|dc.description.abstract||ADEKUNLE ADEBOWALE Ph.D. thesis, University of KwaZulu-Natal, 2014
Strychnos L. is the largest genus of the pantropical or subtropical family
Loganiaceae with about 200 species. Their habits range from trees and shrubs in
open areas to lianas in rain forests. The genus is well-known as a source of alkaloids
such as strychnine and brucine and other allied compounds, all of which have been
used medicinally and in curare formulation for centuries. While taxonomic
circumscription of the genus has never been contentious, there is no consensus
about infrageneric affiliations, the latest of which recognises 12 sections based on
morphological characters. Recent molecular evaluation of the genus on a global
scale with the internal transcribed spacer (ITS) marker suggests that many of the
currently recognised sections are not monophyletic.
An understanding of regional patterns of evolution, which is relevant for biodiversity
conservation, requires an in-depth study of the focus group on a regional scale.
Using a multiplicity of approaches from morphological and molecular to
biogeographical, this study is an attempt at elucidating diversity patterns at different
levels among the southern African species of Strychnos.
Various combinations of morphological attributes from branches, leaves, flowers and
fruits distinguish seemingly homologous clusters of species, sometimes supported
by molecular data. A lack of molecular support for a hypothetical relationship may
indicate case(s) of convergent evolution in these features across the taxa involved.
Molecular phylogenies based on the ITS and chloroplast markers confirm the nonmonophyletic
nature of all but section Spinosae. Proposals for sectional recircumscriptions
of the genus are provided.
Patterns of speciation within Strychnos suggest a Miocene origin in the rain forests
along the South America/Guinea-Congolian axis. Within the southern African
subcontinent, the evolution of the genus carries a strong ecological signature along
either the forest or savanna biome, with many accompanying morphological
adaptations for the respective habitats. The non-synonymy of S. gerrardii with S.
madagascariensis is demonstrated with multiple sources of data, as a case of
integrative taxonomy succeeding where single-source data approaches might have
failed. Routes to current distribution of the genus in southern Africa are hypothesised
to involve a combination of palaeo-climatic oscillations and allopatric speciation,
consistent with the process indicated in many other plant groups for the region.
The findings are discussed in the wider context of their implications for taxonomy
and biodiversity conservation in the face of climate change, food security and other
relevant issues in systematics.||en