A combined approach using allozymes and volatiles for the characterization of Tunisian Thymbra capitata (L.) Cav. (Lamiaceae)

Abstract

The genetic diversity and the essential oil variations within and among 12 Tunisian natural populations of Thymbra capitata (=Thymus capitatus Hoffm. et Link.=Coridothymus capitatus Rch. f.), from different geographic regions and bioclimates, were assessed using eight isozymes and thirty nine terpenoids. Volatiles were analyzed by gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). A high genetic diversity within population which was revealed by isozymes (P=66.64%, Ap=1.85 and He=0.227) resulted from the great number of genotypes in the ancestral population before fragmentation, favored by the outbreeding of the species. Population structure depends on geographic distance between sites rather than bioclimate indicating isolation by distance. Cluster constructed using Nei's genetic distances showed mainly geographic related population groupings. Essential oil composition varied among populations. UPGMA dendrogram based on Euclidean distance matrix among pairs of populations showed two major population aggregates. The chemical population structure is linked to the geographic distance among populations and ecological factors indicating that the local selective environmental factors act on chemical composition. The correlation between matrices of allozymes and terpenoids was not significant. Population differentiation performed on the two sets of data was similar to that probed by isozymes. The conservation strategies of populations should be made according to their level of genetic and chemical diversity in relation to geographic location of populations.