Characterization of Worldwide Olive Germplasm Banks of Marrakech (Morocco) and Córdoba (Spain): Towards management and use of olive germplasm in breeding programs.

Olive (Olea europaea L.) is a major fruit crop in the Mediterranean Basin. Ex-situ olive management is essential to ensure optimal use of genetic resources in breeding programs. The Worldwide Olive Germplasm Bank of Córdoba (WOGBC), Spain, and Marrakech (WOGBM), Morocco, are currently the largest existing olive germplasm collections. Characterization, identification, comparison and authentication of all accessions in both collections could thus provide useful information for managing olive germplasm for its preservation, exchange within the scientific community and use in breeding programs. Here we applied 20 microsatellite markers (SSR) and 11 endocarp morphological traits to discriminate and authenticate 1091 olive accessions belonging to WOGBM and WOGBC (554 and 537, respectively). Of all the analyzed accessions, 672 distinct SSR profiles considered as unique genotypes were identified, but only 130 were present in both collections. Combining SSR markers and endocarp traits led to the identification of 535 cultivars (126 in common) and 120 authenticated cultivars. No significant differences were observed between collections regarding the allelic richness and diversity index. We concluded that the genetic diversity level was stable despite marked contrasts in varietal composition between collections, which could be explained by their different collection establishment conditions. This highlights the extent of cultivar variability within WOGBs. Moreover, we detected 192 mislabeling errors, 72 of which were found in WOGBM. A total of 228 genotypes as molecular variants of 74 cultivars, 79 synonyms and 39 homonyms as new cases were identified. Both collections were combined to define the nested core collections of 55, 121 and 150 sample sizes proposed for further studies. This study was a preliminary step towards managing and mining the genetic diversity in both collections while developing collaborations between olive research teams to conduct association mapping studies by exchanging and phenotyping accessions in contrasted environmental sites.

Cultivated Olive Diversification at Local and Regional Scales: Evidence From the Genetic Characterization of French Genetic Resources.

Molecular characterization of crop genetic resources is a powerful approach to elucidate the origin of varieties and facilitate local cultivar management. Here we aimed to decipher the origin and diversification of French local olive germplasm. The 113 olive accessions of the ex situ collection of Porquerolles were characterized with 20 nuclear microsatellites plus their plastid haplotype. We then compared this collection to Mediterranean olive varieties from the Worldwide Olive Germplasm Bank of Marrakech, Morocco. High genetic diversity was observed within local French varieties, indicating a high admixture level, with an almost equal contribution from the three main Mediterranean gene pools. Nearly identical and closely related genotypes were observed among French and Italian/Spanish varieties. A high number of parent-offspring relationships were also detected among French varieties and between French and two Italian varieties ('Frantoio' and 'Moraiolo') and the Spanish variety ('Gordal Sevillana'). Our investigations indicated that French olive germplasm resulted from the diffusion of material from multiple origins followed by diversification based on parentage relationships between varieties. We strongly suggest that farmers have been actively selecting olives based on local French varieties. French olive agroecosystems more affected by unexpected frosts than southernmost regions could also be seen as incubators and as a bridge between Italy and Spain that has enhanced varietal olive diversification.

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.

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

BACKGROUND: Plant-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. RESULTS: A 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. CONCLUSIONS: Olive 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.

QTL mapping of flowering and fruiting traits in olive.

One of the challenge fruit growers are facing is to balance between tree production and vegetative growth from year to year. To investigate the existence of genetic determinism for reproductive behaviour in olive tree, we studied an olive segregating population derived from a cross between 'Olivière' and 'Arbequina' cultivars. Our strategy was based on (i) an annual assessment of individual trees yield, and (ii) a decomposition of adult growth units at the crown periphery into quantitative variables related to both flowering and fruiting process in relation to their growth and branching. Genetic models, including the year, genotype effects and their interactions, were built with variance function and correlation structure of residuals when necessary. Among the progeny, trees were either 'ON' or 'OFF' for a given year and patterns of regular vs. irregular bearing were revealed. Genotype effect was significant on yield but not for flowering traits at growth unit (GU) scale, whereas the interaction between genotype and year was significant for both traits. A strong genetic effect was found for all fruiting traits without interaction with the year. Based on the new constructed genetic map, QTLs with small effects were detected, revealing multigenic control of the studied traits. Many were associated to alleles from 'Arbequina'. Genetic correlations were found between Yield and Fruit set at GU scale suggesting a common genetic control, even though QTL co-localisations were in spe`cific years only. Most QTL were associated to flowering traits in specific years, even though reproductive traits at GU scale did not capture the bearing status of the trees in a given year. Results were also interpreted with respect to ontogenetic changes of growth and branching, and an alternative sampling strategy was proposed for capturing tree fruiting behaviour. Regular bearing progenies were identified and could constitute innovative material for selection programs.