Genetic diversity and population's structure in Tunisian strawberry tree (Arbutus unedo L.)

A growing interest in the use of the Strawberry tree (Arbutus unedo L., Ericaceae) has been recently reported for industrial, pharmaceutical and chemical fields. However, the bulk material comes from natural populations because of the lack of selection of interesting cultivars. In Tunisia, A. unedo populations are severely destroyed due to deforestation and over-collecting. The species occurs in small scattered populations decreasing progressively in size. Yet, no conservation or improvement programs are attempted to preserve and promote the potential value of this resource. In this work, we assessed the genetic diversity of nine Tunisian populations of A. unedo L. from different bioclimates, using 65 polymorphic RAPD loci. The analysis of the genetic variation within and among populations is primordial to elaborate conservation and improvement programs. A low genetic diversity within a population estimated by both Nei's (He) and Shannon's diversity (H′) indices (0.155 < He < 0.248; 0.229 < H′ < 0.364) was observed due to genetic drift and selfing. At the species level, the amount of the within population variation estimated by Shannon's index (H_POP/H_SP = 0.686) and the molecular variance (80.67%) was higher than that among populations. A moderate genetic differentiation among populations (Φ_ST = 0.193 and G_ST = 0.314) which could be attributed to the long seed distance dispersal was detected. The UPGMA dendrogram based on Φ_ST values showed three clusters each including populations without relationship to bioclimatic or geographical origin indicating that differentiation occurs at a local space scale. The in situ protection measures should be made appropriately according to a population within bioclimates. The ex situ conservation and the selection of genotypes should involve extensive collection of seeds or cuttings from the within populations rather than among them.