Viscum Album Group

The greater part of this book is concerned with the biology and uses of Viscum album. Populations extend further into temperate regions than any other species of the genus. The species has a number of remarkable characters, some of which seem to be

Table 2 Morphological groups of Asian and Australian species of Viscum. Adapted and updated from Danser (1941).

Group

Characters

Species

V. album group

V. mysorense group

V. ovalifolium group

V. orientale group

V. capitellatum group

Dioecious. Inflorescences terminal.

Monoecious. Inflorescences mostly lateral, with terminal female flower and subsidiary cymules male or female

Monoecious. Flowers in triads, middle female, laterals male (or female).

Monoecious. Flowers in triads augmented by adventitious flowers, middle female, laterals male or female

As V. orientate group but middle flower male (or all female)

V. album L.; V. dryophilum Rech.f.; V. alni-formosanae Hayata; V. fargesii Lecomte; V. nudum Danser; V. cruciatum Sieber ex Boiss. V. mysorense Gamble; V. articulatum Burm.f.; V. nepalense Sprengel; V. liquidambaricum Hayata; V. stenocarpum Danser; V. angulatum Heyne; V. ramosissimum Roxb. ex DC.; V. loranthi Elmer V! ovalifolium DC.; V. wrayi Gamble; V. acaciae Danser; V. indosinense Danser; V. scurruloideum Barlow; V. exile Barlow V. orientale Willd.; V. heyneanum DC.; V. monoicum DC.; V. multinerve Hayata V. trilobatum Talbot; V. capitellatum Smith; V. yunnanense H.S. Kiu, V. katikanum Barlow; V. whitei Blakely; V. bancroftii Blakely adaptations to more severe climates. The most obvious feature is the strictly terminal inflorescences, the male and female flowers borne on different plants. Growth is continued by axillary branches, bearing inconspicuous scale-like prophylls at the base and one pair of leaves at the tip. This gives the characteristic dichotomous branching pattern to the fertile parts of the plant. The petiolar region of the leaf has a cavity, which forms a pocket to encase the buds before the leaves expand. There are several other species in Eurasia with exactly the same sort of inflorescence and branching pattern (Table 2), but with modifications in the vegetative parts, notably the Chinese species, V. fargesii, with narrow leaves, and V. nudum, with the leaves all reduced to small scales (Danser 1941). V. alniformosanae in Taiwan and the more recently described V. dryophilum, from Afghanistan and Pakistan, are segregates of more questionable status. The variation of V. album will be described in more detail in subsequent chapters, but includes a reasonably distinctive subspecies, subsp. meridianum (Danser) D.G.Long, in the Himalayan region (Grieson & Long, 1983) and well-known forms with coloured fruits further east (Park, this book).

Viscum Album Parts

Figure 1 Schematic Representation of Inflorescences in Species of Viscum. 1 V. album (a male; b female plant); 2 V. cruciatum (a male, b female plant); 3 V. mysorense; 4 V. ovalifolium; 5 V. orientale; 6 V. capitellatum; 7 V. triflorum; 8 V. congolense (a male, b female plant). O=male flower; solid O=female flower.

Figure 1 Schematic Representation of Inflorescences in Species of Viscum. 1 V. album (a male; b female plant); 2 V. cruciatum (a male, b female plant); 3 V. mysorense; 4 V. ovalifolium; 5 V. orientale; 6 V. capitellatum; 7 V. triflorum; 8 V. congolense (a male, b female plant). O=male flower; solid O=female flower.

Viscum cruciatum, from North Africa and Spain eastwards to Afghanistan, can also be included in the V. album group, but the inflorescence and branching pattern are not quite so modified. In V. album and its closest allies the male peduncle is short and swollen, with a pair of bracts subtending three flowers Figure 1.1a). In V. cruciatum the peduncle is longer and more flattened, with one to three sessile flowers in the bracteal cup (Figure 1.2a). There are terminal inflorescences in bifurcations, but also lateral inflorescences there and at many nodes below, often up to four together. The branching pattern is correspondingly less obviously dichotomous. The male flowers are, however, up to 6-8 mm long, the largest known in the genus (the tropical species rarely have flowers more than 3 mm, those of V. album are about 4 mm long).

The female inflorescences of V. album have shorter peduncles than the male, bearing 3-5 flowers, the terminal usually subtended by a pair of bracts, below which further bracts subtend the other flowers (Figure 1.1b). In V. cruciatum the peduncle is extended in the female like the male and there are only three flowers, the central one sometimes with a short pedicel and then sometimes with its own small bracts (Figure 1.1b). In all species of the group there is a tendency for supplementary inflorescences to develop from the axils of the bracts and thus appear to be superposed.

The variations of inflorescence structure in the V. album group provide some clues to their derivation. All the other species of Viscum in Asia are monoecious, with male and female flowers generally borne in the same inflorescence. Sometimes there is a shift in sex ratios with a high proportion of female flowers. On the basis of their inflorescences, Danser (1941) divided the monoecious Asian species into four groups (Table 2). The V. mysorense group has axillary inflorescences with a sessile cymule of a single flower, subtended by a bracteal cup, with subsidiary cymules developing lateral to the first one. The first-formed flower is female and the lateral flowers are female or male (Figure 1.3). V. mysorense is a species of southern India and there are about half a dozen further species with leaves reduced to scales, including the common V. articulatum and V. nepalense. The group extends from India to China and thinly southwards to Australia.

The Viscum ovalifolium group has the inflorescence reduced to a cymule of three flowers, the central female, the laterals male, subtended by a bracteal cup and borne in the axils or on short lateral shoots (Figure 1.4). Subsidiary cymules often develop around the first one and some cymules may have flowers of only one sex. The small group is restricted to the South-East. Asia and Australia. V. ovalifolium extends from Myanmar (Burma) to Hong Kong and southwards to Queensland. It is polymorphic (Barlow 1997) and allied to two other species of the group, V. wrayi from Peninsular Malaysia, Borneo and Sumatra, and V. exile from the Celebes. The species occur mostly in lowland forests below 1300 m. V. scurruloideum, a little known species from western Java, can probably be included in this group.

The V. orientale group is restricted to continental Asia and its immediate islands. It is similar to the V. ovalifolium group, but the inflorescences are enlarged by the development of adventitious flowers in the bracteal cup (Figure 1.5). The sex ratios are often skewed towards a high proportion of females. V. orientale and V.

heyneanum grow in the drier parts of Sri Lanka and India, V. monoicum extends from India to China and V. multinerve occurs in China and Taiwan.

Table 3 Groups of African species of Viscum. Main features and list of species.

Group 1. Viscum triflorum group. Young internodes slightly flattened, broadened upwards, ribbed. Leafy. Monoecious. Dichasia mostly lateral, mostly 3-flowered, central male, lateral or all female. Bracteal cups pedunculate. Berries sessile, smooth, n=14. Widespread in diverse forest types. V. triflorum DC., V. petiolatum Polh. & Wiens.

Group 2. Viscum congolense group. Young internodes as V. triflorum group. Leafy, the base with a slight cavity. Dioecious, with skewed sex ratios. Dichasia mostly lateral, often more than 3-flowered, especially in female. Bracteal cups pedunculate. Berries subsessile, smooth to minutely tuberculate. 2n=22 in female, 23 in male. Forests of Congolian region and East Africa. V. congolense De Wild., V. fischeri Engl., V. luisengense Polh. & Wiens.

Group 3. Viscum rotundifolium group. Young internodes subterete, ribbed. Leafy. Monoecious. Dichasia 3-flowered, central male, in sessile to pedunculate bracteal cups. Berries stipitate, smooth, n=14. South African shrublands., V. rotundifolium L.f., V. pauciflorum L.f., V. schaeferi Engl. & K. Krause.

Group 4. Viscum tuberculatum group. Stems terete to angular, ribbed. Leafy. Monoecious. Dichasia 3-flowered, unisexual or the central flower female, in sessile bracteal cups. Berries sessile, tuberculate when young, n=12 or 23. Eastern Rift zone and SE. African coastal region, in drier forests and associated bushland. V. tuberculatum A. Rich., V. obovatum Harv.

Group 5. Viscum decurrens group. Internodes strongly flattened, ribbed. Leafy. Monoecious, with sex ratio skewed to female. Dichasia usually unisexual, 3(-5)-flowered in pedunculate bracteal cups (longer in female). Berries stipitate, smooth. Congolian forest, only on Symphonia. V. decurrens (Engl.) Baker & Sprague.

Group 6. Viscum album group. Young internodes rounded to only slightly flattened. Leafy, the base with a distinct cavity. Dioecious, with skewed sex ratios. Dichasia terminal and lateral or more commonly all terminal, the branching then strongly dichotomous, 3-flowered or males single, sometimes superposed. Bracteal cups pedunculate. Flowers unusually large. Berries stipitate, smooth, n=10. Atlas Mts. of North Africa., V. album L., V. cruciatum Sieber ex Boiss..

Group 7. Viscum obscurum group. Internodes angled and flattened at first, becoming rounded. Leafy. Dioecious. Dichasia of male (1-)3(-5)-flowered in sessile bracteal cups; female 1-flowered in sessile to pedunculate cups. Berries sessile to generally stipitate, smooth, n=15, 14, 12 or 11. South African woodlands and shrublands. V. obscurum Thunb., V. oreophilum Wiens, V. crassulae Eckl. & Zeyh., V. subserratum Schltr.

Group 8. Viscum longiarticulatum group. Internodes strongly flattened, slightly ribbed. Leafy, but deciduous. Dioecious. Bracteal cups shortly pedunculate. Male flowers in 3s; female single. Berries sessile, tuberculate when young. East African basement-complex mountains (Usambaras). V. longiarticulatum Engl..

Table 3 continued.

Group 9. Viscum shirense group. Stems strongly flattened, often ribbed. Leafless. Dioecious. Dichasia with male flowers in 3s or single; females single. Berries smooth or tuberculate, stipitate. n=14. Eastern and southern African forests, woodlands and shrublands. V. engleri Tieghem, V. anceps E. Mey. ex Sprague, V. combreticola Engl., V. shirense Sprague, V. goetzei Engl., V. cylindricum Polh. & Wiens, V. congdoni Polh. &: Wiens.

Group 10. Viscum menyharthii group. Stems rounded, except sometimes on youngest internodes. Leafless. Dioecious. Dichasia with male flowers in 3s, rarely 1 or more than 3; female single. Bracteal cups sessile. Berries sessile to shortly stipitate, smooth or tuberculate. n=14. Zambezian and East African woodlands. V. menyharthii Engl. & Schinz, V. chyuluense Polh. & Wiens, V. littorum Polh. & Wiens, V. verrucosum Harv., V. subverrucosum Polh. & Wiens, V. calvinii Polh. & Wiens, V. gracile Polh. & Wiens, V. griseum Polh. & Wiens, V. continuum E. Mey. ex Sprague, V. hildebrandtii Engl., V. tenue Engl...

Group 11. Viscum bagshawei group. Stems flattened and ribbed to terete and smooth, occasionally (V. minimum) very reduced and largely endophytic. Leaves rudimentary or lacking. Monoecious, with male flowers often sparse, or dioecious (V. capense, V. dielsianum). Dichasia usually 1-flowered (3-flowered in V. minimum) in sessile or rarely pedunculate (V. grandicaule) bracteal cups. Anthers often fused into synandria in monoecious species. Berries sessile to stipitate, smooth to tuberculate. n=14 (V. minimum), 28 (V. bagshawei, V. schimperi) or 10 (V. capense, V. hoolei). Widespread in forest and drier habitats, but often with restricted host ranges., V. dielsianum Dinter ex Neusser, V. capense L.f., V. hoolei (Wiens) Polh. & Wiens, V. minimum Harv., V. grandicaule Polh. & Wiens, V. schimperi Engl., V. bagshawei Rendle, V. loranthicola Polh. & Wiens, V. iringense Polh. & Wiens

Finally in Asia, there is a small group of species ranging from Sri Lanka to Australia, similar to the V. orientate group, but with the middle flower of the cymule male, the laterals female, rather than vice versa (Figure 1.6). There are apparently six species in the V. capitellaum group and they are mostly epiparasites of other mistletoes. V. capitellatum occurs in India and Sri Lanka, V. trilobatum in southern India, V. yunnanense in southern China, V. katikanum in Papua New Guinea and V. whitei in northern and north-eastern Australia. V. bancroftii, has the leaves reduced to scales, but is sympatric with and possibly even conspecific with V. whitei (Barlow 1983c).

Among the Asian groups of species, the Viscum album group is not closely matched and all are are monoecious. Among present-day species, the V. album group is most similar to a small group of dioecious species in tropical Africa, the V. congolense group (Table 3; Figure 1.8 and Figure 2). The three species of this complex have discrete ecogeographic ranges, V. congolense in the Guineo-Congolian lowland forests of equatorial Africa, V. fischeri and V. luisengense in the montane forests of East Africa. Like the V. album group, these species are dioecious and have skewed, strongly female biased, sex ratios. In Africa the affinities of the V. congolense group have been associated with V. triflorum, a protean species widespread on the continent and offshore

Figure 2 Fruiting branches, xl, of 1, A, Viscum congolense; 2, B, V. fischeri; 3, C, V. luisengense. Drawn by Christine Grey-Wilson and reproduced from Polhill and Wiens (1998).

islands. V. triflorum has the flowers generally in triads in a bracteal cup, with the central flower male and the laterals female or the triads unisexual (Figure 1.7 and Figure 3), as in the other groups of monoecious species in Africa. The V. congolense group has inflorescences comparable to the V. triflorum group except that they are

Sexy Tepal
Figure 3 Viscum triflorum. 1, A, fertile nodexl; 2, B, staminate triadx6; 3, C, tepal and porate antherx8; 4, D, pistillate triad x6; 5, E, detail of female flowe rx12; 6,F, fruitx4. Drawn by Christine Grey-Wilson and reproduced from Polhill and Wiens (1998).

always unisexual and on separate plants and for a tendency to have more than three flowers in the bracteal cup, especially on female plants. The adventitious flowers and the unbalanced sex ratios compare well with the V. orientale group of continental Asia and suggest that perhaps the V. congolense group may have more affinity with Asian elements than previously supposed.

These putative affinities receive strong support from the cytology. V. triflorum has a basic chromosome number of n=14, characteristic of the family as a whole. Progressive dysploid increase and reduction is commonplace in the genus. Dioecious species in the genus characteristically have sex-associated and floating chromosome translocations that are virtually absent in monoecious species (Wiens and Barlow 1979, Barlow 1981, 1983a). This suggests that the translocations are primarily associated with the origin and establishment of dioecy, by bringing non-allelic male-and female-determining factors into genetic linkage (Barlow 1997). The smallest and simplest translocations occur in other African dioecious species with x=14 (Wiens and Barlow, 1979) and these belong to several morphological groups mentioned in the discussion below.

The three African species of the Viscum congolense group are dioecious with skewed, strongly female biased sex ratios, and a unique sex-determining system in which the male sex-determining factors are located on a translocation chain of chromosomes instead of rings, as in other dioecious species of the genus. These have been described for V. fischeri (Barlow and Wiens 1976) but the nature of the chromosome chains and general features of the meiotic genomes in Viscum congolense and V. luisengense have yet to be described. Male plants of V. fischeri have 2n=23 and constantly produce seven bivalents and a multivalent chain of nine chromosomes at meiosis. Regular assortment results in transmission of 11- and 12-chromosome genomes via the pollen. Female plants have the chromosome number 2n=22 and are homozygous for the 11-chromosome genome. The multivalent chain in the males is a consequence of reciprocal translocations, one of which was Robertsonian and one of which involved the chromosome carrying the sex-determination factors. There is a constant female-predominant sex ratio of approximately 1:2, possibly maintained by gamete selection; the genes involved may have been linked with the sex-determination mechanism through the translocation system

V. album and V. criuciatum have x=10, a sex-associated ring of 8 is most common, 10 occurs and 12 is rare (Barlow et al. 1978, Aparicio 1993). The process of enlargement of the translocation complexes through incorporation of additional translocations has thus occurred along with the dysploid reduction in chromosome number.

On present evidence it would seem that the Viscum album group diverged at about the same time as the Viscum congolense group at a fairly early stage in the evolution of dioecy, which initiated a significant secondary radiation of the genus in Africa and Madagascar. The Asian groups seem relatively discrete, but there is little obvious sequential arrangement among them or with the African elements. The spread of the Viscum triflorum group may have occurred relatively early. V. triflorum has a very scattered distribution in the African forests and also occurs at considerable distance from the main distribution on off-shore islands. Much of the radiation in

Madagascar probably stems from the V. triflorum group (Wiens 1975; Balle 1964a, b) and has resulted in numerous dioecious species, some with x=13. On the mainland V. triflorum occurs in forests of very different types, both wetter and drier, lowland and montane. This sort of distribution is characteristic of species in many groups of plants, including Loranthaceae, that seem to have established in Africa prior to the major rifting and uplift of the continent around and somewhat prior to the Miocene (Polhill and Wiens 1998). By contrast the species of the V. congolense group are restricted to different modern ecosystems, V. congolense in the equatorial lowland forests of the Guineo-Congolian region, V. fischeri and V. luisengense in the Afromontane region along the eastern Rift Valley, V. fischeri in the drier rain-shadow forests in the Kenya highlands and V. luisengense in the wetter types of forest in southern Tanzania. Both have rather small present-day distributions. It seems possible that the progenitors of this group migrated from continental Asia around the Miocene, some 15 million years ago, when the land-surface of Africa had been transformed by major uplifting and rifting and land-connections were re-established with Asia after a separation of some 25 million years by the Tethys Sea (Grove 1983). The Viscum album group could well have originated in the general Sino-Himalayan region about this time or a little earlier and spread with the Laurasian floristic components outwards to Japan and North Africa and subsequently into northern regions. It is not possible to speculate further from what we now know, but techniques in molecular systematics are rapidly developing to trace affinities at species level and a more rigorous assessment of the position of the V. album group should soon be feasible.

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