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Loranthaceae are parasitic plants in about 76 genera that are predominantly found in subtropical and temperate regions of the Southern Hemisphere as branch parasites. Australia is an area of high diversity with about 11 genera and 65 species, most of which are endemic. Loranthaceae branch parasites are also morphologically diverse having both radial and zygomorphic flowers that are typically bird pollinated and each of the four basic haustorial types. Haustorial types include epicortical roots (ERs) that grow along the host branch surface and at intervals form secondary attachments to their host, clasping unions where parasite tissue enlarges partially encircling the host branch, wood roses where host tissue proliferates forming a placenta where the parasite is attached, and bark strands that spread within the outer tissues of the host branch. We hypothesized that those haustoria where parasitic tissue proliferated, such as ERs and clasping unions, would occupy more mesic environments. To test this hypothesis and investigate other relationships among ecological parameters and haustorial form we used 17,753 sets of occurrence and ecological data from the Atlas of Living Australia (ALA) online repository for 42 species of Loranthaceae. We analyzed haustorial forms through comparative studies of haustoria housed at the UC Herbarium, relevant literature, and collections in public repositories. Biogeographical and environmental data were analyzed using mapping and statistical methods in the R environment. Our preliminary research suggests that bark strands are found in climatic regions across Australia, including deserts, while both epicortical roots (ERs) and clasping unions are mostly restricted to mesic coastlines of eastern Australia (21 of 22 species with ERs occur only along eastern coastlines of Australia or in the Cape York Peninsula). Wood roses are less common in Australia with few data points. Haustoria are sometimes complex, especially clasping unions where bark strands are occasionally also produced. An interesting finding was that Amyema sanguinea has a wide distribution in arid as well as mesic climates even though it has ERs. This species has unusually robust ERs that might contribute to its wider ecological niche. Evolution of haustoria in Australia is discussed based on phylogenetic hypotheses of Loranthaceae genera.
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Ecological niche modeling of Australian Loranthaceae mistletoes using a novel haustorial perspective
Mistletoe spatial patterns are poorly understood on a macroecological scale. Previous research conducted at the family-level on Loranthaceae from Australia demonstrated that unlike most plants, mistletoe species richness patterns do not correlate significantly with water and energy input. However, field studies suggested a relationship between the structure of the host-parasite union (haustorium) and environment. We hypothesize that haustorial type influences relationships between the abiotic environment and mistletoe spatial patterns. To investigate this hypothesis, we constructed ecological niche models for individual haustorial types. We have previously compared the distributions of haustorial types in both geographic and environmental space using geographic mapping and PCA, respectively. Here, we expand on our study by examining species richness, constructing predictive models, and emphasizing habitat types. Using the haustorial specimen collection housed at the UC Herbarium and relevant literature, we identified the haustorial type of 55 of the 73 Australia Loranthaceae mistletoe species. Using geographic distributional data from the Atlas of Living Australia and environmental data from WorldClim, we plotted haustorial groups in both geographic and environmental space, compared clusters in principle component space, and calculated Hutchinsonian niche overlap. We used regression to analyze the relationship between species richness and environmental variables at the haustorial level. Lastly, we constructed maximum entropy models to estimate the probability of occurrence of each haustorial group, analyzing the relative contributions of each variable to each model. We discovered that haustorial type is relatively conserved among the Australian Loranthaceae mistletoe genera, with seven out of nine genera exhibiting one haustorial type. Species with epicortical roots (ER), the ancestral character, are exclusively associated with coastal regions while those with derived haustorial types occur across the continent, including desert regions. Environmental analyses confirmed that species with ER are found in regions with milder temperatures and higher precipitation than derived types. Species richness patterns of some haustorial types, including ER, are significantly correlated with most environmental variables, while derived haustorial types are not. Maxent models for species with ER haustoria predict the highest probability of occurrence for coastal regions, while models constructed for derived types feature less bias for coastal regions. Our models demonstrate that relationships between the abiotic environment and mistletoe spatial patterns depend in part on the haustorial type. Hypotheses proposed to explain relationships between abiotic constraint on distribution and haustorial type include differences in water uptake efficiency, exposure of haustoria to the environment, longevity of haustoria, and host preference of species.
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- Award ID(s):
- 1756351
- PAR ID:
- 10336576
- Date Published:
- Journal Name:
- Botany 2021
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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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.more » « less
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