Tunisia occupies the fourth place on world scale in terms of olive oil production. Tunisian olive plantations count about 63 million trees covering 1.6 million of hectares and estimated to include more than 50 cultivars. Despite this economic importance, little is known about the olive germplasm of this country and about the genetic relationships among them. In fact Tunisian olive tree is dominated by the variety-population ‘Chemlali’ that occupies more than 2/3 of the total olive growing area. Whereas a wide range of minor cultivars are cultivated around their presumed areas of origin and have a limited diffusion outside these areas. The study of these less-common cultivars appears of particular interest because they may have agronomic characteristics which can be relevant to olive growers.

In this study, we used AFLP (amplified fragment length polymorphism) and SSR (simple sequence repeat) markers and agronomic traits to characterize a set of 26 Tunisian olive accessions, to assess the correlation between distance estimates based on agronomic characterisation and AFLP and SSR molecular markers, and to classify the cultivars into groups based on molecular profiles and agronomic traits.

The analysis of variance of the agronomical data revealed significant differences among accessions for all measured traits.

The two molecular approaches could uniquely fingerprint each of the 26 olive accessions. A higher level of polymorphism was obtained for SSR than for AFLP which highlights the discriminating power of the former.

The mean phenotypic dissimilarity was low in comparison to dissimilarity calculated using AFLP and SSR markers. SSR and AFLP markers can better differentiate pairs of accessions than agronomic traits that show a low level of genetic variation.

The estimated correlation similarities for the two molecular systems was significant but relatively low. The type of genetic polymorphism detected by the two markers and the number of primers used may affect the correlations among them. The correlation between the agronomical dissimilarity matrix and the matrices of genetic dissimilarity based on SSR and AFLP markers was very weak. When compared with DNA fingerprinting techniques, agronomic traits are relatively less reliable and efficient for precise discrimination of closely related accessions. Despite this limitation, numerical analysis of olive agronomic traits can be used as a general approach to establish from a practical viewpoint a first order of accessions classification within germplasm collections. Although both marker methods did not provide exactly the same description of relationships between the analysed accessions, there was some consistency. The best agreement between the two methods was present for accession pairs that were very distant or very close.

A dendrogram generated from the standardized phenotypic data is presented in Figure 1. The UPGMA cluster analysis revealed three main groups. Group 1 consisted of ‘Meski’, ‘Injassi’, ‘Besbassi’ and ‘Marsaline’, four accessions featuring large-sized fruits (4.95-9.22 g); the first three accessions are among the most important Tunisian table cultivars, whereas the letter is used for both oil and canning. ‘Swabaa Algia’, a medium-sized table olive clustered also within this group. Group 2 comprised two subgroups. One of them comprised six accessions used for both oil and canning (‘Ain Jarboua’, ‘Mongar Ragma’, ‘Regregui’, ‘Chemchali’, ‘Zarrazi’ and ‘Sayali’) that have medium-sized fruits (2.4-4 g), medium oil content (42-56%), high oleic (70.9-81%) and low linoleic (3.7-13.3%) acids contents. Five oil-producing accessions ‘Rakhami’, ‘Oueslati’, ‘Semni’, ‘Chétoui’ and ‘Dhokkar’, as well as the accession ‘Gerboui’, which has dual uses (oil and table) clustered together in the second subgroup. Small fruit size (1.3-2.4 g) and high linoleic acid content (14.6-22.5%) characterized accessions in this subgroup. The accessions ‘Unknown’, ‘Neb Djemel’ and ‘Toffahi’ also branched in the second group at a higher value of dissimilarity. Group 3 included 6 accessions, all used for oil production, with very small fruits (1-1.5 g) and oils rich in palmitoleic acid (2.1-2.3%).

The UPGMA clustering (Figure 1) obtained from the AFLP-SSR distance matrix showed that most of Tunisian olive accessions clustered according to their fruit size. For instance small-fruited accessions clustered in Group A and B. Accessions that have medium to large sized-fruits clustered in Group C. The relationship between electrophoretic profiles and the common utilisation of fruits (oil, canning, or both uses) may be due either to a single origin of varieties with big fruits or to their less-close proximity with wild populations due to a stronger or longer selection towards fruit size (Besnard et al. 2001). A trend of clustering of cultivars originating from the same or adjacent regions was also detected. The clustering of the cultivars from the same or nearby region suggests a common genetic base and autochthonous origin for these cultivars as well as their limited diffusion from their centres of origin. Cultivar intercrossing and crosses with wild accessions, along with local selection of outstanding seedlings and subsequent vegetative cloning, could have led to a large number of varieties around their possible original areas of cultivation.

The data obtained can be used for the varietal survey and construction of a database of all olive varieties grown in Tunisia and providing also additional information that could form the basis for the rational design of breeding programs.

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