Plant Press, Vol. 22, No. 4

THE PLANT PRESS Department of Botany & the U.S. National Herbarium New Series - Vol. 22 - No. 4 October-November 2019 Parasitic plants: Important components of biodiversity By Marcos A. Caraballo-Ortiz arasitic organisms are generally viewed in a negative way itats. Only a few parasitic plants yield economically impor- because of their ability to “steal” resources. However, tant products such as the sandalwood, obtained from the Pthey are biologically interesting because their depend- tropical shrub Santalum album (order Santalales). Other pro- ency on hosts for survival have influenced their behavior, mor- ducts are local and include traditional medicines, food, and phology, and genomes. Parasites vary in their degree of crafts like “wood roses”. Many parasites are also considered necessity from a host, ranging from being partially independent agricultural pests as they can impact crops and timber plan- (hemiparasitic) to being complete dependent (holoparasitic). tations. Some parasites can live independently, but if they find potential It is difficult to describe a typical parasitic plant because hosts, they can use them to supplement their nutritional needs they possess a wide diversity of growth habits such as trees, (facultative parasitism). terrestrial or aerial shrubs, vines, and herbs. The largest Parasitism is not a phenomenon unique to animals, as there Continued on page 2 are plants parasitic to other plants. Current biodiversity estimates indicate that approximately 4,700 species of flowering Tropical mistletoes are very plants are parasitic, which account for about 1.2% of the total inferred number of plant species in the world. About half of the diverse but still poorly known. known species of parasitic plants belong to a single order, San- Herbaria house many unidentified talales, which is diverse and mainly composed of hemiparasites. However, parasitism has evolved independently in 11 lineages and misidentified specimens. of angiosperms comprising 27 families, some of them small and consisting of only one species. Parasitic plants have been able to Marcos A. Caraballo-Ortiz, colonize almost every corner of the world (except the polar re- Smithsonian Botanist gions), and many are common in lowlands and disturbed hab- Department of Botany & the U.S. National Herbarium Mistletoes Orders and families containing parasitic flowering plants, with estimated numbers Continued from page 1 of genera and species. Names within quotes include taxa not properly recognized yet by the Angiosperm Phylogeny Group IV (APG Bot. J. Linn. Soc. 181:1–20; 2016). known parasitic plant is the tree Okoubaka Estimates obtained and adapted from “The Parasitic Plant Connection” website by aubrevielli (order Santalales) from tropical Daniel Nickrent (2019; https://parasiticplants.siu.edu). Africa, which can reach up to 40 m tall and parasitize many species of trees, ap- Order Families Parasitic Genera Parasitic Species parently killing the closest neighbors to minimize competition for light. The small- Boraginales Lennoaceae 2 4 est parasite is likely the miniature mistletoe Viscum minimum (also in order Cucurbitales Apodanthaceae 2 10 Santalales), whose tiny stems and inflores- cences measure up to 3 mm long and in Ericales Mitrastemonaceae 1 (Mitrastemon) 2 nature, only grows on two species of spurges (Euphorbia horrida and E. polyg- Lamiales Orobanchaceae 100 2,100 ona, Euphorbiaceae) from South Africa. It is notable that the largest flower in the Laurales Lauraceae 1 (Cassytha) 20 world, the corpse lily (Rafflesia arnoldii, Rafflesiaceae), is a parasite that grows -em Malpighiales Rafflesiaceae 3 35 bedded into the stems of a woody vine Malvales Cytinaceae 2 12 (Tetrastigma spp., Vitaceae) in the rainfor- ests of Sumatra. Many other parasitic Piperales “Hydnoraceae” 2 12 plants grow as vines such as dodders (Cus- cuta spp., Convolvulaceae) and the laurel Santalales 16 “Families” 166 2,284 dodder (Cassytha spp., Lauraceae), which form dense masses of twining yellowish Saxifragales Cynomoriaceae 1 (Cynomorium) 1 stems wrapping their hosts. Terrestrial parasites like broomrapes (Striga spp., Oro- Solanales Convolvulaceae 1 (Cuscuta) 215 banchaceae) are known for their capacity to affect or even destroy agricultural crops Zygophyllales Krameriaceae 1 (Krameria) 23 such as rice, maize, sugarcane, and sorg- hum. Some parasitic herbs are not aggres- Total Number of 27 282 4,718 sive and are used as ornamentals in Parasitic Plants gardens such as the Indian paintbrush (Castilleja coccinea, Orobanchaceae) and some species of louseworts (Pedicularis they are also considered obligate parasites. spp., Orobanchaceae). The relationships between mistletoes and The most recognized parasitic plants hosts is complex and involves compatibil- are probably mistletoes which are aerial ity at the physical, physiological, and most hemiparasitic shrubs. Mistletoes are part likely genetic levels. Mistletoes, as well as of the folklore from many countries and other parasitic plants, can have unusual are still included in modern traditions chloroplast genomes with major alteration such as the Christmas custom of kissing or losses of genes and rearrangements of under the mistletoe for enduring love. The their genomes due to their dependence on Christmas mistletoe involves two species: hosts. the European Viscum album and its Amer- Most mistletoes also have intimate in- ican counterpart Phoradendron leucarpum. teractions with birds, depending on them However, mistletoes are a diverse group for seed dispersal and sometimes pollina- comprising about 1,663 species in 90 gen- tion services. In fact, studies have shown era distributed around the globe, especially that there are lineages of birds and mistle- in the tropics. All mistletoes belong to toes that have coevolved, where birds spe- order Santalales where they are classified cialize in feeding on mistletoe fruits and in five families: “Amphorogynaceae”, Lo- track them across the landscape. In a simi- ranthaceae, Misodendraceae, Santalaceae, lar way, some mistletoes have evolved spe- and “Viscaceae”. cialization to grow only in one or a few Aerial shrubby habit of a mistletoe All mistletoes have the capacity to species of trees. For example, the dwarf (Dendropemon purpureus, Loranthaceae) create their own food through photosyn- mistletoe (Arceuthobium, Viscaceae) para- on the branches of a calabash tree thesis, hence the term “hemiparasites”. Ho- sitizes pines (Pinaceae), junipers and cy- (Crescentia cujete) in Gonave Island, Haiti. wever, since mistletoes depend on hosts to presses (Cupressaceae) exclusively. In spite (photo by M. Caraballo) obtain water and some mineral nutrients, of being parasites, mistletoes are important Page 2 showcase for the study of biogeography and diversification in the region. During an integrative taxonomic revi- sion in preparation, I have discovered sev- eral new species of Dendropemon and a series of nomenclatural and taxonomic changes in the genus, highlighting the im- portance of combining herbarium specimens, fieldwork, and modern molecular techniques to revise the taxonomy of this poorly known and diverse group. My work also incorporates the conservation of en- dangered parasitic plants, especially when they are island-endemic and depend on rare host trees for their survival. In summary, parasitic plants such as mistletoes offer an excellent opportunity to study the evolution of interactions between plants from the genomic and eco- logical perspectives. My research emphasizes the need to conduct integrative taxonomic studies of poorly known groups of organisms using traditional herbarium studies and modern molecular techniques to assess their global diversity. The Plant Press New Series - Vol. 22 - No. 4 Inflorescence of the American cancer-root (Conopholis americana, Orobanchaceae) Chair of Botany parasitizing the roots of a red oak tree (Quercus rubra, Fagaceae) at the Shenandoah Eric Schuettpelz National Park, Virginia. Note the yellowish color of the plant which reflects its ([email protected]) holoparasitic nature. (photo by M. Caraballo) EDITORIAL STAFF components of ecosystems and are even harbor a rich endemic flora, and the Ca- Editor considered keystone species because of ribbean Islands are not an exception. The Gary Krupnick their capacity to attract and maintain bio- Caribbean island archipelago is one of the ([email protected]) diversity. world’s top five hotspots of biodiversity, Copy Editors My current research project at the Na- containing one the highest concentrations Robin Everly, Bernadette Gibbons, and tional Museum of Natural History is fo- of endemic plant and animal species on Rose Gulledge cused on the genetics and taxonomy of the planet, but much of the endemic diver- The Plant Press is a quarterly publication provided mistletoes from tropical America. With a sity remains little studied. An outstanding free of charge. To receive notification of when new concentration on the Loranthaceous mis- component of these island-endemic floras pdf issues are posted to the web, please subscribe tletoes, I am exploring the structure of are mistletoes in the genus Dendropemon to the listserve by sending a message to their chloroplast genomes and building a (Loranthaceae), which is one of the most [email protected] containing only the following in the body of the text: SUBSCRIBE phylogeny that will include almost all species-rich of the 180 island-endemic PLANTPRESS-NEWS Firstname Lastname. known Neotropical genera. The results plant genera

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