in interactions with vectors by quantifying pollinator visitation, reward production, pollen receipt, and fruit consumption. Mistletoes on mesquite produced more pollinator rewards per flower (1.86 times the nectar and 1.92 times the pollen) and received ~ 2 more pollen grains per flower than those on acacia. Mistletoes on the two host species interacted with distinct but overlapping pollinator communities, and pollinator taxa differed in visitation according to host species. Yet, mistletoes of neither host showed uniformly greater reproductive success. Fruit set (0.70) did not differ by host, and the rates of fruit ripening and removal differed in contrasting ways. Altogether, we estimate strong but asymmetric pre-zygotic isolating barriers between mistletoes on the two hosts. These host-associated differences in reproduction have implications for interactions with mutualist vectors and population genetic structure. 

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5. Systematic shifts in the variation among host individuals must be considered in climate-disease theory

   https://doi.org/10.1098/rspb.2024.2515  

   Mihaljevic, Joseph R; Páez, David J (February 2025, Proceedings of the Royal Society B: Biological Sciences)

   To make more informed predictions of host–pathogen interactions under climate change, studies have incorporated the thermal performance of host, vector and pathogen traits into disease models to quantify effects on average transmission rates. However, this body of work has omitted the fact that variation in susceptibility among individual hosts affects disease spread and long-term patterns of host population dynamics. Furthermore, and especially for ectothermic host species, variation in susceptibility is likely to be plastic, influenced by variables such as environmental temperature. For example, as host individuals respond idiosyncratically to temperature, this could affect the population-level variation in susceptibility, such that there may be predictable functional relationships between variation in susceptibility and temperature. Quantifying the relationship between temperature and among-host trait variation will therefore be critical for predicting how climate change and disease will interact to influence host–pathogen population dynamics. Here, we use a model to demonstrate how short-term effects of temperature on the distribution of host susceptibility can drive epidemic characteristics, fluctuations in host population sizes and probabilities of host extinction. Our results emphasize that more research is needed in disease ecology and climate biology to understand the mechanisms that shape individual trait variation, not just trait averages. 

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