Population genetic structure of Grauer’s Swamp Warbler Bradypterus graueri, an Albertine Rift endemic

Abstract

The endangered warbler Bradypterus graueri is endemic to the Albertine Rift, where it is restricted to montane swamps above 1900 m across the region. We studied genetic structure among six populations sampled across the species’ distribution in northern Burundi, Rwanda, Uganda and the eastern Democratic Republic of Congo. A total of 2117 base pairs of mitochondrial data were sequenced. Phylogenetic analyses and network reconstruction of B. graueri haplotypes recovered three clades with a defined geographical pattern: clade 1, Virunga Volcanoes and Kigezi Highlands; clade 2, Rugege Highlands; and clade 3, Kahuzi-Biega Highlands; clades 2 and 3 are sisters to each other. Both landscape dynamics and historical climate are likely to have played a role in the diversification of this species. The divergence between clade 1 and clades 2 and 3 (168.5 ka, 95% HPD 108.5, 244.4) coincides with a prolonged period of aridity in tropical Africa between 130 and 270 ka. Similarly, the divergence between clades 2 and 3 (99.4 ka, 95% HPD 55.4, 153.8) corresponds with a period of aridity just prior to 94 ka. Populations sampled from the eastern arm of the central Albertine Rift (Kigezi and Rugege Highlands) show a coincident increase in effective population size after the Last Glacial Maximum at c. 15 ka, whereas those sampled from Kahuzi-Biega on the western arm of the rift do not. Despite the perceived higher vagility of bird species relative to other vertebrates, the degree of phylogeographical structure among populations of B. graueri is similar to that reported for small mammals (Hylomyscus vulcanorum, Lophuromys woosnami, Sylvisorex vulcanorum) and a frog Hyperolius castaneus sampled across the central Albertine Rift. Collectively our results suggest that climate dynamics associated with late Pleistocene cycles had a significant influence on driving the population genetic structure and associated levels of genetic diversity in B. graueri and other small terrestrial vertebrates. Our results have implications for the conservation of B. graueri and other endemics to the Albertine Rift, particularly in the context of other phylogegeographical studies centred on this biodiversity hotspot.