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 9Nonpareil9 and 9Lauranne9. Allele-specific marker assays were developed for 309 tag pairs. Application of these assays to 231 9Nonpareil9 × 9Lauranne9 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 9Nonpareil9 crosses, providing the basis for a composite linkage map of 9Nonpareil9. 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.

Loss of genetic variability is an increasing challenge in tree breeding programs due to the repeated use of a reduced number of founder genotypes. However, in almond, little is known about the genetic variability in current breeding stocks, although several cases of inbreeding depression have been reported. To gain insights into the genetic structure in modern breeding programs worldwide, marker-verified pedigree data of 220 almond cultivars and breeding selections were analyzed. Inbreeding coefficients, pairwise relatedness, and genetic contribution were calculated for these genotypes. The results reveal two mainstream breeding lines based on three cultivars: “Tuono”, “Cristomorto”, and “Nonpareil”. Descendants from “Tuono” or “Cristomorto” number 76 (sharing 34 descendants), while “Nonpareil” has 71 descendants. The mean inbreeding coefficient of the analyzed genotypes was 0.041, with 14 genotypes presenting a high inbreeding coefficient, over 0.250. Breeding programs from France, the USA, and Spain showed inbreeding coefficients of 0.075, 0.070, and 0.037, respectively. According to their genetic contribution, modern cultivars from Israel, France, the USA, Spain, and Australia trace back to a maximum of six main founding genotypes. Among the group of 65 genotypes carrying theS_fallele for self-compatibility, the mean relatedness coefficient was 0.125, with “Tuono” as the main founding genotype (24.7% of total genetic contribution). The results broaden our understanding about the tendencies followed in almond breeding over the last 50 years and will have a large impact into breeding decision-making process worldwide. Increasing current genetic variability is required in almond breeding programs to assure genetic gain and continuing breeding progress.