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1. 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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2. 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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3. 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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4. Susceptibility to parasitism by the mistletoe Phoradendron quadrangulare (Kunth) Griseb on its host Guazuma ulmifolia Lam may increase with host size

   https://doi.org/10.1016/j.fooweb.2023.e00327  

   Santiago-Rosario, Luis Y.; Espinoza-Espinoza, Nicole; Gómez, Quimey; Martínez de Zorzí, Victoria; Ramírez-Ortiz, Ramón A.; Rodríguez, Karla (December 2023, Food Webs)

   Host-parasite interactions and host susceptibility are key traits in understanding trophic energy transfer, nutrient movement and general macro-ecoevolutionary dynamics of mistletoe systems and plant-plant interactions. This research investigates host susceptibility and size-dependent interactions of the mistletoe Phoradendron quad- rangulare, a widely distributed species, on Guazuma ulmifolia. We studied the interplay between mistletoe load and host tree size, while also exploring the allometric relationship between host branch size and mistletoe size. A field surveys on 67 trees revealed varying mistletoe loads, with most trees showing no occurrence of P. quadrangulare. Parasitized trees had significantly larger diameters at breast height (DBH) than non-parasitized trees. The susceptibility of host trees to mistletoe parasitism increased with increasing DBH, indicating a positive relationship between host size and mistletoe prevalence. Furthermore, mistletoe stem diameter was found to be influenced by the diameter of the host branch suggesting that larger host trees provide more substrate for larger-sized parasites and surface area for mistletoe colonization, potentially contributing to the parasite’s survival and prevalence. This study also highlights the importance of host size in mistletoe presence and performance and provides insights into the broader eco-evolutionary dynamics and conservation strategies needed to conserve mistletoes, an often-underappreciated keystone taxa. 

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5. Transformation of western hemlock ( Tsuga heterophylla ) tree crowns by dwarf mistletoe ( Arceuthobium tsugense , Viscaceae)

   https://doi.org/10.1111/efp.12664  

   Calkins, Stephen_J; Shaw, David_C; Lan, Yung‐Hsiang; Hantula, ed., Jarkko (December 2020, Forest Pathology)

   Abstract Dwarf mistletoes (Arceuthobiumspecies) are arboreal, hemiparasitic plants of conifers that can change the structure and function of the tree crown. Hemlock dwarf mistletoe (Arceuthobium tsugensesubsp.tsugense)principally parasitizes western hemlock (Tsuga heterophylla) and effects 10.8% of all western hemlock trees in Oregon, USA. In this study, we climbed 16 western hemlock trees (age 97–321 years, height 33–54.7 m) across a gradient of infection (0%–100% of branches infected) and measured occurrence of all dwarf mistletoe infections, dwarf mistletoe caused deformities, foliage, branch and crown metrics, and sapwood area. We then modelled over 25 different response variables using linear and generalized linear models with three metrics of severity as explanatory variables: total infection incidence, proportion of all live branches infected, and proportion of all live, infected branches with 33 per cent or more foliage distal to infection. A strong effect of dwarf mistletoe intensification was the reduction of branch foliage and an increase in the proportional amount of foliage distal to infections, with severely infected trees having the majority of foliage distal to infections. Increasing severity led to an apparent crown compaction as crown volumes decreased and became increasingly comprised of deformities. Sapwood area was unrelated to infection severity. Branch length and diameters were unrelated to increasing infection severity despite severely infected branches supporting 1–70 infections. The most severely infected tree had 3,615 individual plants in the crown. Our results suggested that shifts in crown structure and branch deformation, foliage amount, and foliage distal to infection, reflected a likely reduction of capacity for tree growth that coincided with a hypothesized increase in resource demand by dwarf mistletoe plants as infection severity intensified. 

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