Comparison of quantitative and molecular variation in agroforestry populations of the shea tree (Vitellaria paradoxa C.F. Gaertn) in Mali.

In this study we investigated the within- and between-population genetic variation using microsatellite markers and quantitative traits of the shea tree, Vitellaria paradoxa, an important agroforestry tree species of the Sudano-Sahelian region in Africa. Eleven populations were sampled across Mali and in northern Côte d'Ivoire. Leaf size and form and growth traits were measured in a progeny test at the nursery stage. Eight microsatellites were used to assess neutral genetic variation. Low levels of heterozygosity were recorded (1.6-3.0 alleles/locus; H(E) = 0.25-0.42) and the fixation index (F(IS) = -0.227-0.186) was not significantly different from zero suggesting that Hardy-Weinberg equilibrium is encountered in all populations sampled. Quantitative traits exhibited a strong genetic variation between populations and between families within populations. The degree of population differentiation of the quantitative traits (Q(ST) = 0.055-0.283, Q(STmean) = 0.189) strongly exceeds that in eight microsatellite loci (F(ST) = -0.011-0.142, F(STmean) = 0.047). Global and pairwise F(ST) values were very low and not significantly different from zero suggesting agroforestry practices are amplifying gene flow (Nm = 5.07). The population means for quantitative traits and the rainfall variable were not correlated, showing variation was not linked with this climatic cline. It is suggested that this marked differentiation for quantitative traits, independent of environmental clines and despite a high gene flow, is a result of local adaptation and human selection of shea trees. This process has induced high linkage disequilibrium between underlying loci of polygenic characters.

Genetic diversity of the shea tree (Vitellaria paradoxa C.F. Gaertn), detected by RAPD and chloroplast microsatellite markers.

RAPDs and chloroplast microsatellites were used to quantify the genetic variation of Vitellaria paradoxa (an economically important tree species in sub-Saharan Africa, north of the equator) and to analyse the geographic distribution of diversity in relation to the refuge theory. A total of 13 locations were sampled in eight countries, covering most of the natural range from Senegal to Uganda. A total of 67 polymorphic and 15 monomorphic RAPD loci were detected in 179 individuals. No relationship was identified between diversity and longitude or latitude. An unrooted neighbour-joining tree suggested a western group and an eastern group, representing 7% (P = 0.000) of the total variation. A Mantel test suggested that genetic distances between populations were correlated to geographic distances (R = 0.88, P = 0.001). The three-chloroplast microsatellite primers, assayed in 116 individuals, revealed 10 different alleles and seven chlorotypes. Most of the populations comprised a single haplotype. It is proposed from these results that the difference between western and eastern populations results from putative refugia separated by the current 'Dahomey Gap' (a semiarid zone that meets the coast around the Ghana-Togo-Benin-Nigeria borders), an area that may have been exceptionally dry during glacial periods. In addition, it is suggested that the haplotype distribution and frequency in the western populations could be due to the more recent impact of humans, particularly shea tree selection and dispersal during traditional agroforestry.