Differences between native and introduced olive cultivars as revealed by morphology of drupes, oil composition and SSR polymorphisms: A case study in Tunisia

Abstract

Olive cultivars are diversified but nothing is known on their origins and if they are local or introduced in any regions. The study aims to determine which traits may help to identify native from introduced cultivars and oleaster trees. We compared cultivars and oleasters from North Tunisia to determine their relationships based on morphological traits, oil composition and SSR genotyping at seven loci. We used those parameters to examine 32 cultivar trees from 17 denominations and 70 oleaster trees sampled. We used multivariate analysis, enabling to retain the best variables, to establish relationships among trees based on morphological and pomological parameters. Gas chromatography was used to determine fatty acid composition of 30 cultivar trees and 13 oleaster trees. We determined for one cultivar Gerboui the steady drupe, pit morphological and oil composition variation ranges in six different contrasted agro-systems. SSR genotyping was performed in polyacrylamide gels after fluorescent labelling. Based on morphology, oleaster trees from agro-ecosystems clustered broadly in an intermediate position between cultivars and oleasters from natural ecosystems. SSR revealed that the feral and genuine oleasters plus cultivars are always overlapping. Relationships between cultivars are displayed in two dendrograms. They revealed six and three main clusters based on Unweighted Pair Group Method (UPGMA) and Ward algorithm, respectively. They mix olive cultivar and oleaster trees suggesting kinship relationships between some cultivar and some oleaster trees. In contrast, on PCA, some morphological parameters split our sample approximately between olive and oleaster trees. Oil composition was similar between cultivar and oleaster trees. Kinship relationships between cultivar and oleaster trees based on molecular polymorphisms suggested that olive cultivars may have origin in local oleasters. Oil composition as fruit descriptors and drupe size appeared inefficient to discriminate between olive and oleaster trees, in comparison to SSR. Our results suggested several domestication events for the olive. It is important to know which cultivars have local origin to promote and sale products from Tunisia as from all around the Mediterranean basin.