Walnut: past and future of genetic improvement

Abstract

Persian or English walnut (Juglans regia L.), the walnut species cultivated for nut production, is one of the oldest food sources known. Persian walnuts, native to the mountain valleys of Central Asia, are grown worldwide in temperate areas. World production exceeds three million tons since 2012, mostly provided by China, the USA, and Iran. Despite very ancient culture of walnut species (Juglans spp.), breeding actually started in the twentieth century. Using a range of methodologies, from morphological markers to the most recent advances in genome analysis, many genetic studies of walnut have been conducted during the past 30 years, including examination of diversity, determination of relationships within or among germplasm collections and populations, phylogenetic and origin elucidation, genetic map construction, and biotic or abiotic stress investigations. The genetic improvement of walnut has undergone great evolution. The producing countries of the Middle East have widely studied morphological characteristics of walnut. The USA and France, for example, are behind important cultivar releases such as BChandler^and BFranquette.^Finally, genomics represents a major breakthrough in walnut improvement, in particular by recent sequencing of both chloroplast and nuclear genomes. This review summarizes worldwide molecular and Bomics^studies and gives an overview of the main walnut breeding programs.

A new genomic tool for walnut (Juglans regia L.): development and validation of the high‐density Axiom™ J. regia 700K SNP genotyping array

“…At the same time, advanced genetic analysis can now be performed for traits of interest in the WIP to more thoroughly understand their genetic basis and to predict individual performance. The Axiom™ J. regia 700K array is also being used in France and Iran to genotype local Persian walnut germplasm collections with interesting phenotypic variation related to biotic and abiotic stresses (Bernard et al ., ). All together, these studies demonstrate that the Axiom™ J. regia 700K array is a powerful genomic tool for providing further insight into the genetics of Persian walnut.…”

“…purpurea' (RA1088) along with its progeny 'Robert Livermore' formed a separate group from 'Lara', 'Fernor' and 'Meylan', which were instead admixed with Californian genotypes. The closer relatedness of California and French genotypes can be explained by the series of controlled crosses made in the WIP between French cultivars, chosen for quality and late leafing traits, and the precocious and lateral fruitful varieties of California (Bernard et al, 2018;Germain, 1990). The Italian accessions ( Figure 5) formed two independent subgroups: all walnuts from Sorrento (Campania Region; South Italy) clustered together, with accessions SO3 showing a first-degree relationship with all the others according to the IBD analysis (k > 0.16; IBS0 < 0.03).…”

“…Genetic improvement of walnut only began in the 20 th century (McGranahan and Leslie, 2012), and today the most important walnut breeding programs worldwide are located in California, France, China and the Middle East (Bernard et al, 2018). In California, which accounts for 99% of the US walnut production, the Walnut Improvement Program of University of California, Davis (UC Davis WIP) was established in 1948 under Eugene F. Serr and Harold I. Forde to develop improved cultivars for California growers (Leslie and McGranahan, 2014).…”

High-quality chromosome-scale assembly of the walnut (Juglans regiaL) reference genome

“…High phenotypic diversity for many traits of interest in walnut, such as phenology, nut quality, and yield, has been observed within and between germplasm collections from Western and Eastern countries (McGranahan and Leslie 1991). Walnut trees from Asia are noted for their lateral fruitfulness and precocity, rarely observed in the USA and western Europe, so that they have been used as a source of these phenotypes in different walnut breeding programs (Bernard et al 2018b).…”

Management of a Walnut Germplasm Collection: Which of SSR or SNP Markers Are Most Suitable to Preserve Biodiversity?

“…Before the development of genomics tools, now based mainly on biallelic Single-Nucleotide Polymorphisms (SNPs), the frequently used multiallelic Simple Sequence Repeats (SSRs) had become the markers of choice because of their high polymorphism. In Persian walnut (Juglans regia L.), a widely disseminated and grown species in many temperate regions, more than 20 publications mention the use of SSRs [8]. Recently, a high-density Axiom™ J. regia 700K SNP genotyping array was developed and validated, initiating a novel genomic area in walnut [9].…”