Population genetics and taxonomy of important medicinal tree species of the genus Warburgia
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Date
2007-07
Authors
Muchugi, Alice Njeri
Journal Title
Journal ISSN
Volume Title
Publisher
Kenyatta University
Abstract
The genus Warburgia (Canellaceae) has only four species (W. elongata Verdc., W.
salutaris (Berto; f). Chiov, W. stuhlmannii Engl. and W ugandensis Sprague,
which occur in east and central Africa. These species are of valuable medicinal
importance to the local communities where they occur and have also gained
interest from international herbal markets. The taxonomy of the species in this
genus is conflicting with some authorities referring to the distinct species as
con specific or synonyms. Genetic diversity present in wild populations of these
species is under great threat due to unsustainable harvesting for medicines and
indiscriminate felling trees and therefore there is an urgent need for conservation
of these species. On farm planting is one of the best alternatives for ensuring the
survival of the species and sustainable harvesting for their medicinal products. This
research aimed to resolve the taxonomic confusion using molecular markers and to
determine the genetic relationships within and among populations of the species in
the genus Warburgia in order to guide strategies for their conservation and
sustainable utilisation. Ecological survey showed reduced populations sizes of
Warburgia species, which has being caused by land clearance for farming and
settlement, timber, charcoal and fuelwood harvesting. The study showed a
discontinuous tree distribution where young plants were not replacing mature trees
coming to the end of their reproductive life. Significant differences were found in
fruit and seed size between W ugandensis and W stuhlmannii. The chromosome
number in W ugandensis and W stuhlmannii was found to be 2n=22 for both
species, which implied that the species have conserved the basic chromosome
number in the genus despite adapting to divergent ecological zones. The genetic
relationships among Warburgia species were determined using the AFLP
technique. Genomic DNA for the AFLP analysis was obtained through the CTAB method, which proved to be the best in efficiency and cost among the tested
methods. Analysis of molecular variance (AMOV A) on data obtained revealed
most genetic variation being among individuals within populations (63%, P<
0.0001), but variation among populations (37%, P< 0.0001) was highly significant
as well. High genetic diversity was found within the studied populations. A
phenetic tree and constrained ordination analysis illustrating the relationship
among populations of Warburgia species showed a clear distinction among
Warburgia species. A populations' subgroup of W ugandensis from Uganda and
western Kenya clustered away from the rest of W ugandensis implying genetic
distinctiveness while the coastal species (W salutaris and W stulhmannii) showed
little genetic differentiation. The utility of AFLP and RAPD techniques to resolve
intraspecies genetic variation in W. ugandensis populations was assessed and the
two techniques showed high correlation (r=78, P< 0.001). However, AFLP
technique had a higher marker index (24.96) than RAPD analysis (19.70).
Assessment of type of mating system in W. ugandensis using the AFLP markers
showed the species to be predominantly outcrossing (88.9%) with a low level of
selfing (11.1 %). Implications of these findings to the taxonomy and genetic
management of species in genus Warburgia are discussed.
Description
A thesis submitted in fulfillment of the requirement for the award of the
degree of Doctor of Philosophy in Molecular Genetics in the department of
Biochemistry and Biotechnology of Kenyatta University, Kenya. July 2007