Exploring Olive Genetic Diversity in the Maltese Islands

Abstract

A comprehensive effort was devoted to exploring, collecting and characterizing the local Maltese olive germplasm, often represented by ancient, monumental trees and by plants of uncertain origin. SSR and cp-SSR analysis of all samples enabled the identification of 46 genotypes and establishment of the correspondence between ancient trees, main local varieties and other Mediterranean cultivars. The application of plastid markers enabled identification of two lineages among Maltese genotypes, with more than 50% represented by lineage E2. Twenty-nine cases of grafting were identified among the various genotypes and lineages. In most cases, E1 canopies were grafted on E2 rootstocks, but reverse cases were also observed. The phylogenetic study of Maltese genotypes, together with hundreds of cultivars from the Mediterranean Basin and beyond, highlights the richness of Maltese olive diversity and drawing attention to the genetic similarity of some Maltese olive genotypes with neighboring Italian and Algerian varieties. These results underline the long-lasting presence of the olive in the country, contributing to the reconstruction of its phylogeny and demonstrating a possible autochthonous origin of many samples. Some still-living ancient trees are at serious risk of extinction due to abandonment, urban expansion and environmental threats. This study supports the preservation of the Maltese olive germplasm and highlights its importance as a rich genetic source to face new agronomical challenges and future climatic constraints.

The Ancient Olive Trees (Olea europaea L.) of the Maltese Islands: A Rich and Unexplored Patrimony to Enhance Oliviculture

“…Fruits of 19 genotypes grown on Maltese territory were collected and evaluated over the course of the 2020 and 2022 crop seasons. Among these, two were traced back to Italian cultivars: Nocellara del Belice (1Plattini) and Ottobratica (3Gudja), while the other 17 represented potentially native Maltese plants [ 24 ] including Bidni (1Bidni, Figure 1 A–C) and the Leucocarpa 1Caritas plant ( Figure 1 D–F).…”

“…The most characteristic terrestrial agro-ecosystems are those represented by the typical species of the Mediterranean, among which the olive tree stands out. Contrary to what was believed until a few years ago [ 9 , 23 ], several potentially indigenous olive genotypes of Malta have recently been brought to light [ 24 ]. Evaluations of the fruit characteristics and the main chemical constituents of the fruits harvested from some of these unknown olive trees suggest their possible use in the production of oils and/or olive processing products, strongly linked to the Maltese territory.…”

“…In this study, some genotypes, such as 2Mtarfa and 1Pembroke (both E2), showed very low oil values, at just over 2% of their fresh fruit weights, also as a consequence of the prevalence of the stone with respect to the pulp. Moreover, three out of these four genotypes belong to lineage E2 [ 24 ], typical of the wild olives of the central Mediterranean [ 40 ]. Conversely, the large and medium sized fruits were traced back to the lineage E1, with the exception of 2Qnotta and 1Bingemma Malta which belonged to lineage E2 [ 24 ], as observed in other Mediterranean cultivars [ 41 ].…”

“…Moreover, three out of these four genotypes belong to lineage E2 [ 24 ], typical of the wild olives of the central Mediterranean [ 40 ]. Conversely, the large and medium sized fruits were traced back to the lineage E1, with the exception of 2Qnotta and 1Bingemma Malta which belonged to lineage E2 [ 24 ], as observed in other Mediterranean cultivars [ 41 ].…”

“…di Rienzo and colleagues [ 23 ], using SSR markers on the varieties of four countries (Italy, Algeria, Syria and Malta), showed that Italian and Maltese populations both appeared to derive from the Syrian one, probably due to the migration routes of the Phoenicians [ 23 ]. In addition, in a recent and in-depth work [ 24 ], it has been highlighted how rich in olive diversity the small islands of Malta still are, representing a center of hybridization with a selection of local olive genotypes, with some samples possibly claiming an autochthonous origin. The combination of nuclear and plastidial markers of a large sampling set of centennial olives spread over the Maltese Islands showed that several trees were grafted, confirming that this technique has been known since ancient times [ 9 , 23 ].…”