Genetic structure of natural Tunisian Hypericum humifusum L. (Hypericacae) populations as assessed by allozymes and RAPDs

Abstract

The genetic diversity within and among seven Tunisian natural populations of Hypericum humifusum L., from different geographic regions and bioclimates, was assessed using 11 isozymic polymorphic loci, and 166 RAPD markers amplified by 8 primers. The genetic diversity within populations based on allozymes was higher ( P = 72.46%, Ap = 2.01 and H e = 0.29), than that revealed by RAPDs (29.52 < P < 39.16% and 0.150 < H < 0.200). Both markers yielded high estimates of genetic differentiation and low gene flow (Nm = 0.257 and 0.508 for RAPD and allozymes, respectively) among populations at all space scales. However, the level of differentiation revealed by RAPDs ( Φ ST = 0.494; G ST = 0.561) was higher than that based on allozymes ( F ST = 0.117). No correlation (Mantel test) among genetic ( F ST and Φ ST matrices) and geographical distance matrices was observed indicating no isolation by distance. Cluster analyses from allozyme and RAPD loci did not completely agree. The dendrogram based on allozymes yielded higher separation among most populations, while that from RAPDs separated populations into three distinct subclusters. Groupings of populations, in both dendrograms, did not reflect spatial geographic or bioclimatic patterns, indicating specific adaptation of populations to local environments. The correlation between matrices of allozyme and RAPD band frequencies was not significant (Mantel test). The dendrogram obtained from combined data yielded similar population groupings to that probed by RAPDs suggesting higher accuracy of these markers. Given the high differentiation among all populations even at a low geographic distance, ex situ conservation should involve extensive seed collection from all populations from all bioclimatic zones. The populations from the upper semi-arid bioclimate exhibiting relatively high level of genetic diversity should be first protected.