Carol A. Wilson

An origin of aerial branch parasitism in the mistletoe family, Loranthaceae

The large mistletoe family, Loranthaceae, contains 75 genera and approximately 1000 species. The family originated in the Southern Hemisphere and dispersed, apparently early, between fragments of Gondwana. It is now widely distributed on land surfaces of the former supercontinent. The Loranthaceae has three terrestrial, root-parasitic genera-a habit considered ancestral-and 72 genera of aerial, branch parasites. For almost two centuries, the origin of the mistletoe habit has been of interest to biologists. Two main evolutionary pathways have been proposed to explain the transition from terrestrial to aerial parasitism in the family. One theorizes the presence of an intermediate climbing ancestor in the path to the aerial habit. The other proposes a direct transfer from terrestrial to epiphytic growth following the germination of seeds on tree branches. Here we present molecular and morphological evidence that (1) the terrestrial species Nuytsia floribunda is ancestral within the Loranthaceae, (2) aerial parasitism has had multiple origins in the family, (3) the first aerial branch parasites had epicortical roots, and (4) the origin of aerial parasitism in one Old World clade involved the direct transfer from terrestrial to epiphytic growth following the germination of seeds on tree branches. Our results suggest that it is not necessary to evoke a climbing intermediate in the origins of aerial parasitism in the Santalales.

Genetic diversity of the rare Asian plant, Trigonobalanus doichangensis (Fagaceae)

Trigonobalanus doichangensis is a national rare and endangered fagaceous plant of China. It is currently restricted to five sites, four in Yunnan province in south-western China and one in northern Thailand. We investigated the genetic diversity of T. doichangensis to provide information for the effective preservation of the genetic diversity of the species. Randomly amplified polymorphic DNA (RAPD) markers were used to assess the genetic variation and structure in each of the five extant populations. Sixteen primers produced 157 fragments, of which 83 (53%) were polymorphic. The estimated species diversity of T. doichangensis, although lower than for most fagaceous species, was within the range of values that have been reported for the family. The presence of private and/or uncommon fragments in each population, the presence of unique phenotypes in all individuals sampled, and estimates of relatively high populational fixation and low gene flow indicated that the species diversity present was mostly distributed among the five isolated extant populations. Pairwise genetic distances between populations were not correlated with geographical distances as depicted by the resolution of one of the Chinese populations as most similar to the Thailand population. Conservation of this species should include preservation of each of the five distinct extant populations.

DEVELOPMENT, TAXONOMIC SIGNIFICANCE AND ECOLOGICAL ROLE OF THE CUTICULAR EPITHELIUM IN THE SANTALALES

All genera in the mistletoe family Viscaceae develop a secondary protective covering, the cuticular epithelium, that replaces the epidermis. The cuticular epithelium also occurs in the Eremolepidaceae and some genera within the related family Santalaceae. This secondary covering, unlike the periderm, lacks lenticels or their functional equivalent. We suggest that the cuticular epithelium provides a greater degree of control over transpirational water loss in older plant parts. The cuticular epithelium may arise in the epidermis, subepidermal layer, or in deeper tissues of the stem. Bark strands of the endophytic system, where they are in contact with either nonliving host tissues or the external environment, also form a cuticular epithelium. The epidermal feature, stomatal orientation, was also studied. All genera in the Viscaceae and Eremolepidaceae have stomata with a transverse orientation. The presence or absence of a cuticular epithelium and stomatal orientation are vegetative characters with potential taxonomic value.

Relationship of the Mistletoe Phoradendron Macrophyllum (Viscaceae) to the Wood of its Host

The importance of direct tracheary element connections between mistletoes and their hosts is controversial. Direct connections have been reported for four genera within the Viscaceae. In the past such connections were considered essential for the movement of water and nutrients from host to parasite. In contrast, recent authors have suggested that the apoplastic continuum provided by the walls of contiguous host and parasite parenchyma cells is the main pathway for the transfer of water and nutrients, with direct connections playing at best a minor role in uptake. Our analysis of the parasite (Phoradendron)/host(Juglans) interface suggests otherwise. Parenchyma predominates at the interface for both host (71%) and parasite (95%). While direct tracheary element connections are less frequent than other types of connections, they occurred in all sinkers analyzed. Further, direct connections were much more abundant in host latewood (3.6%) than in earlywood (1.8%). This, and other evidence indicates that both pathways of transfer are important. We suggest that the apoplastic continuum provided by parasite parenchyma provides for selective uptake of nutrients, whereas direct connections provide for bulk flow between host and parasite.