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Home , Amyema, Antidaphne, Arceuthobium, Atkinsonia, Dendrophthoe, Dendrophthora

Santalales () Introductory article

Daniel L Nickrent, Southern Illinois University, Carbondale, Illinois, USA Article Contents

. Introduction are flowering in the sandalwood order () that parasitize the . and Phylogenetics branches of and . Mistletoes have evolved approximately five times in the order . Morphology, Life Cycle and Ecology and are important in forest ecology, pathology and medicine. . Biogeography and Fossil History of Mistletoes . Importance of Mistletoes Introduction

Among the various nutritional modes displayed by flowering plants, represents one of the most Taxonomy and Phylogenetics successful. This heterotrophic mode has evolved indepen- dently approximately ten times within angiosperms. One Molecular phylogenetic investigations have contributed such group is Santalales, an order composed of six families new evidence to address relationships among members of that includes sandalwoods and mistletoes. Although all the sandalwood order. From these data it appears that root parasitic plants attach to their host via one or more parasitism evolved only once within ‘’, a para- haustoria, they can be further categorized as hemiparasites phyletic assemblage that is traditionally classified as one and holoparasites depending upon the degree of nutri- family in Santalales (Figure 1). From such root parasitic tional dependence upon the host. All members of ancestors, stem parasites evolved on at least four and Santalales are hemiparasites;however, some attach to possibly five separate occasions (Figure 1, clades 1–5). Thus, host roots and others attach to host stems. Aerial (stem) the term mistletoe refers to shrubby branch parasites that parasites, generally called mistletoes, can be found in four are not monophyletic (all sharing a common ancestor) but families of Santalales: , Misodendraceae, that have evolved independently in four families of and (Table 1). All of these plants are Santalales (Table 1). All members of Misodendraceae and obligate hemiparasites, meaning they require attachment Viscaceae are mistletoes, but this is not the case for to the host to complete their life cycle. Some of the more Loranthaceae and Santalaceae where root parasites can advanced mistletoes (e.g. ) approach holo- also be found. As shown in Table 1, the vast majority of parasitism, yet none have achieved this condition because mistletoe genera and species are found in Loranthaceae (74 is required during the seedling stage of of the 89 genera, 900 of the 1436 species). This family shows development prior to host attachment. extensive diversification at the generic level, in contrast to

Table 1. Numbers of mistletoes Clade No. of genera No. of species Examples of genera 1. Misodendraceae 1 c.8 Misodendrum 2. Loranthaceaea 74 c. 900 , , Phthir- usa, , Scurru- la, 3. Santalaceae Cb 311Antidaphne, , Le- pidoceras 4. Santalaceae Dc 437Dendromyza, Dendrotrophe, Dufrenoya, Phacellaria 5. Viscaceae 7 c. 480 Arceuthobium, , , , , , Totals 89 c. 1436 a , Gaiadendron and are root parasites. b Traditionally classified as . c Tribe Amphorogyneae in part.

ENCYCLOPEDIA OF LIFE SCIENCES © 2002, John Wiley & Sons, Ltd. www.els.net 1 Santalales (Mistletoe)

5 Viscaceae

4 ‘Santalaceae’ D

3 ‘Santalaceae’ C

‘Santalaceae’ B

‘Santalaceae’ A

Opiliaceae

2 Loranthaceae

Parasitism 1 Misodendrum ‘Olacaceae’ D ‘Olacaceae’ C ‘Olacaceae’ B ‘Olacaceae’ A Outgroups

Figure 1 Generalized phylogenetic showing relationships among the component clades of Santalales (sandalwood order). Current evidence suggests that parasitism arose once in the order in clades C and D of Olacaceae. Branches above this point with thin lines are root parasites whereas those with thick lines represent mistletoes, a habit that has evolved independently approximately five times. This tree is derived from DNA sequences obtained from nuclear and chloroplast genes.

Viscaceae, which contains only seven genera but several evidence indicates that these mistletoes evolved two times hundred species. The majority of the 480 species of independently (Figure 1). One of these clades has tradition- Viscaceae are attributed to only two genera: Phoradendron ally been classified as Eremolepidaceae and the other and Viscum. represents a part of the tribe Amphorogyneae, which is The molecular tree shows that Schoepfia is not closely otherwise composed of root parasites. The last group of related to other Olacaceae but is sister to Misodendrum, the mistletoes is Viscaceae, a strongly supported clade of seven ‘feathery mistletoes’ that parasitize southern hemisphere genera. beech trees (Nothofagus) in Chile and Argentina. This arrangement indicates that the common ancestor of Misodendrum and Loranthaceae was a root, not a stem parasite. Relevant to this discussion is the fact that three Morphology, Life Cycle and Ecology genera of Loranthaceae (Atkinsonia, Gaiadendron and Nuytsia) are root parasites and are thought to be the most The morphological features of mistletoes are various primitive members of the family. If stem parasitism is a depending upon which taxonomic group is being con- component of the definition of a mistletoe, then technically sidered. Most members of Viscaceae have simple, some- these three genera are not mistletoes. This demonstrates what brittle with entire margins. These occur that the term mistletoe is, in one sense, taxonomic (because opposite on stems with constricted nodes. In some (e.g. all are in Santalales) but in another sense is only a Arceuthobium) the leaves are reduced to scales. The flowers description of a particular habit found in several evolu- are inconspicuous, monochlamydous and unisexual, form- tionary lineages. ing spikes or cymes on monoecious or dioecious plants. In As with Olacaceae, ‘Santalaceae’ are paraphyletic. Of contrast, Loranthaceae generally have opposite or alter- the 37 genera in this family, 30 are root parasites and the nate leaves and large, brightly-coloured, dichlamydous, remaining seven are stem parasitic mistletoes. Molecular bisexual, flowers that display coadaptations to

2 Santalales (Mistletoe) pollination. Older classifications often placed members of Cretaceous pollen flora from Seymour Island, Antarctica, Viscaceae and Loranthaceae together within a broadly which contains Loranthaceae (Cranwellia) pollen as well as defined family. However, considerable evidence now exists that from other tropical and cool temperate families. to treat them as distinct groups (e.g. anther dehiscence, Following extensive rifting during the Tertiary, the loranth pollen shape, embryo sac development, and base chromo- flora differentiated on the isolated island continent of some number). . By mid Miocene, the Australian plate collided An important component of the life cycle of all with the Sunda island arc system, thus initiating an mistletoes is the . Most mistletoe are covered extensive exchange of Loranthaceae, both from Australia by a sticky substance called viscin that allows attachment north and from Malesia south across Wallace’s line. to the host branch. Exceptions include Nuytsia with a In contrast to Loranthaceae, Viscaceae are thought to be winged seed and Misodendrum with a plumed seed, both of a Laurasian group. Origins of five of the seven genera can which are wind dispersed. are the agent of dispersal be traced to eastern . Moreover, the world distribution for the seeds of many mistletoe species in Loranthaceae of the genera is not explained by vicariance but by range and Viscaceae. In the palaeotropics, involves expansions that took place during the Tertiary. For spectacular coevolutionary adaptations with flower peck- example, progenitors of the entirely New World genera ers (Dicaeidae). Seed dispersal is explosive in dwarf Phoradendron and Dendrophthora are thought to have mistletoes (Arceuthobium) by means of hydrostatic pres- migrated from Asia during this time. Arceuthobium is the sure that builds up within the . Seeds are discharged at only of mistletoe that occurs in both the Old and 27 metres per second and may travel as far as 16 metres. New World. The genus probably arrived in Once attached to a host branch (by means of viscin), the via a Bering land connection during the Miocene and then mistletoe seed germinates and the primary root or underwent extensive adaptive radiations on in hypocotyl forms the haustorium. In this phase of the life both the USA and Mexico. cycle, the seedling is particularly vulnerable to desiccation and must rapidly establish contact with the host . Seedlings are also photosynthetic at this stage, from chlorophyllous endosperm and/or cotyledonary tissues. Host specificity is determined at the time of haustorial penetration. Some mistletoes exist at this early stage Importance of Mistletoes entirely within the host branch as an endophyte (e.g. Arceuthobium), whereas others (e.g. Loranthaceae) form In countries with traditions rooted in European culture, more typical epicotyls, shoots and leaves. To move water mistletoe is integrally linked with the Christmas holiday. from the host xylem to the parasite haustorium, mistletoes The tradition of kissing under the mistletoe may be a relic must produce a water potential more negative than their of ancient druidic New Year ceremonies that associated hosts and this is accomplished by higher this with fertility. In , the holiday mistletoe is rates. Pollination of mistletoe flowers may involve wind Viscum whereas in North America it is Phoradendron. and insects (Viscaceae) or birds (Loranthaceae). Viscaceae, particularly , has been exten- sively examined for the presence of compounds (e.g. viscotoxins) that may have either toxic or therapeutic effects. Especially in northern Europe, mistletoe extracts Biogeography and Fossil History of have been used to combat cancer, despite some controversy Mistletoes over the efficacy of this practice. Mistletoe lectin (ML) appears to be the major biologically active component. It is Given the requirement that their seeds land on a suitable a glycoprotein that is classified as a type II ribosome- host, dispersability in Loranthaceae is limited. For this inactivating protein owing to its ribosomal RNA-cleaving reason, and the scattered, continental distribution of ability (similar to but less toxic than ricin). ML induces relictual genera on austral landmasses, vicariance is the apoptosis in human cells by inhibition of protein synthesis. most likely biogeographical model to apply to this family. Mistletoes are also important because of their negative Vicariance assumes that ancestral Loranthaceae popula- impact upon commercially valuable trees. The dwarf tions were separated by plate tectonic events that isolated mistletoes (Arceuthobium) are native components of the Gondwanan continents. Cytology also reflects the forests of North America and Mexico yet they cause the biogeographic history of the family. The primary base loss of 11.3 million cubic metres of wood annually from the chromosome number for the family is 12, which occurs in western US conifers (valued at several billion dollars). Atkinsonia (eastern Australia), Gaiadendron (New World Mistletoes in Loranthaceae (e.g. Amyema, , tropics), and Nuytsia (). By at least the , and Psittacanthus) can also early Cretaceous, Loranthaceae had already attained a become problematic pathogens on ornamental and fruit high degree of differentiation. This is evidenced by the late trees.

3 Santalales (Mistletoe)

Further Reading Nickrent DL The Connection. [http://www.science.siu.e- du/parasitic-plants/index.html] Calder M and Bernhardt P (1983) The Biology of Mistletoes. New York: Press MC and Graves JD (1995) Parasitic Plants. London: Chapman Academic Press. and Hall. Kuijt J (1969) The Biology of Parasitic Flowering Plants. Berkeley, CA: University of Press.

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