An investigation into the systematics and population diversity of Tylosema (Leguminosae)

Abstract

Tylosema esculentum, T. fassoglense, and T. angolense are legume species found across southern Africa that produce highly nutritious seeds and tubers. These species have significant agricultural potential but are severely understudied in the wild. As a result of this neglect the taxonomy of these three species is uncertain. Additionally, little is known about the genetic diversity within and between wild populations of the three species largely due to the fact that very few molecular markers have been developed in Tylosema. Both of these issues will greatly reduce the efficacy of any domestication attempts provided they are not addressed. This MSc addresses the taxonomic issues present in T. esculentum, T. fassoglense, and T. angolense. A molecular phylogenetic assessment using the chloroplast markers trnL-F intron and spacer and psbA-trnH intergenetic spacer, the high copy nuclear marker ITS, and the low copy nuclear gene SHMT was produced using Bayensian inference methods. To aid in future genetic diversity studies within the genus, microsatellite markers were also developed using genomic DNA from T. esculentum provided by Dr Chris Cullis, Case Western Reserve University, Ohio, USA. These markers were tested for amplification and polymorphisms in both T. esculentum and T. fassoglense populations to determine whether they were suitable for genetic assessments across the Tylosema genus. Phylogenetic analyses confirmed the species level description of the recently described T. angolense. Both T. angolense and T. fassoglense belong to a separate lineage to T. esculentum, which itself is split into two lineages: a Kalahari Desert lineage and a South African Highveld grassland lineage. This study has demonstrated that the evolutionary history of Tylosema is likely more complex than current taxonomic treatments indicate. Future research should entail molecular/genomic phylogenetic analyses across the entire genus in order to determine accurate species limits in Tylosema. A total of 46 microsatellite markers were developed using the program MSATCOMMANDER, five were selected for amplification and polymorphism detection through gel electrophoresis. All five markers successfully amplified in both T. esculentum and T. fassoglense, four of which were polymorphic in both species, one showing clear length polymorphisms between the two species. These results confirm that microsatellite markers designed in T. esculentum can likely be used across Tylosema and potentially other closely related genera. The results of this study can provide useful insight into the value of Tylosema species for agriculturalists and researchers investigated in their domestication. Researchers should take note of the phylogenetic relations between southern African Tylosema sp., especially with T. esculentum where Kalahari and Highveld populations belong to different lineages and may have different genetic/chemical properties relavent to food scientists and agricultural breeders. The SSR markers developed can also aid in assessing genetic diversity in these wild populations and help ensure than unique genetic lineages within Tylosema species are protected.