Evolutionary history, genetic diversity and conservation implications of selected afro-alpine taxa
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Date
2012
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Sokoine University of Agriculture
Abstract
The tropical East African and Ethiopian mountains are famous for their exceptionally
unique and high biodiversity. The flora on these mountains offers good examples of
distinct adaptations to different altitudes as well as evolutionary differentiation, hence an
ideal natural laboratory for studies on the dynamics of biodiversity. In this study the
genetic diversity, evolutionary history and conservation implications of selected taxa
occurring on these mountains were assessed. The objectives were to: 1) determine the
level of intraspecific and interspecific genetic diversity of selected afro-alpine plant
species, 2) explore the potential of AFLP markers for delimiting species and
reconstructing the evolutionary relationships among some of the selected afro-alpine
plants by comparing the results with those of previous morphological and molecular
studies and 3) reconstruct the phylogeographic structure of the selected afro-alpine plant
species.
Data for this study were collected from 1 ha (10000 m 2 ) plots on 14 mountains around
the region. Amplified Fragment Length Polymorphism (AFLP) markers were used to
characterize the genetic patterns of the selected taxa. Six hundred eighty nine individuals
from 154 populations (13 species, a total of 1168 AFLP markers with 97.9%
reproducibility) of giant lobelias, 33 individuals, nine populations (two species, 172
AFLP markers, 97.86% reproducibility) of Deschampsia, and 153 individuals 36
populations (458 AFLP markers, 97.4% reproducibility) of Koeleria capensis were
successfully analyzed.
Mean within-species (H T ) and within-population (H S ) genetic diversities were generally
low across all species. Among the thirteen species of giant lobelias, the least diversityiii
was observed in the most widely distributed species, L. giberroa (H T = 0.0751), followed
by L. rhynchopetalum (H T = 0.0832). On the contrary, most diversity was observed in the
narrow endemics L. bequaertii (H T = 0.2522) and L. thuliniana (H T = 0.2118). The low
genetic diversity among L. giberroa populations may be attributed to bottlenecks
following reduction of its montane forest habitat by human activities, which may have
been less influential in the high-alpine Ruwenzori habitat of the local endemic L.
bequaertii. There was however no correlation between the age of mountains and levels of
genetic diversity, suggesting that the current populations on the older mountains
originated from colonization episodes taking place long after their formation.
Except for Deschampsia spp. populations, the rest of the molecular and/or morphology-
based recognized species were found to be genetically distinct. In Deschampsia, the
individuals identified as the endemic D. angusta were not genetically distinct from those
of D. caespitosa sampled in the same mountain, Ruwenzori, suggesting that the
characters used to distinguish these species may reflect phenotypic plasticity rather than
taxonomically significant variation. For giant lobelias, the relationships among species
inferred from the primarily nuclear AFLP data were, with some notable exceptions,
consistent with relationships earlier inferred from morphology and/or plastid DNA
restriction site polymorphisms. High-altitude-restricted Lobelia species were intermixed
with species occurring in the forest zone in the AFLP-based tree, supporting a main
scenario of initial expansions of ancestral forest populations followed by parallel high-
altitude adaptation and speciation in different mountain groups. However, the results did
not support the proposed instances of hybrid speciation in this group while suggesting the
most distinct intermountain vicarious patterns among the giant lobelias to be primarily
high-alpine. For Koeleria capensis, there was neither distinct geographic structuring of
the genetic variation nor support for recognition of infraspecific taxa. The resultsiv
suggested that the afro-alpine populations of Koeleria capensis might have arisen by
long-distance dispersal through Ethiopian mountains followed by intermountain dispersal
into the tropical East African Mountains.
Given the current genetic structure and patterns, monitoring the most diverse and
genetically most distinct populations of each species in situ and gemplasm tests for ex
situ conservation are suggested in order to increase the probability for long-term survival
of the studied plants and afro-alpine ecosystem at large. The study highlights that
different afro-alpine species may have experienced very different phylogeographic
histories and that long-distance dispersal among the isolated afro-alpine 'sky islands' can
be more frequent than traditionally thought. Generally, the study demonstrates the need
for further taxonomic exploration of the afro-alpine flora, in particular of taxa described
as endemic.
This thesis is an outstanding contribution to knowledge as it provides for a refined
evolutionary history and taxonomy of the previous morphology and molecular-based
studies. For example the discovery that earlier proposed hybrid species L. bequaertii and
L. bambuseti are actually not hybrids; the fact that Deschampsia angusta did not separate
from D. caespitosa provides for a new idea that the previously known endemic D.
angusta only from Ruwenzori mountains can actually be D. caespitosa; the knowledge
about little genetic diversity within and among most of the studied species is a crucial
contribution to conservationists for improved conservation strategies and; identification
of areas that need further research such as the phylogenetic position of Lobelia thuliniana
helps increase interests of natural biologists to work more on the studied taxa.
Description
PhD Thesis
Keywords
Afro-Alpine taxa, Taxa evolution history, genetic diversity, conservation implication, intraspecific genetic diversity, interspecific genetic diversity, phylogeographic structure