Genetic diversity and structure of wild Tunisian Thymus capitatus (L.) Hoffm. et Link. (Lamiaceae) assessed using isozyme markers

The genetic diversity and population structure of 25 natural populations of Thymus capitatus, from five ecological areas, were analysed using eight isozymes. For all populations, 11 polymorphic loci were detected, and several of them showed rare alleles. A high genetic diversity within populations (A_p = 1.82; P = 62.88%; H_o = 0.116) and an excess of heterozygosity were observed. A high level of inbreeding within populations was observed (F_IS = 0.471). High differentiation and low gene flow (N_m = 0.821) were detected among populations (F_ST = 0.219). The genetic variation within and among ecological groups varied according to the bioclimate. Population structuration depends on geographic distance between sites rather than on bioclimate (Mantel’s test; r = 0.251; P = 0.004). Nei’s genetic distances (D) values calculated between pairs of populations were globally low with a mean of 0.047. The unweighted pairwise groups method using arithmetic average dendrogram showed fourth sub‐clusters. Population groupings occur with evident relationship to geographic location. The substantial differentiation and the high genetic similarities between populations indicate that populations have been recently isolated as a result of anthropic pressure. In situ conservation must first focus on populations with a high level of genetic diversity and rare alleles. Ex situ preservation should be elaborated by collecting seeds within populations that showed a high level of genetic diversity in each ecological group.