Phytogeography, Population Structure and Phytochemical Analysis of the Afromontane Endemic Prunus Africana (Hook.f. Kalkman)
Kadu, Caroline Anne Cheledi
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Prunus africana is a highly valued Afromontane species for its bark whose products are used for treatment of benign prostatic hypertrophy (BPH). Increasing demand of P. africana bark has led to overharvesting of the species natural populations thus threatening its survival in the wild. Consequently, cultivation of the species on farms offers an alternative to sustain the P. africana herbal market. Information on existing genetic and phytochemical variations within and among the species provenances is crucial in initiating P. africana farming and its conservation. This study proposed to generate such important information by assessing the genetic diversity using cpDNA and SSR markers and phytochemical variations from 32 populations sampled across the species distribution range. The colonisation history of the Afromontane tree P. africana was inferred using seven cpDNA loci of 582 individuals from 32 populations that revealed 22 haplotypes. Bayesian coalescence modelling suggested that ‘east’ and ‘west’ African types split early during southward migration of the species, while furthermore recent splitting events occurred among populations in the East of the continent. The high genetic similarity found between western Uganda and West African populations indicates that a former Afromontane migration corridor may have existed through Equatorial Africa. To gain new insights into the population structure, population history and historical gene flow among populations of this Afromontane species, six nuclear microsatellite markers were used on a total of 459 samples from 30 populations and revealed strong population divergence among Afromontane regions. Five main regions were identified: West Africa, East Africa west of the Eastern rift valley, East Africa east of the Eastern Rift Valley, southern Africa and Madagascar. The eastern Rift Valley posed as the second most important barrier to gene flow in Prunus africana. Discordance between recent and historic gene flow estimates and patterns obtained from the cpDNA study gives evidence for a major shift of gene flow barriers in East Africa. To determine the correlation between phytochemical diversity in active compounds of Prunus africana bark with geographic or genetic distribution, the content of the different antioxidants derived from bark extracts was assayed by means of GC_MSD in 20 of the 32 populations sampled. The average concentration [mg/kg w/w] of the bark constituents assayed in increasing order was: lauric acid (18), myristic acid (22), n-docosanol (25), ferulic acid (49), β-sitostenone (198), β-sitosterol (490), and ursolic acid (743). The concentrations of many bark constituents were significantly intercorrelated. Only haplotypic diversity was positively correlated with the variation of the content of β-sitosterol and the fixation index (FIS) was negatively correlated with the content variation of ferulic acid. While genetic distances based on pairwise ΦPT for chloroplast and RST for nuclear genetic differentiation and Euclidian chemical distances were subjected to Mantel’s tests to associate concentration of bark constituents with the molecular phylogenetic pattern. For n-docosanol and ursolic acid, weak, but significant correlations were detected for both chloroplast as well as nuclear data, revealing that the molecular phylogenetic pattern is co-expressed by means of certain bark constituents. This correlation finding suggest that some populations exhibiting high amount of key phytochemical constituents thought to be important for BHP control could be a focus on P. africana domestication and improvement for on farm planting. The detailed implications of these results on the sustainable use and conservation of Prunus africana are further discussed.