Analysis of genetic diversity and population structure of the endangered Origanum compactum from Morocco, using SSR markers: Implication for conservation

Origanum compactum L. (Lamiaceae) is one of the most popular medicinal and culinary herbs with a long history of traditional use. This species, endemic to Morocco and Southern Spain, is considered as threatened, due mainly to over-harvesting and habitat destruction. Assessment of genetic polymorphism of threatened species constitutes a prerequisite for germplasm conservation. To support and monitor future conservation and management measures, we used 15 microsatellite loci to genotype 670 individuals of O. compactum distributed over 59 localities covering the entire geographic distribution of the species in Morocco. In contrast to what was argued in most previous studies, the genetic diversity of the Moroccan oregano was greater mostly on isolated and declined populations compared to less fragmented accessions. Accordingly, our study identifies the most relevant management units for conservation purposes on the basis of their genetic diversity level. Factors such as the perennity of the species, somatic mutations, or multiple founder events could explain this unexpected level of genetic diversity at the species level in this endangered plant. The population genetic results reflect a strong genetic structure and identify three major clusters among O. compactum populations: a Northern cluster composed of populations near the strait of Gibraltar (Tangier-Tetouan and Chefchaouen regions as well as some adjacent populations from Taounate), and two other clusters with significant mixed lineage. This result supports the hypothesis that the strait of Gibraltar could be a migration route for plants between Southern Spain and Northern Morocco. The high differentiation of populations (Fst = 0.22) and the low rate of gene flow between them (Nm = 0.88) might been caused by recent isolation of the populations through habitat disturbances. Historical demographic patterns inferred with a generalized stepwise mutation model (SSM) and two-phase model (TPM) using maximum likelihood showed significant past demographic changes in 39 of the 59 sampled populations. Implications of these results to develop appropriate conservation strategies of this endangered species are proposed.