Date Published: March 16, 2017
Publisher: Public Library of Science
Author(s): Andrew Chibuzor Iloh, Marco Schmidt, Alexandra Nora Muellner-Riehl, Oluwatoyin Temitayo Ogundipe, Juraj Paule, Tzen-Yuh Chiang.
Processes shaping the African Guineo-Congolian rain forest, especially in the West African part, are not well understood. Recent molecular studies, based mainly on forest tree species, confirmed the previously proposed division of the western African Guineo-Congolian rain forest into Upper Guinea (UG) and Lower Guinea (LG) separated by the Dahomey Gap (DG). Here we studied nine populations in the area of the DG and the borders of LG and UG of the widespread liana species, Chasmanthera dependens (Menispermaceae) by amplified fragment length polymorphism (AFLP), a chloroplast DNA sequence marker, and modelled the distribution based on current as well as paleoclimatic data (Holocene Climate Optimum, ca. 6 kyr BP and Last Glacial Maximum, ca. 22 kyr BP). Current population genetic structure and geographical pattern of cpDNA was related to present as well as historical modelled distributions. Results from this study show that past historical factors played an important role in shaping the distribution of C. dependens across West Africa. The Cameroon Volcanic Line seems to represent a barrier for gene flow in the present as well as in the past. Distribution modelling proposed refugia in the Dahomey Gap, supported also by higher genetic diversity. This is in contrast with the phylogeographic patterns observed in several rainforest tree species and could be explained by either diverging or more relaxed ecological requirements of this liana species.
The African Guineo-Congolian rain forest is the second largest block of rain forest on Earth with about 6400 endemic plant species , and considered a biodiversity hotspot . Repeated fragmentation of the tropical forest was suggested due to climate oscillations for the last one million years [3, 4]. Based on White’s chorological analyses , the African Guineo-Congolian rain forest can be divided into three phytogeographic units: Upper Guinea (UG), Lower Guinea (LG) and Congolia. All three units are characterized by considerable historical contractions, shifts and/or expansions . Thus, current ranges of species or particular lineages are defined by the location of their refugia during the Last Glacial Maximum (LGM) as well as by postglacial migration routes .
Geographic patterns of genetic diversity and differentiation of the African liana Chasmanthera dependens were investigated in this study in order to assess phylogeographic processes in West Africa using a descriptive genetic and distribution modelling approach. Particular focus of the modelling approach was given to populations representing the UG and LG phytogeographical units, and processes possibly accounting for observed patterns are discussed. For the distribution models climate grids at 1 km resolution were used, which are considered well-suited to account for the subcontinental extent of the study area and the objective of modelling past distributions. Details on the extent of microhabitat patches with possibly diverging microclimate were therefore not considered, which may lead to overestimations in the drier parts of the species range.
Results from this study show that past historical factors played an important role in shaping the distribution of Chasmanthera dependens across West Africa. Cameroon Volcanic Line seems to represent a barrier for gene flow in the present as well as in the past, and a uniform gene flow across Nigeria and the Dahomey Gap was observed. Distribution modelling proposed refugia in the Dahomey Gap, supported also by higher genetic diversity and the presence of the derived endemic haplotype H1. This is in contrast to the phylogeographic patterns observed in several tree species and could be explained by either diverging or more relaxed ecological requirements of this liana species.