Plastid and nuclear DNA polymorphism reveals historical processes of isolation and reticulation in the olive tree complex (Olea europaea)

The olive tree is considered one of the best indicators of the Mediterranean climate. The species' distribution is associated with geographical and bioclimatic factors, as well as being influenced by a long period of cultivation. Despite concerted efforts of different research groups, the origin of the Mediterranean olive tree still remains elusive. In the present study, relationships between taxa and populations covering the entire range of Olea europaea were investigated using both maternal (plastid genome) and biparental (nuclear genome) markers to disclose evolutionary patterns in the olive complex. Phylogenetic and phylogeographical results of the two-genome analyses were interpreted in a biogeographical context. Mediterranean, temperate and subtropical floristic regions of the Old World. Phylogeographical reconstructions of plastid DNA polymorphism were performed using microsatellites, restriction sites and indels on a wide sample of 185 representative trees across the Old World, including 28 herbarium specimens from remote areas. Additionally, the potential utility of one ITS-1 pseudogene for phylogenetic analyses was explored using Bayesian and maximum parsimony approaches on a subsample of 38 olive trees. Forty plastid haplotypes were recognized and split into two lineages and seven sublineages. The analysis of ITS-1 sequences also allowed the identification of seven well differentiated groups. Distribution of plastid and ribosomal DNA lineages was congruent, but particular cases of phylogenetic incongruence were disclosed (particularly in the Sahara and Madeira). Lastly, two divergent ITS-1 copies were isolated from the same sample of four individuals of different subspecies. Phylogenetic congruence of both ITS-1 and plastid lineages suggested an evolutionary scenario of predominant isolation during the Plio-Pleistocene in Macaronesia, the Mediterranean, southern Africa, eastern Africa and Asia. The Saharan desert appeared to have played an important role of vicariant barrier between southern and northern African populations in early times. Incongruence of some plastid and nuclear results, as well as intermingled ITS-1 copies of different lineages in single individuals, was interpreted as a result of recurrent reticulation events in the olive complex. We identified an ancient hybrid zone from the Sahara to north-eastern African mountains, where divergent plastid and nuclear lineages still co-exist. Results of this paper, and previous studies, suggest that the cultivated olive originated from a pre-Quaternary Mediterranean ancestor, with no evidence for a recent hybrid origin. In contrast, a continuous process of olive domestication through local hybridization events of cultivated trees with natural populations may have brought about a remarkably high genomic diversity among cultivated trees across the Mediterranean.