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Abstract
Vegetation history in tropical Africa is still to date hardly known and the drivers of population differentiation and speciation processes are little documented. It has often been postulated that population fragmentations following climate changes have played a key role in shaping the geographic distribution patterns of genetic diversity and in driving speciation. Here we analyzed phylogeographic patterns (chloroplast-DNA sequences) within and between eight (sister) species of widespread rainforest herbs and lianas from four genera of Marantaceae (Halopegia, Haumania, Marantochloa, Megaphrynium), searching for concordant patterns across species and concordance with the Pleistocene refuge hypothesis. Using 1146 plastid DNA sequences sampled across African tropical lowland rainforest, particularly in the Lower Guinean (LG) phytogeographic region, we analyzed intra- and interspecific patterns of genetic diversity, endemism and distinctiveness. Intraspecific patterns of haplotype diversity were concordant among most species as well as with the species-level diversity pattern of Marantaceae. Highest values were found in the hilly areas of Cameroon and Gabon. However, the spatial distribution of endemic haplotypes, an indicator for refuge areas in general, was not congruent across species. Each proposed refuge exhibited high values of endemism for one or a few species indicating their potential role as area of retraction for the respective species only. Thus, evolutionary histories seem to be diverse across species. In fact, areas of high diversity might have been both refuge and/or crossing zone of recolonization routes i.e., secondary contact zone. We hypothesize that retraction of species into one or the other refuge happened by chance depending on the species' distribution range at the time of climate deterioration. The idiosyncratic patterns found in Marantaceae species are similar to those found among tropical tree species, especially in southern LG.
Highlights
Vegetation history in tropical Africa is still to date hardly known and the drivers of speciation and population differentiation processes are little documented
GENETIC POLYMORPHISM The numbers of individuals sequenced per species ranged from 75 to 166, totaling 1046 individuals (991 in Lower Guinea, Table 2) sequenced for trnC-petN1r and 110 for psbA-trnH (Lower Guinea only, Table 2)
Genetic diversity measured as v taking genetic distance between haplotypes into account was highest in Marantochloa monophylla and endemism per grid cell (End) was highest in M. incertifolia
Summary
Vegetation history in tropical Africa is still to date hardly known and the drivers of speciation and population differentiation processes are little documented. Hypotheses on the diversification of the Afrotropical flora include allopatric differentiation/speciation driven by population fragmentation following Pleistocene climate changes (Robbrecht, 1994; Sosef, 1994; Maley, 1996) and parapatric differentiation/speciation across ecological gradients (e.g., temperature and precipitation gradients; Fjeldsa and Lovett, 1997; Vande Weghe, 2004; Heuertz et al, 2013). This might have led to population fragmentation followed by the independent evolution of the isolated populations through mutation and drift and the establishment of species. And/or simultaneously, isolated populations might have adapted to different climatic conditions forming ecologically different species. Within Lower Guinea (LG, i.e., the western part of the Central African rainforest block, identified as a phytochorion by White, 1979, Figure 1), climatic heterogeneity is characterized by a marked W-E precipitation gradient from the Coast to the inland and a North-South seasonal inversion at a latitude c. 2◦N (Leroux, 1983; Vande Weghe, 2004)
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