Genotyping-by-Sequencing in Almond: SNP Discovery, Linkage Mapping and Marker Design

In crop plant genetics, linkage maps provide the basis for the mapping of loci that affect important traits and for the selection of markers to be applied in crop improvement. In outcrossing species such as almond [Prunus dulcis (Mill.) D.A. Webb], application of a double pseudo-testcross mapping approach to the F1 progeny of a bi-parental cross leads to the construction of a linkage map for each parent. Here, we report on the application of genotyping-by-sequencing to discover and map single-nucleotide polymorphisms in the almond cultivars 'Nonpareil' and 'Lauranne'. Allele-specific marker assays were developed for 309 tag pairs. Application of these assays to 231 'Nonpareil' × 'Lauranne' F1 progeny provided robust linkage maps for each parent. Analysis of phenotypic data for shell hardness demonstrated the utility of these maps for quantitative trait locus mapping. Comparison of these maps to the peach genome assembly confirmed high synteny and collinearity between the peach and almond genomes. The marker assays were applied to progeny from several other 'Nonpareil' crosses, providing the basis for a composite linkage map of 'Nonpareil'. Applications of the assays to a panel of almond clones and a panel of rootstocks used for almond production demonstrated the broad applicability of the markers and providing subsets of markers that could be used to discriminate among accessions. The sequence-based linkage maps and single nucleotide polymorphism assays presented here could be useful resources for the genetic analysis and genetic improvement of almond.