Viscaceae is a family of seven genera and about 400 species. The world distribution of the genera is mapped by Barlow (1983a). All the evidence suggests that this is a close-knit group. The circumscription of genera has been relatively uncontroversial, but there is no clear indication of the relationship between them (Barlow 1997). The adaptations to a parasitic mode of existence are mostly common to all the genera. The structure of the flowers and fruits is relatively simple, without a great deal of variation in the family. The anthers open in various ways, but this does not seem closely correlated with other features. The inflorescences have become modified to a considerable extent, the components often condensed and reduced to minute structures that are somewhat difficult to interpret. The leaves are sometimes reduced to no more than scales and specialised prophylls and cataphylls are commonplace but uninformative. Unlike Loranthaceae, the chromosome complement provides little information. Recent analyses by Nickrent (1996) using the chloroplast gene rbcL and the ribosomal gene 18S rDNA, reaffirm a close-knit group without a clear resolution of relationships within it. Both genes do indicate, however, a link between Arceuthobium and Notothixos and between Ginalloa and Korthalsella. Viscum tended to be basal to the whole family in the 18SrDNA cladograms and linked as a polytomy with Notothixos and Arceuthobium in the rbcL trees. The two American genera Dendrophthora and Phoradendron were closely associated, as is evident from their morphology (Kuijt 1959, 1961).
Viscum has about 100 species. About twenty of these are found in mainland tropical Asia and considerable diversity occurs there. Most species are now found in Africa, which has 45 species, and Madagascar, which has a further 30 species. The few species belonging to the Viscum album group have adapted to more temperate regions in Eurasia and several of the Asian groups extend sparsely southwards to eastern Australia. The genus has spread into all sorts of wooded habitats and some species have become very specialised parasites of particular hosts, sometimes occurring only on other mistletoes, principally Loranthaceae.
Korthalsella has minute seeds that seem to have permitted long-distance dispersal on the feet and feathers of birds. The genus is now widely distributed from tropical Africa and Indian Ocean islands to Japan, Australia, New Zealnd and the Pacific. The plants are small, inconspicuous and often overlooked. They are difficult to classify and some of the species formerly recognised may prove to be no more than races of a few widespread species. The size of the genus is estimated at somewhere between 7 and 25 species (Barlow, 1997).
The dwarf mistletoes, belonging to the genus Arceuthobium, are also often small and with complexities in their taxonomy, but have been studied to a much greater extent because of their economic importance (Hawksworth and Wiens 1996). Unlike almost all other mistletoes, they have become parasites of pine trees. The 40 species range from the Sino-Himalayan region to the Mediterranean, North and eastern Africa and the Canary Islands, then concentrated in North America in Mexico and northern California, with a few species extending north and then eastwards in the Great Lakes region. As indicated above, the genus has a very specialised dispersal technique, the tiny seeds ejected by water pressure up to 10 m or so. The seeds are prone to lodge among pine needles and after rain tend to slide into the axils where they germinate. The genus may well have spread westwards from the Himalayan region and entered North America when the continents were still joined in the Eocene, about 50 million years ago (Hawksworth and Wiens, 1996; Lavin and Luckow, 1993).
Ginalloa has 9 species from Sri Lanka to the Philippines and south-eastwards to Papua New Guinea and the Solomon Islands (Barlow 1997). The species occur mostly in rain-forest communities and there is little host specificity. Nothothixos, with eight species, has a similar distribution, extending a little further, from Sri Lanka to eastern Australia and the Santa Cruz Islands. Some species of Noththixos, towards the edge of the range, are adapted to drier habitats and tend to be more host specific (Barlow 1983b, 1997).
Phoradendron, with about 200 species, and Dendrophthora, with about 50 species, are wholly American and widely distributed in the warmer parts of both continents. They are closely related to each other and show trends that can be related to those found in Notothixos and Ginalloa (Kuijt 1959). They may have migrated on angiosperm hosts about the same time as Arceuthobium.
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