Genetic variation of naturally growing olive trees in Israel: from abandoned groves to feral and wild?

Abstract

Background: Naturally growing populations of olive trees are found in the Mediterranean garrigue and maquis in Israel. Here, we used the Simple Sequence Repeat (SSR) genetic marker technique to investigate whether these represent wild var. sylvestris. Leaf samples were collected from a total of 205 trees at six sites of naturally growing olive populations in Israel. The genetic analysis included a multi-locus lineage (MLL) analysis, Rousset's genetic distances, Fst values, private alleles, other diversity values and a Structure analysis. The analyses also included scions and suckers of old cultivated olive trees, for which the dominance of one clone in scions (MLL1) and a second in suckers (MLL7) had been shown earlier.

Results: The majority of trees from a Judean Mts. population and from one population from the Galilee showed close genetic similarity to scions of old cultivated trees. Different from that, site-specific and a high number of single occurrence MLLs were found in four olive populations from the Galilee and Carmel which also were genetically more distant from old cultivated trees, had relatively high genetic diversity values and higher numbers of private alleles. Whereas in two of these populations MLL7 (and partly MLL1) were found in low frequency, the two other populations did not contain these MLLs and were very similar in their genetic structure to suckers of old cultivated olive trees that originated from sexual reproduction.

Conclusions: The genetic distinctness from old cultivated olive trees, particularly of one population from Galilee and one from Carmel, suggests that trees at these sites might represent wild var. sylvestris. The similarity in genetic structure of these two populations with the suckers of old cultivated trees implies that wild trees were used as rootstocks. Alternatively, trees at these two sites may be remnants of old cultivated trees in which the scion-derived trunk died and was replaced by suckers. However, considering landscape and topographic environment at the two sites this second interpretation is less likely.

Keywords: Crop domestication; Cultivated old olive trees; Gene flow; Grafting; Historical agriculture; Oleaster; var. sylvestris.

Fig. 1

Location of the six naturally growing olive populations sampled (a); naturally growing olive trees in the Galilee at Idmit, where trees are exposed to strong herbivore pressure (b) and in a typical garrigue formation at Zurit (c)

Fig. 2

Mean number of alleles per locus as a function of sample size of the populations analyzed and of suckers and scions of old cultivated trees

Fig. 3

Heat-map illustration of Rousset’s genetic distances between naturally growing populations and multi-locus lineages MLL1 and MLL7, common to scions and rootstocks of grafted old olive trees [33]

Fig. 4

Inferred genetic structure of scions and rootstocks of grafted old olive trees and naturally growing populations of olive trees in the southeast Mediterranean. Bayesian clustering with the admixture model implemented in Structure was used to assign individual MLGs to genetic clusters (K = 3). Individual MLGs within each group are represented by vertical bars and genetic groups are shown in different colors