Scientists at the University of California, Davis, have for the first time sequenced the genome of a commercial walnut variety. The information should accelerate the rate of breeding and variety improvement in walnuts and help breeders select for desired traits such as insect and disease resistance, and drought tolerance.
UC Davis geneticists David Neale and Charles Langley worked with the California Walnut Board to develop genetic markers for use with classical walnut breeding. The first step was to sequence the walnut genome, which, unlike most major agricultural crops, had never been sequenced. This represents the first reference genome sequence for a nut crop. The walnut variety Chandler was used for the sequencing project because it is the leading variety of walnut in California.
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.
In addition to the completed genome sequencing of the Chandler walnut variety, Neale and Langley are 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.
“The large repository of walnut genetic material at UC Davis along with new genomic resources lay the groundwork for future breeding of more-sustainable walnut varieties that meet the growing demand for walnuts,” said Neale.
UC Davis and the California Walnut Board, which funded the project, have made the walnut genome sequencing information publicly available.
