Genetic diversity and population structure of wild olives from the North-Western Mediterranean assessed by SSR markers

Abstract

Background and aims: This study examines the pattern of genetic variability and genetic relationships of wild olive (Olea europaea subsp. europaea var. sylvestris) populations in the north-western Mediterranean. Recent bottleneck events are also assessed and an investigation is made of the underlying population structure of the wild olive populations.

Methods: The genetic variation within and between 11 wild olive populations (171 individuals) was analysed with eight microsatellite markers. Conventional and Bayesian-based analyses were applied to infer genetic structure and define the number of gene pools in wild olive populations.

Key results: Bayesian model-based clustering identified four gene pools, which was in overall concordance with the Factorial Correspondence Analysis and Fitch-Margoliash tree. Two gene pools were predominantly found in southern Spain and Italian islands, respectively, in samples gathered from undisturbed forests of the typical Mediterranean climate. The other two gene pools were mostly detected in the north-eastern regions of Spain and in continental Italy and belong to the transition region between the temperate and Mediterranean climate zones.

Conclusions: On the basis of these results, it can be assumed that the population structure of wild olives from the north-western Mediterranean partially reflects the evolutionary history of these populations, although hybridization between true oleasters and cultivated varieties in areas of close contact between the two forms must be assumed as well. The study indicates a degree of admixture in all the populations, and suggests some caution regarding genetic differentiation at the population level, making it difficult to identify clear-cut genetic boundaries between candidate areas containing either genuinely wild or feral germplasm.

Fig. 1.

Locations of the wild olive populations sampled.

Fig. 2.

Factorial correspondence analysis (FCA) of 171 olive trees belonging to 11 populations. Each individual genotype is indicated by a small symbol, while the population barycentres are represented by larger ones.

Fig. 3.

Unrooted Fitch–Margoliash tree based on Nei's standard genetic distance between 11 olive populations and the proportions of ancestry of each population in each of the four gene pools defined with the model-based clustering method from Pritchard et al., (2000). Numbers above branches indicate bootstrap support percentage over 50 % in 10 000 pseudoreplicates. Inferred gene pools: A₁, southern Spanish genepool; A₂, insular Italian genepool; B₁, north-eastern Spanish genepool; B₂, gene pool detected predominantly in Catalonia 2 and Umbria.