Molecular diversity and gene flow within and among different subspecies of the wild olive (Olea europaea L.): A review

Six subspecies are currently recognized in the wild olive (Olea europaea L.): cuspidata, europaea, laperrinei, maroccana, cerasiformis, and guanchica. These occur in different natural distribution ranges in Europe, Africa, and Asia. Here we present an overview and summary of the accumulated information on the molecular diversity and gene flow in the species. Past climatic oscillations and geographical barriers have been identified to govern the pattern of genetic diversity and gene movement within and between taxa. Seed dispersal by animals and cultivar transport over vast geographical distances by man are important factors which have prevented the complete differentiation of subspecies. Available evidence has shown that subsp. cuspidata is the early diverging taxon of the olive complex. However, results were not fully consistent concerning the relationships between the other subspecies. Molecular studies using nuclear and cytoplasmic markers show that the oleasters (subsp. europaea var. sylvestris) are ancestors to the cultivated forms (subsp. europaea var. europaea). However, due to hybridization in geographical proximate regions sometimes other subspecies also have influenced the cultivars. This complex process has blurred the domestication history of the species. The wild relatives of the cultivated olive are endangered in many parts of their range, albeit being important for forest restoration. Overall, molecular studies have answered a number of important questions, but most of the genetic studies so far focused on the Mediterranean oleasters, while the widespread cuspidata has not been sufficiently investigated. Using genomic approaches to understand the pattern of genetic variation will be helpful to solve open questions. Moreover, future genetic studies should focus on quantifying the local effects of disturbance factors on genetic diversity and mating system parameters of wild olive.