A preliminary study of the morphology and genetic characterisation of olive cultivars in the Maltese islands

Abstract

The use of morphological and genetic characterisation of olive tree cultivars has been widely used internationally by olive producing countries to characterise and identify olive cultivars to create olive germplasm databases. Locally, the literature on olive trees is limited, and a local olive germplasm database is lacking. During this study, sampling of 79 ancient olive trees (from 32 different sites in Malta, Gozo and Comino) was carried out from August 2020 till September 2020 and this was repeated in 2021. Samples were taken from both the canopy and the rootstock of each tree. Samples consisted of 30 leaves from both canopy and rootstock, which were stored at -80°C for genetic testing. Of these, 35 different genotypes were identified, of which four were known olive cultivars belonging to “Nocellara del Belice” Sicilian cultivar, “Lumiaro” Sicilian cultivar, “Ottobratica” Italian cultivar and “Zalmati” Tunisian Cultivar. The remaining 31 genotypes, were unique and not included in the international genotype database of the BeFOre project. Of these, 16 genotypes belong to rootstock and the other 19 belong to the canopy. The 16 rootstock genotypes were not studied further due to the lack of drupes and unviable leaves. The other 19 canopy genotypes were further investigated and morphological studies were carried out on the tree morphology, leaves, drupes and seed pits of the “Leader” cultivar of each genotype. They were characterised and analysed via 13 qualitative and 15 quantitative morphometric parameters through the International Union for the Protection of New Varieties of Plants (UPOV) method. The morphological profiles of the 19 genotypes, were compared to local and international literature and four were found to be identical to the “Bidni”, “Malti”,“ Lumiaro” and “Ottobratica” cultivars. All the other 15 genotypes were found to be unique, both from a morphological and genetic approach. This confirms the usefulness of the morphological profiles in being used as a tool to compare and distingush olive genotypes, and also to back up the genetic results. The morphometric parameters were analysed with One-Way ANOVA and Two-tailed Pearsons correlation by using SPSS v.21. The unique cultivars were compared by morphological parameters via Hierarchical cluster analysis with the Wards method, squared Euclidean distance and depicted via a dendrogram. The SSR marker data analysis for the observed number of alleles (Na), the effective number of alleles (Ne), observed (Ho) and expected (He) heterozygosity estimates and the heterozygous deficiency (F), were calculated by using GenAlEx version 6.5. The Neighbour-joining of the Maltese genotypes and International olive genotypes of the BeFOre database was conducted via MEGA version 7.