Gnidia L., variously estimated to contain 100–160 species, is the largest genus in
the sub-cosmopolitan family Thymelaeaceae. Most species are shrubby, and
occur in tropical and southern Africa, with one species reaching southern India and
Sri Lanka, and 14 species endemic to Madagascar. Assorted segregate genera
have been established using characters considered by some as too few, too trivial
or unreliable. Generic limits have been contentious with authors following either a
narrower concept of Gnidia or a broader circumscription within which segregate
genera are placed in synonymy under Gnidia. Regional treatments for African and
Madagascan floras have been published over the last century until very recently,
but the genus was last revised in its entirety 153 years ago. Today, a broad-based
concept of Gnidia is generally recognised, but there is no modern infrageneric
classification, and species relationships are poorly understood.
Homogenous groups of species are identified by their similarities of leaf length and
width or bract length and width ratios. Species comprising the homogenous groups
for leaf ratios differ to those comprising the homogenous groups based on bract
ratios, and there is no correlation between leaf and bract length and width ratios.
This suggests that the factors influencing leaf diversity differ from those influencing
bract diversity. Bracts differ most from leaves in species with capitate
inflorescences, and involucres of several layers of bracts likely protect
reproductive organs (flowers) in heads. Previously overlooked morphological and
micromorphological details, and morphometric analyses of leaf, bract and floral
dimension data help define individual species, and clades of species derived from
phylogenetic analyses of molecular data. Evidence from a phylogenetic analysis of
nuclear ribosomal and plastid DNA sequence data confirms the polyphyly of
Gnidia. Three lineages contain Gnidia species and species of genera from
southern Africa, southern South America or Australia, while another lineage
corresponds largely to the previously recognized genus Lasiosiphon. The genus
Lasiosiphon is reinstated characterised by flowers mostly in heads, bracts different
from the leaves, and the presence of smooth hairs; it now includes species with
tetramerous flowers as well as ones with pentamerous flowers. Gynodioecy is
recorded for the first time in a single species and represents the first documented
example of sexual polymorphism involving unisexual flowers in Gnidia and sub-
Saharan Thymelaeaceae.
The findings of this thesis are discussed in terms of their phylogenetic value and
contribution to our better understanding of the generic limits of Gnidia and its
relationships with other southern hemisphere Thymelaeoideae. The
circumscription and generic affinities of Gnidia as suggested by results presented
in this thesis are compared to previous classification systems for congruence and
dissimilarity. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/770 |
| Date | January 2010 |
| Creators | Beaumont, Angela Jane. |
| Contributors | Edwards, Trevor J., Van Staden, Johannes. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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