Biological Consequences of Pleistocene Glaciations in East African Afro-Alpine Environment As Inferred From Molecular Data of Some Key Plant Species
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Date
2007-07
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Addis Ababa University
Abstract
The afro-montane/-alpine regions representing biological 'sky islands' comprise the high
mountains of Ethiopia and tropical East Africa. The vegetation of the regions is characterized by
fragmentation among the various mountain massifs with numerous local endemic species. Most
plant species in these ‘sky islands’ are quite different from those occurring in their respective
immediate surrounding lowlands. However, some primarily arctic-alpine plants also occur in the
mountains, as Arabis alpina. It has been suggested that this plant elements are Tertiary relict, but
a recent range-wide study of A. alpina suggested that the species colonized the region twice
during the Pleistocene period. In addition to such immigrant species, some other key plant
species, as Salvia merjamie and Cerastium afromontanum are very common in sub-alpine and
montane forests of the ‘sky islands. In this study, two overlapping phases of investigations have
been carried out on these three species using two basic molecular biological techniques: in the
first phase, the detail colonization history of A. alpina was investigated in the afro-alpine regions
based on chloroplast DNA (cpDNA) sequences, and in the second phase, the phylogeographic
structures and conservation status of the three species were studied using amplified fragment
length polymorphic DNA data (AFLP fingerprinting) obtained from 11 mountain systems, where
present-day populations are often limited to isolated mountain tops. In the first phase, the results
confirm the twice-into-Africa scenario. The Asian lineage is also confined to the mountains
closest to the Arabian Peninsula, on opposite sides of the Great Rift Valley (GRV), suggesting
long-distance dispersal of the A. alpina lineage. The African lineage is divided into two
phylogeographic groups with distinct geographic distribution. The observed pattern is consistent
with isolation of the African lineage in at least two interglacial refugia, located on separated
highlands, followed by range expansion in cooler period(s), when the afro-alpine habitat
extended further down the mountains. Several long-distance dispersal events across the GRV are
also suggested by haplotypes observed outside the area occupied by the phylogeographic groups
they belonged to. In the second phase of the investigation, in the Ethiopian and tropical East
African mountains, A. alpina showed high and clearly structured genetic variability. The
fragmented structure in the mountains indicated that A. alpina disperses little among established
populations. However, occasional long-distance dispersal events were also suggested in all parts
of the ranges, similar to the cpDNA data findings. In S. merjamie and C. afromontanum which
may consist of the subspecies complex, the diversity is so high for the former and very low for
the latter species in certain isolated mountain regions in Ethiopia (i.e. the Simen Mts and Gara
Muleta). The present analyses did also reveal very complex geographical structuring and
phylogeographic patterns among the study areas. Moreover, the AFLP data clearly indicated very
distinct and genetically highly divergent individuals, which might represent possible cryptic
subspecies signifying the call for future detail morphological assessments of the species in the
region.
Key words: cpDNA, AFLP, Arabis alpina, Salvia merjamie, Cerastium afromontanum,
phylogeography, genetic diversity, refugia, afro-alpine, afro-montane, East Africa, Pleistocene.
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Keywords
cpDNA, AFLP, Arabis alpina, Salvia merjamie, Cerastium afromontanum, phylogeography, genetic diversity, refugia, afro-alpine, afro-montane, East Africa, Pleistocene