Tracking down the parents of sweet orange

Valencia, Spain – Detailing the ancestry of sweet orange has been a long-sought prize for citrus scientists. On Friday, the leader of a group that has been analyzing the genetic makeup of citrus said in a scientific talk that the ponkan -- a large, loose-skinned type of mandarin widely cultivated in Asia and Brazil -- was the likely father of the sweet orange.

The mother of the orange, its seed parent, was almost certainly a hybrid that was three-quarters pummelo and one-quarter mandarin, said Fred Gmitter Jr., professor of citrus genetics at the University of Florida, speaking at the International Citrus Congress in Valencia, Spain. That would make the orange roughly five-eighths mandarin and three-eights pummelo, the giant fruit that resembles and is the mother of the grapefruit.

In recent decades molecular marker studies have confirmed that four ancestral citrus species – citron, mandarin and pummelo, along with a species of papeda (a primitive, acrid form) – crossed to create all the other cultivated forms of citrus, such as oranges, lemons, limes and grapefruit. Scientists speculated that sweet orange (so-called to distinguish it from sour orange, used for marmalade) was a hybrid of mandarin and pummelo, deriving orange color from mandarin, and larger size from pummelo.

As the genomes of many citrus species were sequenced in the last few years, it became clear that orange wasn’t a direct cross, because mandarin genes were somewhat preponderant. The news today at Gmitter’s talk, in which he summarized conclusions from the work of the International Citrus Genome Consortium, was the detailed pedigree of sweet orange (five grandparents mandarin, three pummelo), and particularly the ponkan’s posited paternity.

Ironically, both ponkan and Willowleaf mandarin (the first kind to make it from China to the Mediterranean and the New World, in the 19th century), the two kinds selected for analysis by the consortium because they seemed to be pure, prototypical mandarins, turned out to have a small portion of pummelo in their ancestry.

“There must have been some funny business somewhere back in time,” said Gmitter.

There is some evidence, he added, that mandarin and pummelo diverged from each other between 1.4 million and 2.8 million years ago. Once they recombined to form sweet orange, this first individual orange tree was the ancestor of all the many varieties such as Valencias, navels and blood oranges, which evolved primarily by spontaneous mutation. (Oranges don’t readily create new hybrids with each other because they usually produce daughter plants that closely resemble the mother tree.)

A rival Chinese group of scientists led by Guangyan Zhong showed a poster at the Spanish conference that offered similar conclusions to Gmitter’s regarding the orange’s mandarin and pummelo makeup, although it did not conclude that ponkan was an immediate parent of orange. Other scientists remarked that it was extraordinary that the Genome Consortium had sequenced only two potential mandarin parents, among hundreds of possibilities, and happened to select the right one, ponkan.

Citrus experts may well continue to debate the credit for and the details of the consortium’s theory, but if its general conclusions hold up, laymen now will be able to know how oranges came to be.

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