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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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