Abstract
Distantly related lineages of the enigmatic giant rosette plants of tropical alpine environments provide classical examples of convergent adaptation. For the giant senecios (Dendrosenecio), the endemic landmarks of the East African sky islands, it has also been suggested that parallel adaptation has been important for within-lineage differentiation. To test this hypothesis and to address potential gene flow and hybridization among the isolated sky islands, we organized field expeditions to all major mountains. We sampled all currently accepted species and all but one subspecies and genotyped 460 plants representing 109 populations. We tested whether genetic structuring corresponds to geography, as predicted by a parallel adaptation hypothesis, or to altitudinal belt and habitat rather than mountains, as predicted by a hypothesis of a single origin of adaptations. Bayesian and Neighbor-Net analyses showed that the main genetic structure is shallow and largely corresponds to geography, supporting a hypothesis of recent, rapid radiation via parallel altitude/habitat adaptation on different mountains. We also found evidence for intermountain admixture, suggesting several long-distance dispersals by wind across vast areas of unsuitable habitat. The combination of parallel adaptation, secondary contact, and hybridization may explain the complex patterns of morphological variation and the contradicting taxonomic treatments of these rare enigmatic giants, supporting the use of wide taxonomic concepts. Notably, the within-population genetic diversity was very low and calls for increased conservation efforts.
Highlights
The enigmatic giant rosette plants of tropical alpine environments such as the giant lobelias (Lobelia) and the giant senecios (Dendrosenecio) in Africa and Espeletia in South America provide classical examples of convergent evolution
The four groups showed a clear geographic structure, largely corresponding to the four mountain groups (Figs 2 and 3): 1) the WRZ-Ruwenzori group included all plants from the WRZ mountain Ruwenzori; 2) the WRZ/ERZ group included all plants from the WRZ mountain Muhavura, all plants from the western side of the ERZ (Elgon and Cherangani Hills), and some of the plants from Mt Kenya and Mt Aberdare at the eastern side of the ERZ; 3) the ERZ-Kenya/Aberdare group included most of the plants from Mt Kenya and Mt Aberdare; and 4) the ERZ-Kili/Meru is the youngest (Meru) group included all plants from the ERZ mountains Kilimanjaro and Meru
We have shown that repeated parallel evolution, which has been demonstrated for distantly related lineages of the enigmatic giant plants of tropical alpine environments [4, 6, 7], have played a prominent role within the single lineage of the giant senecios in the East/Central African sky islands
Summary
The enigmatic giant rosette plants of tropical alpine environments such as the giant lobelias (Lobelia) and the giant senecios (Dendrosenecio) in Africa and Espeletia in South America provide classical examples of convergent evolution. They are distantly related but have independently developed strikingly similar morphological, physiological and life history traits in response to the harsh tropical alpine climate with its diurnal freeze-thaw cycles [1,2,3]. There are several examples of parallel adaptation along altitudinal gradients in mountains, e.g. in Arabidopsis arenosa [15], Heliosperma pusillum [13], and Primula elatior [21]
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