Researchers Trace Olive Cultivars to Four Major Genetic Founders

Researchers in Spain have created an “olive parentage atlas,” the first of its kind, providing a detailed, sometimes surprising picture of how cultivated olives are genetically connected across the Mediterranean and beyond.

The research reconstructs hundreds of parent-offspring relationships and identifies patterns of descent that, in some cases, have remained obscure for centuries. The results provide new details about the origins and spread of olive diversity, while offering practical data with immediate real-world applications.

We wanted a family tree that breeders and gene bank managers could actually use.– Francisco Jesús Gómez-Gálvez, researcher

According to Francisco Jesús Gómez-Gálvez, the study’s lead author, the work grew out of day-to-day research at the World Olive Germplasm Bank of Córdoba (WOGBC), one of the world’s largest such collections.

Over several years, the team at the WOGBC genotyped the entire collection, along with newly prospected material, using the same panel of 96 next-generation DNA markers. This allowed researchers to distinguish genuinely distinct cultivars from duplicates held under different names.

The resulting dataset, Gómez-Gálvez said, “was something nobody had before: around 840 distinct cultivars from across the Mediterranean and beyond, all genotyped in exactly the same way.”

While reviewing scientific literature, Gómez-Gálvez encountered pedigree studies in grapevine and apple that used SNP markers to reconstruct the family trees of large numbers of cultivars. The idea of creating a similar atlas for olives began to take shape.

Recognizing the project’s practical and scientific value, he began foundational research that eventually led to the study published this year in BMC Plant Biology.

“We wanted a family tree that breeders and gene bank managers could actually use,” he explained. “There is also a practical concern behind it. Many of the new cultivars bred for modern high-density hedgerow orchards descend directly or indirectly from a single cultivar, Arbequina. If you want to keep diversity in breeding programs, you first need to know how cultivars are related.”

The study identified four major founder cultivars: Gordal Sevillana, Safrawi, Toffehi Tataouine and Lechín de Granada.

Together, the four were the most likely parents of 132 of the 280 robustly assigned offspring, or 47.5 percent of the total. Of them, Toffehi Tataouine had never previously been suspected of holding such an important place in the history of olive cultivation.

Gordal Sevillana was inferred as a candidate parent in 91 robust trios and 12 different parental pairs, with just three pairs accounting for most of its offspring.

Gordal Sevillana and Lechín de Granada were identified as the most likely parental pair for 53 cultivars, mainly from the southwestern Mediterranean. These include Manzanilla de Sevilla, Cornicabra, Cordovil de Serpa and Picholine Marocaine, as well as Azapa, a South American cultivar common in Chile and Peru.

The discovery of Azapa’s parentage led the researchers to suggest that ancient Mediterranean cultivars, including some believed to have been lost, may have been preserved in the Americas, where they underwent further local diversification after introduction.

Paired with Toffehi Tataouine in a combination not previously reported, Gordal Sevillana was linked to 19 cultivars, mostly from northern Spain, but also Amargoso in southern Spain, Grossanne in France and Abunara in Sicily.

With Morchiaio, the authors identified 10 offspring, mainly from the Balkans, including Buga and Oblica, as well as the Italian Ascolana Tenera.

Chloroplast data showed that Gordal Sevillana was the pollen donor in its cross with Lechín de Granada, a role the authors believe it played in most of these crosses, reinforcing its classification as a male founder.

Safrawi was inferred as a candidate parent in 14 parental pairs, generating 34 distinct offspring. Its descendants were the most widely distributed among those of the four founders and include Karydolia in Greece, Rowghani in Iran, Gerboui in Tunisia, and Farga in Spain.

Of the four founders, Safrawi was also the one most often inferred in parental pairs that produced more than one offspring.

Toffehi Tataouine, described by Gómez-Gálvez as a forgotten founder and recently genetically matched to an ancient tree in northern Spain, is a little-known cultivar from southern Tunisia.

It appeared in four parental pairs. Apart from its pairing with Gordal Sevillana, its most productive combination was with Safrawi, which produced 11 offspring. Along with the northern Spanish cultivars Claramunt and Domengues, these offspring include the Turkish Ayvalik and several Greek and Albanian cultivars.

Lechín de Granada, synonymous with the Greek cultivar Dafnelia, was identified as a putative parent of 57 cultivars in the southwestern Mediterranean, most of them from its cross with Gordal Sevillana.

Other combinations include Safrawi; the eastern Spanish Genovesa, itself a parent of the commercial cultivar Changlot Real; the Egyptian Sebhawy; and the Italian Rotondella di Melfi.

Gómez-Gálvez said the researchers see a range of practical applications for their work, both immediate and long-term.

As many emerging cultivars for high-density hedgerow systems are direct or indirect descendants of Arbequina, there is concern about a potentially narrowing genetic base.

Breeding low-vigor cultivars suited to high-density hedgerow systems, and repeatedly using Arbequina and related genetic material, can increase the efficiency of such systems. However, repeatedly relying on closely related progenitors may also increase genetic uniformity.

Genetic uniformity can limit plants’ ability to adapt to biotic and abiotic stresses, potentially increasing vulnerability to novel pests, diseases, and environmental changes.

The authors do not assert that such effects have been demonstrated in olives. However, they note that while inbreeding depression has not yet been documented in the crop, it has been widely reported in other woody species, where it can reduce vigor, flowering and fruit set and contribute to fruit abortion, poorer seed germination and seedling survival, abnormal growth and loss of disease resistance.

A comprehensive and reliable reference such as the olive parentage atlas could therefore help breeders broaden the genetic base while maintaining desirable traits by allowing them to select parents that are not too closely related.

“The cultivars Arróniz, Rotondella di Melfi and Zeitoun Boubezzoula, which have shown potential for hedgerow systems, are examples of genetically differentiated material that could be further explored as alternatives to the widespread use of Arbequina,” the authors wrote.

The olive has a self-incompatibility system comprising two groups, G1 and G2. A cultivar from one group is compatible only with cultivars from the other. Reliably identifying a cultivar’s group can therefore increase the efficiency and speed of breeding programs.

Similarly, when a breeder sows seeds produced through open pollination, only the mother is known. Accurate lineage data can help determine the likely father.

In the Xylella fastidiosa-infected zone of Apulia, Italy, for example, two tolerant cultivars used to replace susceptible trees are Favolosa and Leccino.

The atlas matches Favolosa, obtained from Frantoio seed, to a Frantoio-Ascolana Tenera cross with a high score, illustrating how parentage data can provide additional information in cases of particular agronomic importance.

The researchers also see the atlas as a useful tool for gene bank management. Pedigrees can guide collection selection and help establish the origin of material with incomplete or missing records.

They added that the four founders identified in the study could serve as reference points when deciding which cultivars to sequence, since their genomes can help infer missing genetic data in related cultivars.

While the olive parentage atlas is already extensive, Gómez-Gálvez believes much work remains.

He identified combining pedigree information with genomic and phenotypic data — including flowering time, fruit characteristics, disease resistance, oil composition and drought adaptation — as an important next step.

“Knowing how cultivars are related gives a historical and genetic framework,” he said. “But the next question is which traits and alleles have been passed down through the cultivar families.”

The research could also be expanded by incorporating cultivars from regions that remain underrepresented in genetic studies.

“This could reveal additional founder genotypes and diversification pathways,” Gómez-Gálvez said. “We strongly believe that there is still a huge diversity that remains to be characterized in the Eastern Mediterranean.”

The paper calls for the atlas to be continuously updated as additional cultivars are genotyped. It also identifies relationships that fell short of the study’s robustness thresholds and require further investigation.

One example is a possible line of descent from Lechín de Granada and Gordal Sevillana through Picual and Hojiblanca, which the authors said should be re-examined using higher-density SNP arrays or whole-genome sequencing.

“The accumulated diversity of the olive should not simply be regarded as a heritage collection,” Gómez-Gálvez concluded. “It is a living reservoir of genetic options. Understanding how that diversity is related is a first step toward using it more intelligently to breed the cultivars that olive growing may need under future environmental and production conditions.”