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

Abstract

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. 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. 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'. 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.