Protecting the World’s Walnuts
By Marcia Wood – ARS

Food Consumer, IL
Aug 3 2005

About 60 million years ago, walnut trees began to form vast primeval
forests on our planet. Today, many of those prehistoric species are
extinct. But most that remain are safeguarded at a unique living
collection.

Headquartered in Davis, California, about a 90-minute drive north of
San Francisco, this genebank serves as America’s official collection
of walnut species-and their distant, rare, and unusual relatives.
Containing specimens from both the Old and New World, it is the
largest, most genetically diverse walnut collection anywhere.

Some 80 trees grow in containers, protected inside a screen-enclosed
greenhouse. Only 10 minutes away, 1,600 other trees prosper in neat
rows at a carefully tended research orchard.

Some Juglans regia trees from Armenia are among the collection’s
most remarkable members. That’s because of their shells, which are
somewhat larger than a golf ball. This should be a walnut-lover’s
dream, but unfortunately, the nutmeat inside the jumbo shell isn’t
any larger than usual.

Many specimens are also different kinds, called cultivars or
varieties, of J. regia, the so-called English or Persian walnut. J.
regia is the most widely marketed walnut type in the United States.
J. regia varieties are the mainstay of California’s walnut industry,
which produces nearly all of the nation’s $300 million walnut harvest.

Known formally as the ARS National Clonal Germplasm Repository
for Tree Fruit and Nut Crops and Grapes, this genebank belongs
to a nationwide network of ARS-managed collections. These centers
“protect the natural genetic richness of hundreds of plant species,”
says Davis repository curator Ed W. Stover, an ARS horticulturist.

Fingerprinting Walnut Trees

The genebank serves as an invaluable resource for breeders, growers,
the nursery industry, and researchers-including, of course, ARS’s
own scientists at the repository. For instance, walnut collection
manager and geneticist Mallikarjuna K. Aradhya worked with University
of California at Davis colleagues in using samples from repository
trees to pinpoint telltale stretches of genetic material called SSRs,
or simple-sequence repeats.

SSRs are unique, natural duplications in the nucleic acids that
comprise the building blocks of genetic material, or DNA. As such,
SSRs serve as distinctive fingerprints, suitable for differentiating
among individual walnut trees within the same species. Also known as
microsatellite markers, the 14 SSRs that the scientists tested “provide
a fast, reliable means of positive identification,” says Aradhya.

Adapted from a set of microsatellite markers developed by researchers
elsewhere for accurately distinguishing one black walnut tree (J.
nigra) from another, the new assay for J. regia offers an excellent
way to prevent costly mixups at tree nurseries, growers’ orchards,
and even at the ARS walnut collection itself. The markers are an
example of how ARS investigations help everyone who is involved in
growing walnuts and preserving these magnificent trees.-By Marcia Wood,
Agricultural Research Service Information Staff.