High Genetic Diversity Detected in Olives beyond the Boundaries of the Mediterranean Sea

et al. (2014) High Genetic Diversity Detected in Olives beyond the Boundaries of the Mediterranean Sea. PLoS ONE 9(4): e93146. doi:10.1371/journal.pone.0093146 High Genetic Diversity Detected in Olives beyond the Boundaries of the Mediterranean Sea Mehdi Hosseini-Mazinani 0 Roberto Mariotti 0 Bahareh Torkzaban 0 Massoma Sheikh-Hassani 0 Saeedeh Ataei 0 Nicolo` G. M. Cultrera 0 Saverio Pandolfi 0 Luciana Baldoni 0 Manoj Prasad, National Institute of Plant Genome Research, India 0 1 National Institute of Genetic Engineering and Biotechnology , Tehran , Iran , 2 CNR - Institute of Biosciences and Bioresources , Perugia , Italy Background: Olive trees (Olea europaea subsp. europaea var. europaea) naturally grow in areas spanning the Mediterranean basin and towards the East, including the Middle East. In the Iranian plateau, the presence of olives has been documented since very ancient times, though the early history of the crop in this area is shrouded in uncertainty. Methods: The varieties presently cultivated in Iran and trees of an unknown cultivation status, surviving under extreme climate and soil conditions, were sampled from different provinces and compared with a set of Mediterranean cultivars. All samples were analyzed using SSR and chloroplast markers to establish the relationships between Iranian olives and Mediterranean varieties, to shed light on the origins of Iranian olives and to verify their contribution to the development of the current global olive variation. Results: Iranian cultivars and ecotypes, when analyzed using SSR markers, clustered separately from Mediterranean cultivars and showed a high number of private alleles, on the contrary, they shared the same single chlorotype with the most widespread varieties cultivated in the Mediterranean. Conclusion: We hypothesized that Iranian and Mediterranean olive trees may have had a common origin from a unique center in the Near East region, possibly including the western Iranian area. The present pattern of variation may have derived from different environmental conditions, distinct levels and selection criteria, and divergent breeding opportunities found by Mediterranean and Iranian olives.These unexpected findings emphasize the importance of studying the Iranian olive germplasm as a promising but endangered source of variation. - . These authors contributed equally to this work. The olive (Olea europaea L. subsp. europaea var. europaea) is known as a symbol of the Mediterranean basin, but it also grows towards the east, to western Asia, and on to Georgia, Azerbaijan and Iran, which is known as one of the most eastern olive-producing countries. There is a long history of olive cultivation documentation in the Middle East, including citations in religious texts and descriptions by archaeo-botanists [1]. Recent biogeographical and archaeological studies have limited the area of origin for olive tree domestication to the western Mediterranean basin during the late Neolithic and Chalcolithic period [2,3]. Other works have placed it in the Near East [4,5,6], although this area was broader in the past and spanned the southern Caucasus to the Iranian plateau [7]. Olive cultivation developed significantly approximately 5,000 ya (years ago) in the eastern Mediterranean, spreading either to the island of Cyprus and towards Anatolia or from the island of Crete towards Egypt. Olives subsequently expanded into the western Mediterranean and were conveyed by Phoenicians, Greeks and Romans [8,9]. A recent study based on chloroplast markers [6] demonstrated that as many as 90% of present-day cultivars are characterized by the same haplotypes, suggesting a human-mediated dispersal of this chlorotype from the eastern Mediterranean into western localities. In addition, archaeological and historical evidences support the spread of cultivated olive trees from the Near East to the western Mediterranean [1,10]. Conversely, the early history of olive trees in Iran is shrouded in uncertainty. Pollen remains demonstrated the presence of olives in the western area of Iran starting approximately 4,300 ya, coinciding with the onset of the Bronze Age civilization, when olive pollen is appeared simultaneously with other tree crop species, such as Pistacia, Juglans and Platanus [11]. This evidence may support the idea that the first domesticated olive was more likely to have spread with other crops, first to the whole Levant [12] before being progressively disseminated to the western Mediterranean. During the 10th and 11th centuries, olives were cultivated in the Iranian areas of Nisapur, Gorgan, Deylam, Ramhormuz, Arrajan, and Fars. This distribution likely reflects the agricultural situation as it existed in pre-Islamic Persia [13]. Olives were later most likely neglected throughout the Iranian plateau for a very long time for various historic and economic reasons. In the 19th century, olive cultivation in Iran was mainly concentrated along the Caspian provinces, and during the first half of the 20th century, olives were grown solely in the Manjil, Rudbar and Tarom districts of the Elburz Mountains. At present, the olive crop area in Iran covers approximately 110,000 hectares, and it is still established along the Caspian Sea and northern provinces from Zanjan to Golestan. Thus, the country has a relatively small olive growing area [14]; furthermore, a massive introduction of allochthonous varieties has recently been reported [15]. Discrimination of olive genotypes is widely performed by the use of SSR markers [16], and numerous Iranian olive accessions have been analyzed in this way. In particular, SSR markers, used in a survey of 92 accessions of Iranian olive varieties, from the northern provinces of Gilan, Zanjan and Qazvin, uncovered the presence of cases of identity among genotypes carrying different names, as well as cases of variations within cultivars with the same name [17]. Another work, on different genotypes corresponding to the three most common names of Iranian cultivars (Golooleh, Shengeh and Rowghani), showed again high variability within accessions carrying the same name, underlining the discrimination power of SSR markers and the need to clarify the cases of homonymy [18]. It was also previously observed that a large set of Iranian cultivars grouped separately when compared with 6 [19] and 30 Mediterranean cultivars [20]. Samples belonging to O. europaea subsp. cuspidata occurred as spread plants in southern and south-eastern Iran. It was demonstrated that cuspidata samples from Kerman contain a chlorotype common to samples from China, Pakistan, Oman and Ethiopia [21,22], whereas other cuspidata representatives collected from the Hormozgan province showed two different forms that were distinguished by morphological and RAPD marker data [23]. Apart from cultivated and cuspidata trees, many other olive plants of unknown origin exist in Iran, most of which are found in regions far from cultivated areas a (...truncated)