Olive

Abstract

The scarce knowledge about the genetics of the olive tree is not comparable to the great impact of its cultivation on the economy and culture of Mediterranean countries. Actually, the polyploid nature of some Olea europaea subspecies has been recently confirmed by the use of new techniques and methodologies, like microsatellite markers and flow cytometry analyses. The most extended idea among the researchers is that the origin of olive cultivation goes back to the Prehistory in the Eastern Mediterranean. The use of cytoplasmic DNA markers to trace olive migration routes has allowed identifying, at least, two possible centres of origin for the olive tree, located to the east and the west of the Mediterranean Sea, Near East and Maghreb. Nowadays, the olive tree cultivation is concentrated in Mediterranean-type climate regions with benign winters and dry and hot summers. Modern olive oil industry requires more competitive cultivars better adapted to the new trends in olive growing. Breeding programmes undertaken have focused in obtaining new cultivars with a combination of superior characteristics, like high productivity, low vigour and compact plant architecture, earliness of flowering and fructification, resistance to pathogens and pests (i.e., leaf spot, Verticilium wilt and olive knot), among agronomic traits; and high oil content and quality, as oil traits. The detection of a large number of mislabellings, homonyms and synonyms has raised the need of easy and accurate cultivar identification methods to manage properly the rich olive biodiversity. Up to date, morphological traits are the only markers accepted and used by the International Plant Genetic Resources Institute (IPGRI, Rome) and the International Olive Oil Council (IOOC), though their usefulness is being constantly strengthened by molecular markers to unambiguously discriminate among individuals. The use of molecular markers can speed the breeding programmes up, not only being used in identification and compatibility studies, but in the selection of individuals with desirable agronomic characteristics in an early stage (marker-assisted selection, MAS). Isozymes became the biochemical markers most widely used in plant breeding, though they have been superseded by genetic markers. Most of them have been used with identification purposes, some cases of homonyms and synonyms being solved, and to estimate the genetic distances among very diverse sources of material (wild, feral and cultivated forms). In this sense, microsatellite markers have revealed the exotic germplasm as a source of new variability, wild genotypes being grouped together in a different gene pool than the cultivated forms. Clusterings of olive cultivars according to economically important traits have been described, what could be very useful when it comes to design breeding crosses. And the genetic relationships among olive cultivars and genotypes selected from a breeding programme that ultimately has rendered a new variety have been elucidated. Furthermore, microsatellites have become tremendously useful for checking the paternity of olive progenies from controlled crossings and exploring the compatibility relationships among olive cultivars, which is vital to design effective crosses in breeding programmes. Linkage maps in olive are needed, so markers linked to the traits of interest can be identified. Up to date, restriction fragment length polymorphism (RFLP), random amplified polymorphic DNA (RAPD), amplified fragment length polymorphisms (AFLP) and microsatellite markers have been used to construct linkage maps. Genetic transformation can significantly contribute to plant breeding by generating additional genetic diversity and introducing alleles that encode desirable traits into superior cultivars. The progress in the genetic transformation methodologies in olive must be accompanied by the design of efficient regeneration protocols, via organogenesis and somatic embryogenesis. Real-time quantitative PCR (qPCR) and real-time quantitative reverse-transcription PCR (qRT-PCR) have contributed to monitor the sanitary status of olive plants that is essential to undertake successful breeding programmes. These techniques have also been used to infer the resistance or susceptibility level of particular cultivars to olive leaf spot, this application being very valuable as a breeding tool. From MAS to expression studies, without forgetting genetic transformation, the olive research community has used these technological innovations to acquire a deeper knowledge of the species and to transfer it to breeding programmes, what is providing the first promising results.