What's juicy, sweet, seedless, fits in your pocket and peels well? It's the Tango, the newest mandarin variety from the citrus breeding program at the University of California, Riverside.

Mikeal Roose, a professor of genetics in the Department of Botany and Plant Sciences at UCR, led the decade-long research effort, to develop the seedless selection of the parent mandarin variety, W. Murcott (Afourer), now known as the Tango.

The method used to develop Tango isn't new but has not been widely used to develop seedless mandarin varieties in the past.

"It's one of the first mandarins to be irradiated to generate this particular mutation," Roose said of its seedless quality. "There's been a growing public demand for seedless fruit, which has been moving research in this direction."

A recent study by the UC Sustainable Agriculture Research and Education Program shows that mandarin oranges are a growing global market with very good growth potential in the U.S., where Americans' per capita consumption of mandarins is about three pounds per year.

The trick for Roose, was finding an existing variety that showed promise, and that's where the W. Murcott mandarin emerged as a viable parent variety. The tasty, easy to peel, mid- to late-season mandarin, which reaches maturity in February and March, has been widely planted throughout California, with two to three million trees estimated to be in production.

The variety's major problem, according to Roose, has been an increasing problem with seedy fruit caused by cross pollinations with other citrus varieties, such as other mandarins, oranges, tangelos and lemons. To stem the problem, in 1995, Roose, and staff scientist Tim Williams, began irradiating bud wood and seeds of several seedy but otherwise promising mandarin varieties. Advanced trials including 3,160 trees from 35 irradiated mandarin selections and six hybrid selections were planted at UCR, Irvine, Indio, Santa Paula, Arvin, Lindcove and Woodlake.

The most promising selection derived from irradiated W. Murcott buds was known during testing as W. Murcott IR1 - but is now formally named Tango.

"Maybe we had a certain amount of luck in selecting a variety that, in its seeded form, was already a winner," Roose said.

Ongoing observation of 15 to 20 trees planted in 2001-2002 show that all the trees produced good crop quality and have been consistently low seeded in trials where cross-pollination occurred. The average number of seeds per fruit has been less that 0.2 in samples of 25 to 50 trees at each of three locations. That compares favorably with unaltered W. Murcott trees, which averaged eight to 15 seeds per fruit.

Tango matures in Riverside from late January through April. The fruit size is moderately large at about 2.3 inches across, with a weight of about 3.2 ounces per fruit. The fruit are deeply oblate with a deep orange rind color and smooth rind texture. The color of the flesh is deep orange, finely textured, juicy (50 percent juice) with a rich, sweet flavor. Its peel is easy to remove.

UCR has been preparing for Tango's release creating a supply of about 33,000 buds to be released to growers and nurseries. The University of California does not sell trees to the public. A system of distribution of this limited bud wood supply has been developed in consultation with the citrus growers and nursery industries.

Support for Roose's research came from the California Citrus Board, a statewide growers' organization, and the California Citrus Nursery Advisory Board which operates under authority of the California Department of Food and Agriculture.

"The industry funding supported the staffing we needed to do this," Roose said. "That includes the work of scientist Tim Williams, greenhouse workers, tree maintenance staff and laboratory technicians who did the molecular analysis."

UC has filed for patent protection on Tango. A license to propagate and sell the variety will be available to any California nursery that completes the standard license agreement.

Roose has been involved in the development of four previous mandarin varieties. In 2002, he released three related, low seeded, varieties called TDE2 TDE3 and TDE4. In 1999, he released a seedless, sweet tangerine, 30 years in the making, dubbed the Gold Nugget.

UC Riverside has a long tradition in citrus research. In 1907, the University of California established the Citrus Experiment Station in Riverside to support Southern California's growing citrus industry with scientific data to improve production. In 1917, the station moved to its present site, which would become the new University of California, Riverside in 1954.

Now known as the Citrus Research Center-Agricultural Experiment Station, the work of the center has grown to include all aspects of agricultural production in arid and semi-arid subtropical lands. Research on citrus production and development of new varieties remains a major focus of UC Riverside agricultural research.

UC Riverside is also home to the University of California Citrus Variety Collection of some 900 varieties that have been used extensively to solve citrus disease problems and improve commercial varieties.