Scientific Publication

Influence of putative forest refugia and biogeographic barriers on the level and distribution of genetic variation in an African savannah tree, Khaya senegalensis (Desr.) A. Juss

author(s) Sexton, Gary - Frere, Celine - Kalinganire, Antoine - Uwamariya, A - Lowe, Andrew - Godwin, Ian - Prentis, Peter - Dieters, M
from International Center for Agricultural Research in the Dry Areas (ICARDA)
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themes
regions
  • Eastern Africa
  • Sub-Saharan Africa
  • Africa
  • World
  • Western Africa
  • Middle Africa
citation

Gary Sexton, Celine Frere, Antoine Kalinganire, A. Uwamariya, Andrew Lowe, Ian Godwin, Peter Prentis, M. Dieters. (31/10/2015). Influence of putative forest refugia and biogeographic barriers on the level and distribution of genetic variation in an African savannah tree, Khaya senegalensis (Desr. ) A. Juss. Tree Genetics and Genomes, 11: 103

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Abstract

Phylogeographic studies of tropical and subtropical
tree species provide an ideal method to study the role of forest
refugia in the structuring of genetic diversity in contemporary
populations. To date, most studies have examined rainforestdependent
trees, yet the influence of forest refugia on savannah
forest and woodland trees remains poorly understood despite
their potentially important role in forest assemblages
during the last glacial maximum. Here, we examine the level
and structuring of genetic diversity across the geographic
range of the savannah woodland tree, Khaya senegalensis—
African mahogany, from Senegal to Uganda (>5000 km) and
from the steppes of the Sahara desert to the fringes of Africa’s
equatorial rainforests (700 km). Three chloroplast (cpDNA)
loci and 13 nuclear microsatellite (nSSR) loci were genotyped
in 503 individuals. Individual-based clustering of nSSR genotypes
identified that all samples formed two populations that
showed no pattern of geographic structuring. Population level
analysis of nSSR data revealed only very weak genetic structure
(FST=0.013) with most of the genetic diversity contained
within populations. Geographical differentiation (GST=0.096)
was low for the 14 haplotypes identified. However, whereas
all haplotypes occurred in the western populations (including
Cameroon), only two cosmopolitan haplotypes were found in
central-eastern populations. Geographically restricted lowfrequency
haplotypes were found in eight western populations.
Higher genetic diversity in western populations was
confirmed by patterns of allelic richness, which were lower
in central-eastern populations. Taken together, these results
indicate that K. senegalensis displays very little genetic differentiation
across its geographic range and that previously identified
barriers to dispersal across the northern savannah belt of
tropical Africa (e.g. the Dahomey Gap, Mega Lake Chad and the Adamawa Highlands) have not impeded contemporary
gene dispersal in this species. Nevertheless, clustering of rare
haplotypes indicates that the Dahomey Gap has likely been an
important historical barrier to genetic connectivity between
western and eastern regions of the species and that it is likely
that this savannah tree also experienced range contraction
around major forest refugial fringes or riparian incursions during
the last glacial maximum (LGM)

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