Genetic variation of naturally growing olive trees in Israel: from abandoned groves to feral and wild?

Barazani et al. BMC Plant Biology (2016) 16:261 DOI 10.1186/s12870-016-0947-5 RESEARCH ARTICLE Open Access Genetic variation of naturally growing olive trees in Israel: from abandoned groves to feral and wild? Oz Barazani1*, Alexandra Keren-Keiserman1,2, Erik Westberg3, Nir Hanin1, Arnon Dag4, Giora Ben-Ari5, Ori Fragman-Sapir6, Yizhar Tugendhaft4,7, Zohar Kerem7 and Joachim W. Kadereit3 Abstract Background: Naturally growing populations of olive trees are found in the Mediterranean garrigue and maquis in Israel. Here, we used the Simple Sequence Repeat (SSR) genetic marker technique to investigate whether these represent wild var. sylvestris. Leaf samples were collected from a total of 205 trees at six sites of naturally growing olive populations in Israel. The genetic analysis included a multi-locus lineage (MLL) analysis, Rousset’s genetic distances, Fst values, private alleles, other diversity values and a Structure analysis. The analyses also included scions and suckers of old cultivated olive trees, for which the dominance of one clone in scions (MLL1) and a second in suckers (MLL7) had been shown earlier. Results: The majority of trees from a Judean Mts. population and from one population from the Galilee showed close genetic similarity to scions of old cultivated trees. Different from that, site-specific and a high number of single occurrence MLLs were found in four olive populations from the Galilee and Carmel which also were genetically more distant from old cultivated trees, had relatively high genetic diversity values and higher numbers of private alleles. Whereas in two of these populations MLL7 (and partly MLL1) were found in low frequency, the two other populations did not contain these MLLs and were very similar in their genetic structure to suckers of old cultivated olive trees that originated from sexual reproduction. Conclusions: The genetic distinctness from old cultivated olive trees, particularly of one population from Galilee and one from Carmel, suggests that trees at these sites might represent wild var. sylvestris. The similarity in genetic structure of these two populations with the suckers of old cultivated trees implies that wild trees were used as rootstocks. Alternatively, trees at these two sites may be remnants of old cultivated trees in which the scion-derived trunk died and was replaced by suckers. However, considering landscape and topographic environment at the two sites this second interpretation is less likely. Keywords: Crop domestication, Cultivated old olive trees, Gene flow, Grafting, Historical agriculture, Oleaster, var. sylvestris Background The domestication of crop species started 13,000 to 10,000 years before present by gradual selection of desirable traits and of adaptations to agricultural environments [1]. Such artificial selection of individual plants with desirable traits, e.g., high yield, large fruits, loss of shattering seeds, etc., had an artificial selection effect * Correspondence: 1 Institute of Plant Sciences, the Israel Plant Gene Bank, Agricultural Research Organization, Rishon LeZion 75359, Israel Full list of author information is available at the end of the article which resulted in genetic differences between crops and their wild ancestors, both in coding and neutral regions of the genome. However, the long co-existence of crops alongside their wild relatives provided opportunities for hybridization, leading to gene flow between the diverging gene pools. Gene flow between cultivated plants and their wild ancestors has been demonstrated in woody species cultivated for their edible fruits such as almonds (Prunus dulcis and P. orientalis) [2], grapes (Vitis vinifera subsp. vinifera and V. vinifera subsp. sylvestris) [3, 4] and apples (Malus domestica and M. sylvestris) [5]. In addition to © The Author(s). 2016 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Barazani et al. BMC Plant Biology (2016) 16:261 gene flow, dispersal of seeds from cultivated trees into natural surroundings can result in feral populations of natural aspect [6], as shown for several plants introduced to Australia, including Olea europaea [7, 8]. Both these processes can result in substantial difficulties when trying to identify populations as truly wild. It is generally accepted that the cultivated olive Olea europaea subsp. europaea var. europaea originated from wild var. sylvestris (Mill) Lehr by artificial selection from wild populations [9]. Recently, analysis of plastid DNA diversity among 1,263 supposedly wild olive trees from 108 localities across the Mediterranean area and 534 cultivars suggested that the north Levant (i.e., the area close to the Syrian/Turkish border) was the primary domestication centre of olives [10]. However, one of the earliest indications of the use of olives and possibly also of its cultivation was found in the southeastern Mediterranean area (i.e., in the area of modern Israel) and dated to 6,500 B.C. [11]. Wild var. sylvestris, often called ‘oleaster’, resembles cultivated olives except for its shrubby growth and smaller leaves and fruits [12]. These characters, however, are highly variable and do not allow reliable distinction between the wild and cultivated varieties. Thus, the identification of olives growing in natural surroundings in the southeast Mediterranean area as var. sylvestris is often questionable [13]. However, using an ecological niche model based on current climatic parameters, Besnard et al. [10] could identify the natural distribution range of var. sylvestris and could show that current conditions are suitable for its presence in the southwest Levant, i.e., modern Israel. Studies employing different molecular marker techniques to investigate the relationship between cultivated and wild olives and to map the distribution of wild olives in the Mediterranean area have been conducted before, e.g. [14–22]. In several cases, genetic similarity between trees growing in natural surroundings and cultivated olives was interpreted as evidence for the feral nature (i.e., descended from cultivated trees) of the former [14, 15]. However, the studies by Baldoni et al. [14] and Belaj et al. [15] also revealed the existence of genetically distinct populations in Italy and Spain, respectively, which were interpreted as evidence for the continued existence of isolated populations of wild var. sylvestris in the Mediterranean area. Supporting this hypothesis, o (...truncated)