Genetic and phenotypic evidence suggest the existence of indigenous olive population of wild var. sylvestris in the Carmel coast, southern Levant

Abstract

Background: Populations of Olea europaea subsp. europaea var. sylvestris, the ancestor of cultivated olives, are scattered across the Mediterranean Basin. However, after millennia of possible hybridization with cultivated varieties, the genetic identity of many of these populations remain questionable. In the southern Levant, the plausible primary domestication center of olives, many of the naturally growing olive (NGOs) are considered feral, having developed from nearby olive groves. Here, we investigated the genetic identity of NGOs population in the Carmel region, hypothesizing that their specific location, which limit anemophily, provided an opportunity for the persistence of genuine var. sylvestris.

Results: We mapped more than 1,000 NGOs on the Kurkar ridge along the Carmel coast, within and outside the residential area of Atlit and used simple sequence repeats of 14 loci to assess the spatial genetic structure of 129 NGOs. Genetic diversity parameters and genetic distances between NGO and cultivated olives, as well as phenotypic and morphometric analyses of their oil content and pits, respectively, indicated the presence of a genuine var. sylvestris population. However, NGOs within the residential area of Atlit and old settlements showed an intermediate admix genetic structure, indicating on hybridization with local varieties, a consequence of their proximity to cultivated trees.

Conclusions: Integrating the results of genetic and phenotypic analyses we provide crucial evidence of the presence of a genuine var. sylvestris population in the southern Levant, in close geographical proximity to archaeological sites with the earliest evidence of olive exploitation in the ancient world. We supplement the results with recommendations for a conservation program that combines municipal requirements and the urgent need to preserve the largest population of var. sylvestris in the southern Levant.

Keywords: Olea europaea subsp. europaea var. sylvestris; Conservation; Genetic variation; Morphometry; Oil quality; Spatial structure.

Fig. 1

Mapping naturally growing olives in the Carmel region and Galilee, the presumed distribution range of O. europaea subsp. europaea var. sylvestris in the southern Levant. (A) Naturally growing olive trees were sampled in two populations, Idmit, and Atlit. (B) In Atlit, naturally growing olives were mapped and sampled in 12 polygons. (C) NGOs appearance in Idmit (D) and Atlit (E) indicate to the seniority of the plants, and thus the long existence of NGOs populations in the region

Fig. 2

Principal coordinate analysis (PCoA) of the two studied NGOs populations (Idmit and Atlit), MLL1 and MLL7 represent the most common southern Levant traditional cultivars

Fig. 3

Pairwise F_ST and G_ST values between NGOs of two investigate populations, Idmit (IDM) and Atlit (ATL), and MLL1 and MLL7 traditional cultivars. G_ST and F_ST values are presented above and below the diagonal, respectively

Fig. 4

Inferred genetic structure of naturally growing olives and of the two main local traditional cultivars MLL1 and MLL7. Bayesian clustering with the admixture model implemented in Structure was used to assign individual MLGs to genetic clusters (K = 8, Supplementary Fig. S3). Individual MLGs within each group are represented by vertical bars and genetic groups are shown in different colors

Fig. 5

Box-and-Whisker diagrams showing the means with upper and lower variability of the oil content in the fruit (A) and average fruit weight (B) of naturally growing olives in Atlit and MLL1. Asterisks represent significant differences at P < 0.001 level

Fig. 6

Results of the principal component analysis (PCA) of chemical compounds in the oil of the two groups of olive, cultivated MLL1 and naturally growing olives in Atlit (red and blue symbols, respectively) (see Table S2). Percent of variance explained by each axis is indicated in the axis titles

Fig. 7

Box-and-Whisker diagrams showing the median with upper and lower variability in the relative content of linoleic (A), oleic (B), palmitic (C) and gondoic (D). FAs of naturally growing olives in Atlit and the MLL1 cultivar. Asterisks represent significance at P < 0.01(**) and P < 0.001 (***) levels

Fig. 8

Results of the principal component analysis (PCA) of morphometric characteristics of NGOs olive pits NGOs from Atlit and those of the cultivated traditional cultivar reference groups: MLL1 and MLL7. Percent of variance explained by each axis in the PCA is indicated in the axis titles

Fig. 9

Representative pits of naturally growing trees from Atlit in comparison to those of the cultivated reference varieties