Abdelmajid Moukhli

Substantial genetic diversity in cultivated Moroccan olive despite a single major cultivar: a paradoxical situation evidenced by the use of SSR loci

To assess the genetic diversity in Moroccan cultivated olive, Olea europaea L. subsp. europaea, we performed molecular analysis of olive trees sampled in four geographic zones representing all areas of traditional olive culture. The analysis of 215 trees using 15 simple sequence repeat (SSR) loci revealed 105 alleles distributed among 60 SSR profiles. The analysis of chloroplast deoxyribonucleic acid polymorphism for these 60 olive genotypes allowed to identify four chlorotypes: 42 CE1, one CE2, nine COM1 and eight CCK. Among the 60 SSR profiles, 52 corresponded to cultivated olive trees for which neither denomination nor characterisation is available. These local olive genotypes displayed a spatial genetic structuring over the four Moroccan geographic zones (northwest, north centre, Atlas and southwest), as pairwise Fst values ranged from 0.0394 to 0.1383 and varied according to geographic distance. As single alleles detected in local olive were also observed in Moroccan oleaster populations, results suggest that plant material was mainly selected from indigenous populations. The assumption that Picholine marocaine cultivar is a multi-clonal cultivar was not supported by our data because we found a single genotype for 112 olive trees representing 31 to 93% of the olives sampled locally in the 14 different areas. Picholine marocaine and the few other named cultivars do not seem to belong to the same gene pools as the unnamed genotypes cultivated only locally. The situation is paradoxical: a substantial genetic diversity in Moroccan olive germplasm, probably resulting from much local domestication, but a single cultivar is predominant.

Genetic structure and core collection of the World Olive Germplasm Bank of Marrakech: towards the optimised management and use of Mediterranean olive genetic resources

The conservation of cultivated plants in ex-situ collections is essential for the optimal management and use of their genetic resources. For the olive tree, two world germplasm banks (OWGB) are presently established, in Córdoba (Spain) and Marrakech (Morocco). This latter was recently founded and includes 561 accessions from 14 Mediterranean countries. Using 12 nuclear microsatellites (SSRs) and three chloroplast DNA markers, this collection was characterised to examine the structure of the genetic diversity and propose a set of olive accessions encompassing the whole Mediterranean allelic diversity range. We identified 505 SSR profiles based on a total of 210 alleles. Based on these markers, the genetic diversity was similar to that of cultivars and wild olives which were previously characterised in another study indicating that OWGB Marrakech is representative of Mediterranean olive germplasm. Using a model-based Bayesian clustering method and principal components analysis, this OWGB was structured into three main gene pools corresponding to eastern, central and western parts of the Mediterranean Basin. We proposed 10 cores of 67 accessions capturing all detected alleles and 10 cores of 58 accessions capturing the 186 alleles observed more than once. In each of the 10 cores, a set of 40 accessions was identical, whereas the remaining accessions were different, indicating the need to include complementary criteria such as phenotypic adaptive and agronomic traits. Our study generated a molecular database for the entire OWGB Marrakech that may be used to optimise a strategy for the management of olive genetic resources and their use for subsequent genetic and genomic olive breeding.

Construction of core collections suitable for association mapping to optimize use of Mediterranean olive (Olea europaea L.) genetic resources.

Phenotypic characterisation of germplasm collections is a decisive step towards association mapping analyses, but it is particularly expensive and tedious for woody perennial plant species. Characterisation could be more efficient if focused on a reasonably sized subset of accessions, or so-called core collection (CC), reflecting the geographic origin and variability of the germplasm. The questions that arise concern the sample size to use and genetic parameters that should be optimized in a core collection to make it suitable for association mapping. Here we investigated these questions in olive (Olea europaea L.), a perennial fruit species. By testing different sampling methods and sizes in a worldwide olive germplasm bank (OWGB Marrakech, Morocco) containing 502 unique genotypes characterized by nuclear and plastid loci, a two-step sampling method was proposed. The Shannon-Weaver diversity index was found to be the best criterion to be maximized in the first step using the Core Hunter program. A primary core collection of 50 entries (CC50) was defined that captured more than 80% of the diversity. This latter was subsequently used as a kernel with the Mstrat program to capture the remaining diversity. 200 core collections of 94 entries (CC94) were thus built for flexibility in the choice of varieties to be studied. Most entries of both core collections (CC50 and CC94) were revealed to be unrelated due to the low kinship coefficient, whereas a genetic structure spanning the eastern and western/central Mediterranean regions was noted. Linkage disequilibrium was observed in CC94 which was mainly explained by a genetic structure effect as noted for OWGB Marrakech. Since they reflect the geographic origin and diversity of olive germplasm and are of reasonable size, both core collections will be of major interest to develop long-term association studies and thus enhance genomic selection in olive species.

Menara gardens: a Moroccan olive germplasm collection identified by a SSR locus-based genetic study

The Menara gardens in Marrakech (Morocco), established in the 12th century, comprise more than 2000 olive trees. To assess the genetic diversity within these gardens, we performed SSR analysis of 128 randomly sampled olive trees. Using 15 SSR loci which revealed 70 alleles, we identified 16 distinct genotypes. The analysis of chloroplast DNA polymorphism allowed to identify 2 chlorotypes: COM1 for 2 genotypes and CE1 for all others. Among the 128 trees analysed, Picholine marocaine was the prevalent cultivar represented by 100 trees (78%), the 28 remaining trees being classified into 15 distinct genotypes. However, four genotypes were closely related to Picholine marocaine from which they probably derived through somatic mutations. The genotypic and genetic diversity observed in Menara gardens compared with the genotype diversity from different areas (North and Atlas); these facts support the hypothesis that these gardens have been planted with trees probably originating from different areas of Morocco. Our results point out an important concept for ex situ conservation: the assumption that local Moroccan olive germplasm was empirically, but not purposively, conserved already in Menara gardens.

Elucidation of the genetic architecture of self-incompatibility in olive: Evolutionary consequences and perspectives for orchard management

The olive (Olea europaea L.) is a typical important perennial crop species for which the genetic determination and even functionality of self‐incompatibility (SI) are still largely unresolved. It is still not known whether SI is under gametophytic or sporophytic genetic control, yet fruit production in orchards depends critically on successful ovule fertilization. We studied the genetic determination of SI in olive in light of recent discoveries in other genera of the Oleaceae family. Using intra‐ and interspecific stigma tests on 89 genotypes representative of species‐wide olive diversity and the compatibility/incompatibility reactions of progeny plants from controlled crosses, we confirmed that O. europaea shares the same homomorphic diallelic self‐incompatibility (DSI) system as the one recently identified in Phillyrea angustifolia and Fraxinus ornus. SI is sporophytic in olive. The incompatibility response differs between the two SI groups in terms of how far pollen tubes grow before growth is arrested within stigma tissues. As a consequence of this DSI system, the chance of cross‐incompatibility between pairs of varieties in an orchard is high (50%) and fruit production may be limited by the availability of compatible pollen. The discovery of the DSI system in O. europaea will undoubtedly offer opportunities to optimize fruit production.

Olive Genetic Resources

As one of the most important and ancient fruit crops in the Mediterranean Basin, olive is characterized by a huge genetic patrimony, represented by cultivated and wild germplasm, ancient trees and related forms. The richness of this germplasm represents an unusual case among horticultural crops, due to species longevity, lack of new better performing genotypes, and the millennial tradition of cultivation. Focusing on a wide spectrum of genetic resources, their conservation, characterization, and management, this chapter tries to give an insight into the achievements and the necessities of this type of works in olive. Knowledge of existing diversity among the olive genetic resources is essential to maximize their conservation, safeguard, and exploitation.

Plant-parasitic Nematodes Associated with Olive Tree in Southern Morocco

Plant-parasitic nematodes affect significantly the production of susceptible plants, including olive trees. In this context, nematode communities were determined in soil samples collected from 23 olive growing sites in the Haouz and Souss regions (southern Morocco). These sites corresponded to various modalities: wild (Olea europaea L. ssp. sylvestris) or cropped olive (Olea. europaea L. ssp. europaea), traditional or high-density cropping, rainfed or irrigated. Even free-living nematodes prevailed in most of the sites, high population levels of plant-parasitic nematodes were observed in rainfed cropping systems than in irrigated systems. Ten nematode families and 14 genera were identified. The most important plant-parasitic nematodes detected, in order decreasing frequency of infestation (percentage of samples), were spiral nematodes Helicotylenchus spp. (100%) and Rotylenchus spp. (87%), lesion nematodes Pratylenchus spp. (100%) and root-knot nematodes Meloidogyne spp. (40%). Most of the nematode species were assigned to more or less colonizer nematodes, whereas only one family (Longidoridae) was assigned to persistent nematodes. Considering the two producing areas, plant-parasitic nematodes were significantly more abundant in the Souss region than in the Haouz region, but nematode diversity was higher in the second one. The prevalence of Meloidogyne spp. in the Haouz region would be related to the predominance of irrigated cropping systems in this region. Hoplolaimidae nematodes (Helicotylenchus spp. and Rotylenchus spp.) are better adapted to rainfed conditions that prevail in the Souss region. Co-inertia analysis showed the importance of soil physic-chemical characteristics (e.g., pH, texture and nutrients) on the structure of the plant-parasitic nematode community patterns. The high occurrence of Helicotylenchus spp. in olive orchards may be induced by fertilization. Aphelenchoides spp., Gracilacus spp., Pratylenchus spp., Rotylenchidae and Tylenchidae were supported by coarse textures as it was observed in the Souss region

How anthropogenic changes may affect soil-borne parasite diversity? Plant-parasitic nematode communities associated with olive trees in Morocco as a case study

BackgroundPlant-parasitic nematodes (PPN) are major crop pests. On olive (Olea europaea), they significantly contribute to economic losses in the top-ten olive producing countries in the world especially in nurseries and under cropping intensification. The diversity and the structure of PPN communities respond to environmental and anthropogenic forces. The olive tree is a good host plant model to understand the impact of such forces on PPN diversity since it grows according to different modalities (wild, feral and cultivated olives). A wide soil survey was conducted in several olive-growing regions in Morocco. The taxonomical and the functional diversity as well as the structures of PPN communities were described and then compared between non-cultivated (wild and feral forms) and cultivated (traditional and high-density olive cultivation) olives.ResultsA high diversity of PPN with the detection of 117 species and 47 genera was revealed. Some taxa were recorded for the first time on olive trees worldwide and new species were also identified. Anthropogenic factors (wild vs cultivated conditions) strongly impacted the PPN diversity and the functional composition of communities because the species richness, the local diversity and the evenness of communities significantly decreased and the abundance of nematodes significantly increased in high-density conditions. Furthermore, these conditions exhibited many more obligate and colonizer PPN and less persister PPN compared to non-cultivated conditions. Taxonomical structures of communities were also impacted: genera such as Xiphinema spp. and Heterodera spp. were dominant in wild olive, whereas harmful taxa such as Meloidogyne spp. were especially enhanced in high-density orchards.ConclusionsOlive anthropogenic practices reduce the PPN diversity in communities and lead to changes of the community structures with the development of some damaging nematodes. The study underlined the PPN diversity as a relevant indicator to assess community pathogenicity. That could be taken into account in order to design control strategies based on community rearrangements and interactions between species instead of reducing the most pathogenic species.

Olive tree varieties cultivated for the great Baetican oil trade between the 1st and the 4th centuries ad: morphometric analysis of olive stones from Las Delicias (Ecija, Province of Seville, Spain)

During the excavations of a Roman amphora workshop and oil mill of the 1st–4th century ad in Las Delicias, Genil valley, Ecija, Spain, large quantities of charred olive stones were recovered. The assemblages discovered in the pottery kilns demonstrate the use as fuel of olive residues, which were obtained from the extraction of the oil in the nearby mill. The abundance of material offered the opportunity to study the infra-specific diversity of the olives growing in the province of Baetica, which is known to have been an important oil-producing region during the Roman Empire. In total, 335 intact charred archaeological olive stones were analysed using geometric morphometry (outline analysis) and compared with several current morphotypes. These have been identified within a set of dimensional references of the stones established from the morphometric study of current varieties and wild populations, including genuinely wild and feral forms of olives, from various areas around the Mediterranean. The morphotype mainly found in wild populations was widely represented among the olive stones from Las Delicias. A large proportion of the archaeological stones were however close to various domesticated forms, which reflect the history of the region and of its varied cultural Mediterranean influences, Punic, Greek and Roman. Moreover, intermediate forms between two distinct morphotypes were identified. They suggest that hybrid olive trees derived from crosses among domesticated varieties and also between domesticated and wild forms, were grown in Las Delicias. In the Genil valley, Roman olive cultivation was based on a set of local olives which included wild and domesticated varieties from various origins, and whose diversity arose from breeding for improvement of varieties.