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Genetics in the olive grove
By Birgit Reinert

Genuinely wild olive trees, though rare, can still be found in some forests around the Mediterranean Sea. In a survey, researchers used gene analysis to distinguish between cultivated and wild trees. Most genuinely wild olive trees were found in Morocco, Portugal, and Spain.

The Olive Trees by Vincent van Gogh. Saint-Rémy, France 1889, oil on canvas.

Olives are one of the oldest cultivated plants in the world. Many olive trees that seem to grow wild on the hillsides of countries bordering the Mediterranean are, in fact, descendants of cultivated trees returned to the wild. These feral types are difficult to distinguish from genuinely wild trees.

Roselyne Lumaret, of the Centre National de la Recherche Scientifique in Montpellier, France, and Noureddine Ouazzani, of the Ecole Nationale d'Agriculture in Meknès, Morocco, surveyed ten forests in seven Mediterranean countries to identify surviving genuinely wild olive trees. They scored the trees for genetic markers associated with characters unsuitable for domestication.

"These wild stocks are genetically distinct and more variable than either the crop strains or their derived feral forms," the researchers write in a brief communication in Nature. They report that wild types still survive in the western part of the Mediterranean area, whereas none were identified in Egypt, Syria, Turkey, Crete, or Greece—areas where olive trees have been extensively cultivated for a longer period of time.

Geographical distribution of forests tested for the presence of genuinely wild olive trees. Five different alleles (each represented by a different symbol) peculiar to these trees were found to be present at frequencies of 4–32%, 8–14%, 20–24%, 10–22% and 4–15%, respectively; these were present exclusively in the 10 forests suspected of containing genuinely wild olives. Arrows, population locations tested. For each country, the sequence of four numbers indicates populations of wild oleasters, feral oleasters, formal cultivars and locally cultivated clones, respectively, that were scored genetically.

"The domesticated olive represents a sample of the genetic variation in genuinely wild olive populations that persist today," the researchers write. This should encourage both conservationists and plant breeders alike to preserve these trees.

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Lumaret, R. & Ouazzani, N. Ancient wild olives in Mediterranean forests. Nature 413, 700 (October 18, 2001).

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