The molecular evolution of the spiral-horned antelope (Mammalia: Tragelaphini)

Willows-Munro, Sandi (2003-12)

Thesis (MSc)--Stellenbosch University, 2003.

Thesis

ENGLISH ABSTRACT: The evolutionary history of the African tribe Tragelaphini (spiral-horn antelope) is controversial. Past phylogenetic relationships among species were based on morphology or limited fossil evidence and are in conflict with mitochondrial DNA sequencing studies that have been conducted more recently. Although the group is distinguished from other African ungulates by the presence of spirally-twisted horns, the nine recognized extant species differ considerably in morphology, feeding habits and their habitat preference. The present study aims to resolve the phylogenetic uncertainties of the Tragelaphini using nuclear DNA sequence data derived from four independent DNA loci (MGF, PRKCl, SPTBN and THY). These data were combined with all previously published DNA sequences to produce a molecular supermatrix comprising approximately 6000 characters. Both parsimony and model based phylogenetic analyses of the nuclear DNA support the associations resulting from the analysis of mitochondrial genes. These findings suggest that the morphological characters previously used to delimit species within the group are subject to convergent evolution. The molecular phylogeny presented herein suggests that early members of Tragelaphini diverged from the other bovids during the mid-Miocene approximately 15.7 million years before present (MYBP). The common nyala (Tragelaphus enqest; and lesser kudu (Tragelaphus imberbis) representing the most basal species, separated from the other tragelaphids approximately 7.1 MYBP. This was subsequently followed by the radiation of those species adapted to a more tropical environment and they include the mountain nyala (Tragelaphus buxtom), bongo (Tragelaphus euryceros), sitatunga (Tragelaphus spekel) and bushbuck (Tragelaphus scriptus), and the arid adapted clade comprising the giant eland (Taurotragus derbianus), common eland (Taurotragus oryx) and greater kudu (Tragelaphus strepsiceros). It is thought that this split occurred at the Miocene-Pliocene boundary approximately 5.4 MYBP. The timing of evolutionary events within the tribe suggests climatic oscillations and subsequent biotic shifts as the major driving forces underpinning speciation in the tribe Tragalaphini.

AFRIKAANSE OPSOMMING; Die evolusionêre geskiedenis van die ras Tragelaphini (spiraalhoringwildsbokke) is kontroversieël. Vorige filogenetiese verwantskappe tussen die spesies is gebaseer op morfologie of beperkte fossiel bewyse. Meer onlangse studies, gebaseer op mitochondriale ONS nukleotieddata, is in teenstryding met baie van die evolusionêre hypotese afkomstig van morfologiese studies. Alhoewel die groep van die ander hoefdiere uitgeken kan word deur die aanwesigheid van spiraalvormige horings, verskil die nege hedendaagse spesies grootliks ten opsigte van morfologie, voedingswyse en habitat. Die hoof doelwit van hierdie studie was om die filogenetise verwantskappe tussen die Tragelaphini spesies te ontleed deur gebruik te maak van nukluêre ONS nukleotieddata afkomstig van vier onafhanklike ONS merkers (MGF, PRKCl, SPTBN en THY). Die data verkry is saamgevoeg by vorige gepubliseerde ONS nukleotidedata om 'n "supermatris" van sowat 6000 karakters te produseer. Parsimonie en modelgebaseerde filogenetise analise van die nukluêre ONS nukleotieddata het ooreengestem met die resultate van vorige mitochondriale studies. Hierdie bevindings dui daarop dat die morfologiese karakters wat voorheen gebruik is om die evolusionêre verwantskappe tussen die Tragelaphini spesies te ontleed onderhewig is aan konvergente evolusie. Die molekulêre filogenie wat hierin beskryf word stel voor dat die ras Tragelaphini gedurende die mid- Miocene, omtrent 15.7 miljoen jaar (MJ) gelede van die ander lede van die subfamilie Bovinae geskei het. Tragelaphus angasi en Tragelaphus imberbis, die mees basale spesies in die filogenie, het omtrent 7.1 MJ gelede van die ander lede van die Tragelaphini geskei. Hierdie skeiding is gevolg deur 'n split tussen die spesies aangepas vir 'n meer tropiese habitat (Tragelaphus buxtoni, Tragelaphus euryceros, Tragelaphus spekei en Tragelaphus scriptus) en die spesies aangepas vir 'n droë habitat (Taurotragus derbianus, Taurotragus oryx en Tragelaphus strepsiceros) Hierdie finale skeiding het gedurende die Miocene-Pliocene oorgang plaasgevind. Die tydsberekening van die evolusionêre gebeurtenisse wat binne die Tragelaphini ras plaasgevind het, gekoppel aan paleoklimaatdata, dui aan dat veranderinge in klimaat en die geassosieerde habitatveranderinge verantwoordelik was vir die spesiasie patroon wat ons vandag in die Tragelaphini ras waarneem.

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