Protected areas of Spain preserve the neutral genetic diversity of Quercus ilex L. irrespective of glacial refugia

Abstract

Quercus ilex L. (holm oak) is a wind-pollinated, sclerophyllous tree that copes with the environmental variability of the Mediterranean climate and that displays flexible ecophysiological adaptability in relation to hydric and thermic stresses. The holm oak dominates Mediterranean woodlands on both acidic and calcareous soils and has been exposed to management (dehesas) for thousands of years. Both protected areas and glacial refugia are supposed to preserve a substantial fraction of the genetic diversity of Iberian species. Genetic diversity was examined for 68 populations sampled throughout Spain using ptDNA SNPs, ptDNA microsatellites, and primarily nuclear AFLPs. Protected populations did not significantly differ from nonprotected populations by any of the measures of levels of genetic diversity. The three-level hierarchical AMOVA indicated that a low number of protected populations harbor most of the species’ genetic diversity. In addition, we found no evidence from either ptDNA or AFLP variation to support that populations from putative glacial refugia are divergent genetic groups as expected during isolation. Outcrossing, anemophilous long-distance pollen dispersal, acorn transport by animals, tree reliance, and habitat availability in Spain probably played a primarily role in homogenizing allele frequency among populations. This result leads us to suggest that extensive gene flow has been prevalent across Spanish populations. We conclude that glacial refugia have not been essential to maintain the neutral genetic makeup of Q. ilex. Nevertheless, conservation of the holm oak in protected areas ensures protection of the species’ genetic diversity, the most widespread woodland ecosystem in Iberia and indirectly the four iconic, endangered animal species (black stork, cinereous vulture, Iberian lynx, western imperial eagle).