Diversity gradients and phylogeographic patterns in Santiria trimera (Burseraceae), a widespread African tree typical of mature rainforests.

PREMISE OF THE STUDY: New insights into the history of the African rainforest can be gathered from the phylogeographic structures of their constituent species, but few studies have been performed in this ecosystem. We studied the phylogeographic structure of Santiria trimera, a primate- and bird-dispersed, dioecious tree typical of mature African rainforests.
METHODS: We sequenced three chloroplast DNA (cpDNA) regions (trnL-F, rbcL, and rpl36-infA-rps8) in 377 individuals from 42 populations.
RESULTS: Sequence chromatograms regularly displayed double peaks of unequal heights. Cloning of PCR products and sequencing of outgroup taxa led to assigning the taller peak in ambiguous sequence positions to cpDNA. A total of 14 polymorphic cpDNA sites and 12 haplotypes were detected. Populations from three distinct biogeographic regions, namely, Upper Guinea, Lower Guinea, and the volcanic island of São Tomé, did not share any haplotype, indicating allopatric divergence. In Lower Guinea, Gabonese forests had high diversity mainly from the sympatry of two genetically divergent morphotypes, whereas forests of eastern Cameroon were less diversified. The two haplotypes of the morphotype without stilt roots were distributed north and south of the Ogooué River, suggesting refuges on both sides of the river bed.
CONCLUSIONS: The divergence between Upper and Lower Guinean rainforests is explained by the discontinuity of forest between those regions throughout most of the Quaternary. The distribution of rare endemic haplotypes concurred with proposed Pleistocene rainforest refuges in west and southwest Cameroon. Overall, phylogeographic structure is consistent with the biogeographic hypotheses largely based on patterns of species diversity.