Genetic structure of wild and cultivated olives in the central Mediterranean basin

Abstract

Background and aims: Olive cultivars and their wild relatives (oleasters) represent two botanical varieties of Olea europaea subsp. europaea (respectively europaea and sylvestris). Olive cultivars have undergone human selection and their area of diffusion overlaps that of oleasters. Populations of genuine wild olives seem restricted to isolated areas of Mediterranean forests, while most other wild-looking forms of olive may include feral forms that escaped cultivation.

Methods: The genetic structure of wild and cultivated olive tree populations was evaluated by amplified fragment length polymorphism (AFLP) markers at a microscale level in one continental and two insular Italian regions.

Key results: The observed patterns of genetic variation were able to distinguish wild from cultivated populations and continental from insular regions. Island oleasters were highly similar to each other and were clearly distinguishable from those of continental regions. Ancient cultivated material from one island clustered with the wild plants, while the old plants from the continental region clustered with the cultivated group.

Conclusions: On the basis of these results, we can assume that olive trees have undergone a different selection/domestication process in the insular and mainland regions. The degree of differentiation between oleasters and cultivated trees on the islands suggests that all cultivars have been introduced into these regions from the outside, while the Umbrian cultivars have originated either by selection from local oleasters or by direct introduction from other regions.

Fig. 1

Map of the sites where oleaster populations were collected. The regions under study are shown in grey. The temperate region is distinguished from the Mediterranean region by grainy shading.

Fig. 2

(A) Estimated population structure for oleasters. (B) Estimated population structure for olive cultivars. (C) Estimated population structure for all individuals. Each individual is represented by a vertical coloured line, which is partitioned into K segments that represent the individual's estimated membership fractions in K clusters. Different colours indicate different populations. Colours do not correspond to the same groups in the various figure parts (A, B and C). Long black lines indicate the separation among a priori assigned populations, labelled above each figure. The figure shown for a given K is based on the highest probability run at that K. In all cases, the highest probability runs are shown.

Fig. 3

Results of the principal co-ordinate analysis on all AFLP data. The different genotypes and regions of diffusion are distinguished.