Dr Yoshan Moodley
Pan-African phylogeography and adaptation in the bushbuck (Tragelaphus scriptus)
Tragelaphus scriptus is a phenotypically variable pan-African complex, collectively known as bushbuck. The smallest of the spiral-horned antelopes, the bushbuck is the most widely distributed ungulate on the African continent; occurring from as far west as southern Mauritania and Senegal, east across the Sahel to Ethiopia and Somalia and south in all countries to South Africa. Bushbuck are singularly unusual in their ubiquity.
Throughout their extensive distributional range, they inhabit, and appear to be adapted, to a wide variety of ecotypes including rainforest, wooded savanna, semi-arid to arid savanna, sub-desert, fynbos and montane forest. Bushbuck are dependent only on the cover offered by forests, dense bush and thickets (Dorst and Dandelot, 1970). They are usually found in the vicinity of permanent water but are able to subsist on dew in waterless areas (Kingdon 1997).
Evolution and adaptation
Adaptive phenotypic variation across their wide distribution is accordingly high with over 40 subspecies described (Haltenorth 1963, Dorst and Dandelot 1970, Ansell 1972, Skinner and Smithers 1990). In general (see Figure), the form of the holotype (T. s. scriptus) described in Senegal extends throughout West and Central Africa and is believed ancestral. Conspicuousness of striping pattern lessens as bushbuck radiated out of dense forests into the savannas of the Sahel (T. s. pictus, T. s. bor, T. s. uellensis) and South-Central Africa (T. s. ornatus).
The Rift Valley system prevents west to east radiation between Lake Albert in the north and Lake Tanganyika in the South. T. s. ornatus therefore continued to lose its striping pattern and increase in size as it radiated south, resulting in a morphocline (Grubb 2000), the end of which is inhabited by the stripeless T. s. sylvaticus of South Africa. It is unclear whether the lightly striped, larger bushbuck of East Africa were a result of the radiating intermediate forms T. s. ornatus from south of Lake Tanganyika or T. s. bor through the Bahr-el-Ghazal and Uganda.
The arc of mountains stretching from Mt. Meru in Tanzania to the Imatong Mountains in Sudan is a hotspot for bushbuck diversity, with five subspecies (T. s .barkeri, T. s. heterochrous, T. s. meruensis, T. s. insularis, T. s. haywoodi) described. The Ethiopian Highlands east and west of the Omo River and south of the Blue Nile are inhabited by T. s. meneliki. All montane forms are characterised by large size and longer hair.
The many phylogeographic hypotheses arising from such a wide geographic distribution can be tested with mitochondrial DNA sequences of the variable control region as well as single nucleotide polymorphisms (SNPs).
Molecular variation in this species may be linked to the biochemical mechanisms that enable survival in different habitats. A range of candidate genes have been identified in bovine and ovine studies which are potentially applicable to this species and where SNPs have already been characterised both in the introns and exons of genes involved in traits such as disease resistance (e.g. the Major Histocompatibility Complex), coat colour and pattern (e.g. melanocortin-1 receptor and bovine ectodysplasin-A) and fibre structure genes such as Keratin Associated Proteins and Stratifin.
These systems are currently under study in Prof. Bruford’s laboratory in domestic ungulates. This is a novel approach to molecular ecology, since nearly all studies in this field have concentrated on neutral DNA markers.
This study will not only have applied benefits pertaining to the conservation and management of bushbuck, but will also provide us with the rare opportunity of describing molecular evolution within a continentally distributed species right across its distribution. The latter may enable the understanding of fundamental patterns of gene flow, adaptation and incipient speciation in other continentally distributed or ubiquitous mammal species.
Ansell, W. F. H. 1972. Order Artiodactyla. In: Meester, J. and Setzer, H. W., eds, The mammals of Africa: an identification manual 15: 1-84. Smithsonian Institution Press, Washington D. C.
Dorst, J. and Dandelot, P. 1970. A field guide to the larger mammals of Africa. Collins, London.
Grubb, P. 2000. Morphocline evolution in ungulates. Yale University.
Haltenorth, T. H. 1963. Klassifikation der Säugetiere: Artiodactyla 1. Handbuch der Zoologie 8: 1 — 167.
Kingdon, J. 1997. The Kingdon field guide to African mammals. Academic Press, London
Skinner, J. D. and Smithers, R. H. N. 1990. The mammals of the southern African subregion. University of Pretoria.
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