Addressing the Challenge of Cultivars Identification and Authentication in Mediterranean Olive Collections: A Case Study in Morocco

Conservation and use of well-characterized olive (Olea europaea L.) genetic resources are the key to future olive improvement and sustainable production. Yet, authentication of plant materials in ex-situ olive collections throughout the world has received little attention. Here we characterized 95 accessions, from a collection maintained in the experimental station of INRAMeknes, Morocco, by comparing their SSR (14 markers) and morphological (11 endocarp traits) profiles to an international reference dataset with 672 distinct genotypes corresponding to 535 well-described olive cultivars from the two Worldwide Olive Germplasm Banks of Marrakech, Morocco, and Cordoba, Spain (WOGB-M/C). Results revealed 122 alleles in the Meknes collection versus 265 in the reference database, but the difference was not significant. Additionally, forty cultivars were identified in Meknes collection, among which 33 were present in the reference database. Principal Coordinates Analysis revealed that these varieties span the range of all of the 535 varieties in the international database, indicating important genetic diversity within the investigated plant materials. Finally, cases of mislabeling errors, synonyms, and redundant genotypes pertaining mainly to “Picholine marocaine” and “Frantoio” varieties have been encountered in Meknes collection. Overall, our work highlights the power of coupling modern genetic and morphological tools along with exploring reference databases for authenticating genetic cultivars in olive tree collections. &nbsp

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

Primary Selection and Secondary Diversification: Two Key Processes in the History of Olive Domestication

Knowledge on the crop domestication process is important from a cultural and agricultural standpoint since it can shed light on the origin and history of human civilizations as well as the management of genetic resources, while offering guidance for modern breeding. The olive tree (Olea europaea ssp. europaea) is the most iconic of the old crop species of the Mediterranean Basin (MB). Primary domestication from wild olive probably occurred around 6000 BP in the Middle East. However, the question remains as to whether cultivated olive derived from a single domestication event in the Levant, followed by secondary diversification, or whether it was the result of independent domestication events. Here, we analyzed a comprehensive sample collected from 35 wild populations (722 individuals) and 410 cultivars from across the MB using nuclear and plastid DNA markers. Our genetic investigations argue in favor of a single primary domestication event in the eastern MB, followed by diffusion of the first domesticated olive and diversification in the central and western MB as key processes in the olive tree history

Testing different approaches to construct an olive (Olea europaea L.) core subset suitable for association genetic studies

Evaluation of genetic diversity is of great interest for the management of germplasm collections and breeding programs. Management can be efficient when the evaluation is focused on a subset of accessions that represents the variability observed in the whole germplasm collection. Most core sets have been developed for seed crops using different approaches and sampling size to select entries on the basis of genetic and/or phenotypic data, while few studies on perennial crops have been published. Here, we proposed a core collection for cultivated olive (Olea europaea L.) using both Simple Sequence Repeat (SSR) markers and phenotypic traits by testing different sampling approaches including stratified and non-stratified methods. Twelve SSR markers were used to construct a core subset from an initial collection of 505 single genotypes sourced from 14 Mediterranean countries. Among all the sampling methods, we showed that a sample size of 12.5% was most suitable in capturing all the observed alleles using the M-method approach. Based on both SSR and phenotypic data, we established an initial core set, including the main Mediterranean cultivars, which displayed the highest genetic and phenotypic variability. No obvious genetic structure was indicated when the core subset was analyzed with Principal Coordinate analysis (PCoA). Our results gave an efficient basis as a first step for olive association mapping. The constructed core subset could be further evaluated for traits of agronomic interest, leading to association between the allelic variation and the phenotypic variability