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Biology of Mistletoes: Aerial Vascular Parasites
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Biology of Mistletoes: Aerial Vascular Parasites
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Conveners:
Rationale:
The mistletoes comprise a polyphyletic group of nearly 2000 species of predominantly tropical, dicotyledonous parasites within four families in the order Santalales. Mistletoes make up about 50% of all parasitic angiosperms, and represent the largest and most diverse group of aerial parasites. The transition from terrestrial, root parasites to aerial, branch parasites occurred independently at least three times in the early development of the Santalales. This symposium brings together investigations from diverse areas of botany that collectively provide clues as to the processes by which mistletoes colonized and radiated within the forest canopy. Particular emphasis will be placed on recent studies in the two largest mistletoe families, the Loranthaceae and Viscaceae. Reports will summarize advances in research since 1987, the last time mistletoes were the focus of a symposium at an International Botanical Congress. There are several reasons why a symposium on mistletoes is especially relevant at this time: 1) Many advances have been made in our understanding of mistletoe evolution over the past decade. 2) The mistletoes have a mainly tropical distribution, and their habitats are threatened. A discussion of the extent of our current understanding of this unique group of angiosperms, as well as future research needs, is timely. 3) As successful canopy plants, knowledge of mistletoe evolution and specialization is applicable to a better understanding of the constraints and potential of canopy habitats. 4) Another symposium, Current Research on Parasitic Flowering Plants, will also be held at the Botanical Congress. The two symposia complement one another, as the parasitic flowering plants symposium covers a wide range of topics and groups mostly outside of the mistletoes. This symposium, in contrast, is an in-depth examination of the single largest group of parasitic angiosperms, the mistletoes.
Speakers, in order of their presentations:
Abstracts Printed Below:
THE WOODLAND CANOPY AS PLANT HABITAT: CONSTRAINTS AND OPTIONS FOR VASCULAR FLORA. D.H. Benzing (Dept. of Biology, Oberlin College, Oberlin, Ohio 44074)
Between eight and ten percent of all vascular species spend part to all of their life cycle anchored in the crowns of woody hosts without direct access to earth soil, but less than ten percent of this group do so as aerial parasites. Moreover, some 80 families contain at least one autotrophic epiphyte, while the mistletoes belong exclusively to Santalales. I consider this evolutionary asymmetry relative to opportunities for plants to use woody flora simply as perches versus sources of water and mineral nutrients and sometimes fixed carbon as well. Factors that account for the many fewer heterotrophic than autotrophic lineages in woodland canopies appear to be related to aspects of water and ion relations and seedling establishment that distinguish these two life strategies. Nevertheless, aerial parasitism offers certain advantages that permit the mistletoes to exploit a greater variety of kinds of forest habitats than their autotrophic counterparts.
GEOGRAPHICAL AND EVOLUTIONARY LINKS BETWEEN AFRICAN AND ASIATIC LORANTHACEAE. Don Kirkup1 & Bryan Barlow2, 1Royal Botanic Gardens Kew, 2Australian National Herbarium.
Loranthaceae show a classic Gondwanan pattern of radiation which has been extensively discussed in the literature. African Loranthaceae have been assumed to be directly derived from Gondwanan stock, high levels of specialisation in the continent attributed to the early separation of Africa from Gondwanaland and paleoclimatic changes that resulted from the drifting of the African plate to lower latitudes. Recent morphological studies of African Loranthaceae reveal that the most primitive African genera have centres of distribution in Asia and there is a notable paucity in African Loranths of primitive features for the family. This begs the question are there any Loranths at all now extant in Africa that are derived directly from a Gondwanan flora, or are they instead largely the result of colonisation from Asiatic stock?
COMPARATIVE MORPHOLOGY OF HAUSTORIA IN THE OLD WORLD LORANTHACEAE. C. A. Wilson and C. L. Calvin, Dept. Of Biology, Portland State Univ., Portland, OR 97207 USA
The widely accepted model of haustorial evolution in aerial Loranthaceae views epicortical roots as primitive and bark strands as advanced. Solitary unions--wood roses and clasping unions--are regarded as intermediate in specialization. Comparative morphological methods have been used to test this model in Old World species. Our results indicate that a more complex model of haustorial evolution is needed. We found that primitive aerial forms have a small number of robust epicortical roots. In some taxa bark strands may occur concurrently with epicortical roots, as can wood-roses. Some genera with epicortical roots, such as Lepidaria and Ileostylus, form strong primary attachments and few or no secondary attachments. In most species epicortical roots favor basipetal branch growth, have sympodial-like elongation, and form root-born shoots in conjunction with secondary haustoria.The morphological and phylogenetic implications of these findings are illustrated and discussed.
MORPHOLOGY AND PLACEMENT OF THE INFLORESCENCE OF PHORANDENDRON (VISCACEAE). Job Kuijt, Dept. Of Biology, University of Victoria, Victoria, BC V8W 3N5, CANADA
The inflorescence of Phoradendron consists of two or more squamate internodes, the upper one(s) bearing flowers along their length, in uni-, bi-, or triseriate patterns, and uniquely produced by an intercalary meristem. Prophylls and cataphylls, as they may subtend inflorescences (or, otherwise, vegetative shoots), partly determine may also be axillary to expanded leaves. Rarely, the upper cataphylls also subtend either through abortion of the shoot apex, or through its transformation into an inflorescence.
RESOURCE ACQUISITION AND PARTITIONING IN XYLEM-TAPPING WOODY ROOT HEMIPARASITES AND MISTLETOES: SIMILARITIES, CONTRASTS AND THEIR PHYSIOLOGICAL IMPLICATIONS. J. S. Pate and T. L. Bell, Department of Botany, University of Western Australia, Nedlands 6907, Western Australia.
Most root hemiparasites differ obviously from mistletoes in possessing extensive root systems, exploiting many rather than a single host and carrying the potential for uptake of water and nutrients from soil independently of haustorial contacts with hosts. However, both classes of parasites extract organic and inorganic nutrients from xylem streams of hosts by means of apoplastic or symplastic transfer mechanisms. In either case, water relations of the parasite engineer flow more or less consistently in favour of the parasite and sometimes resulting in poorer water use efficiencies than in host(s). Case studies of typical root hemiparasites (Olax, Santalum) and mistletoes (Amyema) will be presented to explore how subtle differences in strategies of functioning in each class impact upon patterns of uptake, partitioning and utilisation of water, nitrogen and other nutrients and relative extents of autotrophic and heterotrophic inputs of carbon by the parasite. Likely effects of parasitism on growth and functioning of hosts will also be considered.
ASPECTS OF HAUSTORIAL ORGANIZATION IN MISTLETOES AND THEIR POSSIBLE EVOLUTIONARY IMPLICATIONS. B. A. Fineran (Dept. Plant & Microbial Sciences, University of Canterbury, Christchurch, New Zealand)
Evolution in aerial mistletoes has involved various structural modifications of the haustorial system. Here the secondary haustorium of a primitive Loranthaceae is compared with that in root hemi-parasitic Santalales. Cellular specialization within the endophytic system is also examined. In Alepis flavida (Loranthaceae) secondary haustoria exhibit collapsed layers, a flask-shaped vasculature, and an interrupted zone with vessels often containing stainable material. These features are absent in the primary haustorium, but occur in root hemi-parasitic Santalalean haustoria. The endophytic tissue usually shows little cellular specialization, with ordinary parenchyma and tracheary elements predominating. However, in Korthalsella and Phoradendron (Viscaceae) flange cells are differentiated. Phorodendron shows greatest diversity with both flange cells and flange cells combined with transfer cells. These are located at the interface against host vessels (or tracheids in conifer hosts) and abutting tracheary elements of the sinker.
SIUC / College of Science / Parasitic Plant Connection
URL: http://www.parasiticplants.siu.edu/MistletoeSymp.html
Last updated: 1-Sept-99 / dln
SIUC / College of Science / Parasitic Plant Connection
URL: http://www.parasiticplants.siu.edu/MistletoeSymp.html
Last updated: 28-Dec-98 / dln