Plant Evolution in the African ‘Sky Islands’: Evidence from Fossil Calibrated Molecular Dating and Amplified Fragment Length Polymorphism
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Date
2012-07
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Addis Ababa University
Abstract
The afro-alpine region in Eastern Africa encompasses the elevated plateaus and
mountains in Ethiopia and the isolated high mountain peaks in Kenya, Tanzania,
and Uganda, often referred to as biological 'sky islands'. The age of the flora on
these mountains has been debated. Based on the high level of endemism, it has
been suggested that the flora is old and might represent Tertiary relicts. Others
have argued that some plant groups are young and have colonized these
mountains during the Pleistocene. The impact of the glacial-interglacial
oscillations of the Pleistocene on the origin and geographical structuring of the
genetic diversity in many temperate plant species has been well documented,
whereas only few studies for the afro-alpine plant speices have so far been
conducted. Here we use a combination of molecular dating and phylogeographic
analysis of selected plant species to provide insight into the evolution of plants
in the afro-alpine region. A fossil calibrated multilocus species tree based on
two nuclear and three plastid DNA regions was used to estimate the age of the
afro-montane endemic Lychnis (Caryophyllaceae). Furthermore, Amplified
Fragment Length Polymorphism (AFLP) fingerprinting was used to assess the
genetic diversity and population genetic structure of seven plant species: two
heather species from the ericaceous zone, Erica arborea, E. trimera (Ericaceae),
and five species from the afro-alpine zone proper: Carex monostachya, C.
runssoroensis (Cyperaceae), Lysimachia serpens (Primulaceae), Dicrocephala
chrysanthemifolia and Haplocarpha rueppellii (Asteraceae). The multilocus
species tree analysis have placed the mean age of the afro-montane Lychnis to
2.28-5.8 Myr (million years) corresponding to late Miocene to the late Pliocene
and thus indicating a Tertiary relicts. The results from the AFLP analysis
showed that all investigated species have very low levels of overall gene
diversity, suggesting bottlenecks in small refugial populations during
unfavourable climatic periods. The populations of E. trimera and H. rueppellii
showed relatively distinct geographical structuring of the genetic variation,
suggesting long-term isolation of the former in at least three separate places and
of the latter in two refugial populations. The lack of common phylogeographic
patterns indicates that different species have responded individually to the
Pleistocene climatic oscillations.
Key words: AFLP, Afro-alpine, Afro-montane, Lychnis, Erica arborea, Erica
trimera, Carex monostachya, Carex runssoroensis, Lysimachia serpens,
Dicrocephala chrysanthemifolia, Haplocarpha rueppellii, Molecular dating,
Phylogeography, Plio-Pleistocene, Tertiary-relicts.
Description
Keywords
AFLP, Afro-alpine, Afro-montane, Lychnis, Erica arborea, Erica trimera, Carex monostachya, Carex runssoroensis, Lysimachia serpens, Dicrocephala chrysanthemifolia, Haplocarpha rueppellii, Molecular dating, Phylogeography, Plio-Pleistocene, Tertiary-reli