Scientists crack open walnut genome

Researchers have made the walnut genome sequencing information available to the public. (Credit: Jochen Wolters/Flickr)

Scientists have for the first time sequenced the genome of a commercial walnut variety. The information should help breeders select for desired traits such as insect resistance and drought tolerance.

For the project, David Neale and Charles Langley, geneticists at the University of California, Davis, used the Chandler walnut because it is the leading variety in California and is grown on about 50 percent of the state’s walnut acreage, accounting for more than 70 percent of the trees sold for new plantings.

California produces 99 percent of US commercial walnuts, and walnuts are the state’s fourth largest agricultural export.

Consumer demand for nuts is growing, and nutritionists are touting the health attributes of moderate nut consumption in lowering the risk for heart disease. Walnuts are high in omega-3 fatty acids, a healthy form of fatty acids.

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Growers are also striving to produce nuts more sustainably, particularly in the areas of pest management and drought tolerance.

Commercial walnut trees are grafted onto rootstocks, usually a California native black walnut species or a hybrid, known to better withstand diseases, pests, and abiotic stress.

For the scion, or nut-bearing portion of the tree, an English walnut species is usually used, and breeders are interested in yield, nut quality, harvest date, and meeting consumer preferences. Having the genome sequence of the walnut should accelerate its rate of breeding and variety improvement.

Dan Kluepfel, a USDA research plant pathologist in the plant pathology department, is heading a project to breed for walnut rootstock disease resistance.

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In addition to the completed genome sequencing of the Chandler walnut variety, Neale and Langley are also sequencing the walnut rootstock species. This composite information will provide additional genomic resources that can be combined with traditional breeding techniques to develop new walnut varieties.

Chandler walnuts, for example, are harvested late in the season, and growers would like earlier harvests. Marker-assisted breeding provides an opportunity to develop early harvesting cultivars with the desirable attributes of Chandler.

UC Davis and the California Walnut Board, which funded the project, have made the walnut genome sequencing information publicly available. Other researchers from UC Davis and from Johns Hopkins University and the University of Connecticut are coauthors of the work.

Source: UC Davis