Scientists Sequence Genomes of English Walnut and Its Wild Relative

A research team led by scientists from the University of California, Davis, has used a unique approach to sequence the genomes of the English walnut (Juglans regia) and its wild North American relative, the little walnut (Juglans microcarpa), by tapping into the capabilities of two state-of-the-art technologies: long-read DNA sequencing and optical genome mapping.

Members of the genus Juglans are wind-pollinated trees in the family Juglandaceae.

Members of the genus Juglans are wind-pollinated trees in the family Juglandaceae.

The English walnut, an important nut crop with 3.8 million tons harvested worldwide in 2017, is native to Asia.

The little walnut, also known as the Texas walnut, is native to North America, where it occurs in riparian areas in the southwestern United States.

Both species are wind-pollinated, highly heterozygous, and intolerant of inbreeding. Their hybrids are infertile.

“By sequencing the genome of their hybrid, we produced complete genome sequences for both parents in the time normally required to produce the sequence of one genome,” said study co-lead author Dr. Ming-Cheng Luo, a research geneticist in the Department of Plant Sciences at the University of California, Davis.

“This approach could be applied to genome sequencing of trees and many other woody perennials, opening the door to a better understanding of the genetic blueprints of almonds, pecans, pistachios and grapes.”

“Like walnut, these other crops naturally cross-pollinate and are therefore highly variable,” said study co-lead author Professor Jan Dvorak, also from the Department of Plant Sciences at the University of California, Davis.

“Variability has always greatly complicated our ability to produce a high-quality genome sequence for such crops, but these new technologies now make it possible.”

The assembled genome sequences of the two walnut species will now help scientists identify genetic markers that breeders can use to develop new varieties with improved pathogen and pest resistance.

“We chose to cross the widely used English walnut specifically with the wild Texas black walnut because of its native resistance to several soil-borne diseases and root nematodes, which are serious pests of walnut in California,” said co-author Dr. Dan Kluepfel, a researcher in the Crops Pathology and Genetics Research Unit at USDA’s Agricultural Research Service.

The results were published in the journal Horticulture Research.

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Tingting Zhu et al. 2019. Sequencing a Juglans regia x J. microcarpa hybrid yields high-quality genome assemblies of parental species. Horticulture Research 6, article number: 55; doi: 10.1038/s41438-019-0139-1

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