Genetic diversity of wild Thymus capitatus (Lamiaceae) in Tunisia using molecular markers

Abstract

In Tunisia, Thymus capitatus L. 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 Thymus capitatus L. from different bioclimates, using 103 polymorphic randomly amplified polymorphic DNA (RAPD) loci. The analysis of the genetic variation within and among populations is primordial to elaborate conservation and improvement programs. The species showed a low diversity within populations (0.276<H’<0.327) due to genetic drift caused by the habitat fragmentation. A high genetic differentiation (GST=0.359 and ΦST=0.284) and a low gene flow (0.435<Nm<0.970) among populations, indicating a short seed and/or pollen dispersal distance, were revealed. The absence of isolation by distance and the high level of differentiation among populations suggested the existence of important historical gene flow between them. The UPGMA dendrogram analysis basedon ΦST distance matrix showed that most populations clustered independently to bioclimate or geographical distance indicating that genetic differentiation mainly occurs at local space scale due to genetic drift. Given the high proportion of the within-population diversity and the high amount of among populations variation, the ex-situ conservation should be made by the collection of seeds/cuttings rather within than among populations. The in-situ conservation should be made appropriately according to populations in their bioclimate