Phenotypic variation and genetic diversity of East African sandalwood trees (Osyris lanceolata Hochst. & Steud.) in Uganda

Abstract

East African Sandalwood tree (Osyris lanceolata Hochst. & Steud) is an aromatic tree species at risk of extinction due to over-exploitation for its essential aromatic oil. Moreover, limited information on its phenotypic variation, genetic diversity, and population structure, is a great impediment to its effective management in Uganda. Thus, the objectives of this study were (i) to characterize phenotypic variation and (ii) to estimate the level of genotypic diversity and population structure within and among O.lanceolata populations in Uganda. Samples were taken from three (3) natural populations in Amudat, Moroto, and Nakapiripirit districts. Eight (8) phenotypic traits were used to characterize the variation, while 5 SSR markers were optimized to estimate the level of genetic diversity and population structure. Phenotypic data were analyzed using R, FigTree and PAST software. Significant differences were revealed in 3 of the 8 traits, including LAA, LL, and LW. Trees in Moroto and Amudat districts were the tallest. LL and LW had the greatest contribution in differentiating phenotypes. Significant correlations were found between RCD and TH, CD, LL, and PL. The genetic data analysis was done using GenAlEx 6.5, Power Marker, FigTree, and PAST software. At the locus level, all genetic diversity indices were high (Ne=1.971, Na=2, Hs=0.493, Fis=-0.020, Fst=0.006, Nm=50.89). At the population level, the means of all genetic diversity indices ranged from moderate to high (Na = 2, Ne = 1.48, I = 0.48, %P = 100%). Based on the Shannon index, Amudat and Moroto populations showed the highest level of genetic diversity; Amudat (I=0.372, h=0.55, %P=100%); Moroto (I=0.48, h=0.31, %P = 100%). There was more variation (98%) within populations compared to variation among populations (2%), and differentiation was low (PhiPT=0.023). The ΔK was plotted against various values of K, and the optimum value of K was 4, indicating that 4 was the most likely number of clusters. The high level of genetic diversity is attributed to the outcrossing nature of Osyris lanceolata and gene flow. The lowest level of genetic divergence was observed between Amudat and Nakapiripirit populations, while the highest level of divergence was observed between Moroto and Nakapiripirit populations. The Moroto and Amudat populations have shown a unique genetic signature and should be managed as in situ conservation areas. Meanwhile, an ex-situ conservation program should also be initiated, targeting healthy trees from all populations. Key words: Conservation, Genetic differentiation, Genetic diversity, Molecular markers, Osyris lanceolata, phenotypic traits, Variation.