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Figure 4 Groups of African Species of Viscum. Strict Consensus Tree from Cladistic Analysis of Characters and Character States in Tables 4 and 5 using Henig 86...og=outgroup (V. triflorum).

Unlike the Asian representatives, the leafless species are not so obviously related to the species with normally developed leaves. As indicated above, the dioecious Viscum congolense and V. album groups have derived features in common and may have their origins in the Sino-Himalayan region. The other groups seem to be more related to the V. triflorum group in Africa, either directly or indirecty.

Four of the derived groups are monoecious. The Viscum rotundifolium group has three species in southern Africa in mixed woodland and bushland. V. rotundifolium is widespread from the Cape to southern Angola and north-eastwards to central Zimbabwe, crossing several phytochoria and occurring on a wide range of hosts. V. pauciflorum occurs only in the mountains of the south-western Cape Province. V. schaeferi links with the V. tuberculatum and subsequent groups in the cladogram, but is closely similar to V. rotundifolium. It occurs in drier woodland and mixed bushland in Namibia, with isolated populations in the northern Cape and North-western Province. It commonly grows on Boscia and mimics that host to some extent in the clustering and shape of its small leaves. V. rotundifolium is closely sympatric with V. pauciflorum in the mountains east of Worcester and with V. schaeferi in the northern Cape and southern Namibia. They all have n=14 and inflorescences like the V. triflorum group, but with ribbed, subterete internodes and relatively small, somewhat thickened leaves.

The Viscum tuberculatum group has just two closely related species. V. tuberculatum is widespread in drier forests and associated bushland the length of the Eastern Rift Valley in a broad zone from Ethiopia to the Northern Province of South Africa and Angola. V. obovatum is closely related and extends the range south-east-wards along the coast from southern Mozambique to the eastern Cape and Swaziland. Both occur on a wide variety of hosts. They are vegetativeiy similar to the V. rotundifloum group, but the inflorescences and berries tend to be sessile rather than stalked and the fruits are tuberculate. More importantly they are evolutionary separated from that group and from each other by their chromosome numbers, n=23 in V. tuberculatum and n=12 in V. obovatum. The group would seem to be independently derived from the V. triflorum group.

Viscum decurrens (Figure 5.A) is an isolated species with unusually flattened internodes for a leafy species. It occurs only on Symphonia globulifera L.f. and, although often overlooked, has quite a wide range in the Congolian forests from southern Nigeria to Angola and western Uganda. It is monoecious like V. triflorum, but the dichasia are usually unisexual, sometimes with more than 3 flowers and the male inflorescences much sparser than the females, suggesting links to the V. orientale and V. congolense groups. Two other rarely collected and apparently local western forest species may belong to this affinity. V. petiolatum, known only from the type gathering made in montane forests on the Western Rift of eatern Zaire, is provisionally included in the V. triflorum group. It is monoecious, with n=14, but usually has unisexual dichasia. V. grandicaule (Figure 6.A & 6.B) is another isolated species from Pagalu (Annobon) Island of the west coast of Gabon. It is also monoecious, with unisexual and 1-flowered dichasia, the internodes are strongly flattened and the leaves are reduced to scales. It might belong in this affinity or with the V. bagshawei group, a provisional assemblage of dioecious and anomalously monoecious species discussed at the end.

Figure 5 Fruiting branches, x1, of A, Viscum decurrens; B, V. longiarticulatum. Drawn by Christine Grey-Wilson and reproduced from Polhill and Wiens (1998).
Figure 6 Viscum grandicaule. A, staminate flowering nodex1; B, fruiting branchletx1. V. calvinii. C, fruiting branchletx1. V. engleri. D, Fruiting branchletx1. Drawn by Christine Grey-Wilson and reproduced from Polhill and Wiens (1998).

Figure 7 Viscum minimum. Habit of plant growing on Euphorbia sp. Drawn by Marguerite Scott and reproduced from Polhill and Wiens (1998).

Viscum minimum (Fig. 7) is also of unknown affinity. It is largely endophytic, growing within the stems of two species of succulent Euphorbia in the eastern Cape Province of South Africa. The visible parts are minute, about 1 mm high including the inflorescence. The fruit is at least twice the height of the plant and many times its volume. Despite the extreme physiological specialisations and reduction in size, the species retains many reproductive features characteristic of the least specialised species in Africa. It is monoecious, with n=14, bearing terminal and lateral 3-flowered dichasia, the central flower male, the laterals female. This places it closest to the V. rotundifolium group, but it is also fairly close to the least specialised representatives of the V. bagshawei group, where it is placed in the cladogram.

There are four reasonably discrete groups of dioecious species, two with normal leaves, two with reduced leaves, each subdivided into groups with the internodes rounded or strongly flattened. Most species examined have n=14, but in the V. obscurum group n=15, 14, 12 or 11. Twelve species examined by Wiens and Barlow (1979) had translocations that were smaller and simpler than in the V. album and V. congolense groups. The sex-associated complex consistently appeared in males as rings of 4 or 6. In addition most had a floating ring of 4.

The Viscum obscurum group, with rounded internodes and well-developed leaves, has four species in South Africa. Each has a different chromosome number. V. oreopbilum, with n=14, occurs in bushland associated with forest edges in the Drakensburg Mountains of Swaziland and the north-eastern provinces of South Africa. V. obscurum, with n=15, occurs in forest associations nearer the coast from near the

Cape to the Northern Province, while V. subserratum, with n=11, occurs in drier woodland from northern Kwa Zulu to the Northern and North-West Provinces. V. crassulae, with n=12, is in the Eastern Cape Province and almost exclusively on Portulacaria afra, rarely on Euphorbia.

Viscum longiarticulatum (Figure 5.B) has not been examined cytologically, but is a seemingly isolated species without clear affinities. It has flattened internodes, the small leaves are deciduous, the bracteal cups are pedunculate, but, like other dioecious groups, the male flowers are in threes and the females are single. It could be envisaged as link between the V. triflorum group and the V. shirense group.

The two groups with leaves reduced to scales, the V. shirense group with flattened stems, and the V. menyharthii group with rounded stems, have eighteen species between them and nearly the same number of species in each. They all have n=14 and inflorescences similar to the V. obscurum and V. longiarticulatum groups. The V. shirense group occurs in the montane and riverine forests of eastern and south-eastern

Figure 8 Fruiting branches, x1, of A, Viscum cylindricum; B, V. congdonii; C, V. goetzei; D, V. combreticola. Drawn by Christine Grey-Wilson and reproduced from Polhill and Wiens (1998).

Africa, with V. combreticola (Figure 8.D) extending further into the drier wodlands, generally on Combretum or legumes. V. engleri, the species with the most robust habit, is restricted to the forests of the Usambara Mountains of NE. Tanzania. V. congdonii (Figure 8.2), V. cylindricum (Figure 8.A) and V. goetzei (Figure 8.C) occur in the montane forests between southern Tanzania and eastern Zimbabwe. V. goetzei is generally, at least, an epiparasite of Loranthaceae. V. shirense and V. anceps occur in riverine forest and associated bushland, V. shirense in tropical Africa from Zaire to Mozambique and V. anceps in South Africa from Kwa Zulu to the eastern Cape Province.

The V. menyharthii group occurs principally in the woodlands of the Zambezian region, with outliers in the Acacia woodlands of the Somali-Masai region and the Karoo. V. tenue is an exception, occurring in the montane forests of Tanzania, but could be linked with the V. shirense group. The main species are separated ecogeographically, either allopatric or on different hosts. Five of them occur only on species of Acacia. V. chyuluense, on the Kenya-Tanzania border region, is apparently an obligate parasite of Loranthaceae. V. menyharthii generally occurs on Ficus and always so in some areas. When on other hosts physiological races may have been established.

There is a residual group of leafless species that seem to have reverted in part to the monoecious condition. The species fall into two subgroups, one tropical, the other in South Africa. The inflorescences are produced with the dichasia one-flowered and the male flowers sparse. The monoecious species tend to have the anthers fused into a central synandriurn (Kuijt et al. 1979). The tropical species of the V. bagshawei group are similar to the V. shirense and V. menyharthii groups and often confused with them. V. bagshawei is a polyploid, with n=28, and superficially similar to V. hildebrandtii in the V. menyharthii group, and like it is a parasite of Acacia. It occurs in mesic Acacia woodland at higher elevations along the Eastern Rift from Ethiopia to the Kenya highlands and westwards to the northern part of the Western Rift in eastern Zaire, Rwanda, Burundi and Uganda. It is replaced by V. iringense in the drier Acacia woodlands of central and southern Tanzania. V. loranthicola occurs a little further west and south in the Zambezian region and, as the epithet implies, is an epiparasite of Loranthaceae. V. schimperi has n=28, like V. bagshawei, but the anthers show no more than a tendency to fuse and the stems are flattened more like the V. shirense group. It occurs in drier forests along the Eastern Rift in Ethiopia and Kenya. V. grandicaule, from Pagalu Island, is much more robust with very flattened stems and has not been examined cytologically. It might belong here or in the affinity of the V. decurrens group, as mentioned above.

In South Africa there is another series showing similar modifications. V. capense is a dioecious species with n=10. It forms small shrublets principally in the winter rainfall areas of South Africa from the Cape to central Namibia, but with outlying populations in the Northern Province of South Africa. The scale-leaves are conspicuous and distinctive. V. hoolei replaces V. capense from the eastern Cape to the southwestern part of the Free State and Lesotho. It is so similar to V. capense that it has not been recognised as distinct until recently, but is monoecious with the anthers forming synandria. The male flowers are very sparse, so that populations often seem to be wholly female. V. dielsianum has also been confused with V. capense in the past. It has similar distinctive scale-leaf rudiments, but the dichasia tend to be 3-flowered. It occurs in the Northern Cape Province to SW. Namibia. It is odd that the secondarily monoecious species all tend to have developed synandria, but the phenomenon does seem to have arisen twice from fairly closely related stocks.

Overall Viscum seems to have a substantial genepool in Africa. The endemic African species may have arisen from two major immigrant stocks, the V. triflorum stock, which has radiated mostly down the Eastern Rift Valley to South Africa, and the V. congolense complex, mostly in the forests of equatorial Africa. The V. album group comes closest to the latter but may have affinities with the groups in tropical Asia also.

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