Phylogeography of the heathers Erica arborea and E. trimera in the afro-alpine ‘sky islands’ inferred from AFLPs and plastid DNA sequences

Abstract

The ericaceous vegetation zone of the unique and highly fragmented afro-alpine environment in the eastern African high mountains is typically dominated by the heather Erica arborea, often in combination with its close relative E. trimera. Both species are shrubs or small trees with tiny seeds, potentially capable of dispersal by wind over long distances. While E. arborea is widely distributed in Africa, the Middle East and Europe, E. trimera is endemic to the afro-alpine region where it is restricted to higher altitudes than E. arborea. We used Amplified Fragment Length Polymorphisms (AFLPs) and variation in non-coding plastid DNA sequences to test whether these two morphologically and ecologically very similar species display similar phylogeographic patterns in the afro-alpine region. We predict that the more high-altitudinal E. trimera shows more distinct genetic structuring than E. arborea, because dispersal of the latter may have been facilitated by formation of interglacial forest bridges between mountains. Based on extensive field sampling in most of the high mountains of Ethiopia and East Africa, we show that the two species are clearly distinct at AFLP and plastid DNA loci. Both showed low levels of overall AFLP diversity, suggesting bottlenecking in small refugial populations during unfavourable climatic periods. However, their genetic structuring and inferred phylogeographic histories were conspicuously different. The more high-altitudinal E. trimera consisted of three to four distinct AFLP groups, which also had different plastid DNA haplotypes and different geographic distributions, suggesting long-term restriction to several refugia (at least one in Ethiopia and two in East Africa). In contrast, E. arborea showed little geographic structuring at AFLP loci and only a single, widespread plastid DNA haplotype, which may suggest recent colonization of the entire study area from a single source population, likely via a combination of gradual expansion via forest bridges and long-distance dispersals. The source population of E. arborea may be situated in (or north of) Ethiopia, which harbours most genetic diversity.