Genetic diversity and gene flow between the wild olive (oleaster, Olea europaea L.) and the olive: several Plio‐Pleistocene refuge zones in the Mediterranean basin suggested by simple sequence repeats analysis

Abstract

Aim The oleaster is believed to have originated in the eastern Mediterranean, implying that those in the western Mediterranean basin could be feral. Several studies with different molecular markers (isozymes, random amplified polymorphic DNA, amplified fragment length polymorphism) have shown a cline between the eastern and the western populations, which supports this hypothesis. To reconstruct the post‐glacial colonization history and establish a relationship between olive and oleaster populations in the Mediterranean basin, analyses were carried out on the genetic variation of chloroplast DNA (chlorotype) and at 12 unlinked simple sequence repeat (SSR) loci, sampling a total of 20 oleaster groves. Location This is the first known large‐scale molecular study of SSR loci based on samples of both oleasters and cultivars from the entire Mediterranean basin. Methods Samples were taken from 166 oleasters in 20 groves of modern populations, and 40 cultivars to represent molecular diversity in the cultivated olive. The Bayesian method and admixture analysis were used to construct the ancestral populations (RPOP; reconstructed panmictic oleaster populations) and to estimate the proportion of each RPOP in each tree. If one tree can be assigned to two or more RPOPs, it can be regarded as a product of hybridization between trees from different populations (i.e. admix origin). Results On this first examination of the SSR genetic diversity in the olive and oleaster, it was found to be structured in seven RPOPs in both eastern and western populations. Based on different population genetic methods, it was shown that: (1) oleasters are equally present in the eastern and the western Mediterranean, (2) are native, and (3) are not derived from cultivars. Chlorotypes (one and three in the eastern and western Mediterranean, respectively) revealed fruit displacement for the oleasters. Main conclusions Oleaster genetic diversity is divided into seven regions that could overlay glacial refuges. The gradient, or cline, of genetic diversity revealed by chloroplast and SSR molecular markers was explained by oleaster recolonization of the Mediterranean basin from refuges after the last glacial event, located in both eastern and western regions. It is likely that gene flow has occurred in oleasters mediated by cultivars spread by human migration or through trade. Animals may have helped spread oleasters locally, but humans have probably transported olives but not oleaster fruits over long distances. We found that cultivars may have originated in several RPOPs, and thus, some may have a more complex origin than expected initially.

Extending glacial refugia for a European tree: genetic markers show that Iberian populations of white elm are native relicts and not introductions

“…Assuming that Iberia acted as a glacial refugium, the phylogeographic pattern observed in U. laevis is similar to those observed in other widely distributed, non-cultivated, European tree taxa (that is, Petit et al, 2002;Magri et al, 2006). Before U. laevis, other tree taxa previously considered introduced in the Iberian peninsula have been proven to be native by the use of genetic markers, like chestnut (Fineschi et al, 2000), olive (Breton et al, 2006), stone pine (Vendramin et al, 2008). The latter, like U. laevis, showed very little genetic diversity across its broad Mediterranean range.…”

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“…Therefore we suggest that these trees are formed either by hybridization between cultivated and brown olive trees of the neighbouring localities or they have escaped from cultivation and have become adapted to the new environmental conditions showing morphological changes as feral trees 32 . Unfortunately, we could not perform RAPD analysis of brown and cultivated olive trees together to check if the Khersan population stands intermediate between brown and cultivated olive trees as in the morphological analysis.…”

Stone diversity in wild and cultivated olive trees (Olea europaea L.)

“…europaea (Green, 2002) and comprise olive cultivars grown for oil or table olives. In addition to these two olive forms, a third form resembling oleasters by their physiognomy, has escaped from cultivation, and has been called feral form (Besnard et al, 2001;Mekuria et al, 2002;Breton et al, 2006a).…”