Population genetics of Mediterranean and Saharan olives: geographic patterns of differentiation and evidence for early generations of admixture.

BACKGROUND AND AIMS: The olive (Olea europaea subsp. europaea) was domesticated in the Mediterranean area but its wild relatives are distributed over three continents, from the Mediterranean basin to South Africa and south-western Asia. Recent studies suggested that this crop originated in the Levant while a secondary diversification occurred in most westward areas. A possible contribution of the Saharan subspecies (subsp. laperrinei) has been highlighted, but the data available were too limited to draw definite conclusions. Here, patterns of genetic differentiation in the Mediterranean and Saharan olives are analysed to test for recent admixture between these taxa. METHODS: Nuclear microsatellite and plastid DNA (ptDNA) data were compiled from previous studies and completed for a sample of 470 cultivars, 390 wild Mediterranean trees and 270 Saharan olives. A network was reconstructed for the ptDNA haplotypes, while a Bayesian clustering method was applied to identify the main gene pools in the data set and then simulate and test for early generations of admixture between Mediterranean and Saharan olives. KEY RESULTS: Four lineages of ptDNA haplotypes are recognized: three from the Mediterranean basin and one from the Sahara. Only one haplotype, primarily distributed in the Sahara, is shared between laperrinei and europaea. This haplotype is detected once in 'Dhokar', a cultivar from the Maghreb. Nuclear microsatellites show geographic patterns of genetic differentiation in the Mediterranean olive that reflect the primary origins of cultivars in the Levant, and indicate a high genetic differentiation between europaea and laperrinei. No first-generation hybrid between europaea and laperrinei is detected, but recent, reciprocal admixture between Mediterranean and Saharan subspecies is found in a few accessions, including 'Dhokar'. CONCLUSIONS: This study reports for the first time admixture between Mediterranean and Saharan olives. Although its contribution remains limited, Laperrine's olive has been involved in the diversification of cultivated olives.

The complex history of the olive tree: from Late Quaternary diversification of Mediterranean lineages to primary domestication in the northern Levant.

The location and timing of domestication of the olive tree, a key crop in Early Mediterranean societies, remain hotly debated. Here, we unravel the history of wild olives (oleasters), and then infer the primary origins of the domesticated olive. Phylogeography and Bayesian molecular dating analyses based on plastid genome profiling of 1263 oleasters and 534 cultivated genotypes reveal three main lineages of pre-Quaternary origin. Regional hotspots of plastid diversity, species distribution modelling and macrofossils support the existence of three long-term refugia; namely the Near East (including Cyprus), the Aegean area and the Strait of Gibraltar. These ancestral wild gene pools have provided the essential foundations for cultivated olive breeding. Comparison of the geographical pattern of plastid diversity between wild and cultivated olives indicates the cradle of first domestication in the northern Levant followed by dispersals across the Mediterranean basin in parallel with the expansion of civilizations and human exchanges in this part of the world.

Spatial genetic structure in the Laperrine's olive (Olea europaea subsp. laperrinei), a long-living tree from the central Saharan mountains.

The Laperrine's olive (Olea europaea subsp. laperrinei) is an emblematic species of the Sahelo-Saharan Mountains. Populations of this tree are locally threatened by extinction due to climatic vicissitudes and human activities, particularly in Niger and Algeria. In order to study the spatial genetic structure and the dynamics of O. e. laperrinei populations, we sampled trees in four isolated mountain ranges (Tassili n'Ajjer and Hoggar (Algeria), Tamgak and Bagzane (Niger)). A total of 237 genets were identified using nuclear microsatellites. Phylogenetic reconstruction based on plastid DNA data supported a maternal origin of O. e. laperrinei populations in South Algeria, where a higher allelic richness was observed. Based on nuclear microsatellite data, two levels of structure were revealed: first, individuals from Niger and Algeria were separated in two distinct groups; second, four less differentiated clusters corresponded to the four studied mountain ranges. These results give support to the fact that desert barriers have greatly limited long distance gene flow. Within populations, pairwise kinship coefficients were significantly correlated to geographical distance for Niger populations but not for Algerian mountains. Historical factors and habitat heterogeneity may explain the differences observed. We conclude that the Hoggar acts as an important genetic reservoir that has to be taken into account in future conservation programmes. Moreover, very isolated endangered populations (for example, Bagzane) displaying evident genetic particularities have to be urgently considered for their endemism.