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From a few wild ancestors, a citrus cornucopia

Whether it's a halved grapefruit sprinkled with sugar, mandarin slices tumbled in a green salad, mouth-puckering lemon wedges or a classic navel orange, there are probably enough kinds of citrus to satisfy any personality or taste.

But scientists with the Agricultural Research Service (ARS) in Riverside, Calif., who recently assessed their extensive collection of Citrus species from around the world, have found that despite the long list of seemingly distinct and different citrus fruits, the majority of those most familiar to us are hybrids that got their start from just a handful of wild citrus species.

For their study, the team of researchers, led by ARS horticulturist Robert Krueger, delved into the genes of nearly 1,000 citrus accessions comprising seeds, fruit, live trees and pollen kept inside greenhouses and in outdoor groves at the University of California-Riverside (UCR).

They wanted to determine the true genetic diversity of the collection by identifying duplicate accessions and linking those that are genetically similar.

The researchers created 13 new molecular markers to help them track the accessions' genetic similarities. Like markers used in forensic cases to determine parental lines, the markers let the scientists draw relationships between the numerous citrus specimens and group together more closely related ones.

Along with Mikeal Roose of the UCR Department of Botany and Plant Sciences and former UCR graduate student Noelle Barkley, Krueger discovered that most of the genetic diversity of the collection's hundreds of citrus accessions was found in only about 50 accessions.

According to ARS research leader Richard Lee, this relatively small subset likely represents much of the diversity of the entire Citrus genome. Using it will help researchers more efficiently pinpoint valuable citrus genes related to pest and disease resistance and high nutrient levels.

In addition to its research function, ARS' citrus collection is a critical resource for safeguarding rare and wild citrus specimens, especially given increasing encroachment pressures facing native citrus stands in Southeast Asia.

Read more about the research in the June 2005 issue of Agricultural Research magazine, available online at:

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