Identification of bloom date QTLs and haplotype analysis in tetraploid sour cherry (Prunus cerasus)

Bloom date is an important production trait in sour cherry (Prunus cerasus L.) as the risk of crop loss to floral freeze injury increases with early bloom time. Knowledge of the major loci controlling bloom date would enable breeders to design crosses and select seedlings with late bloom date. As sour cherry is a segmental allotetraploid, quantitative trait locus (QTL) analysis for bloom date was performed based on haplotype reconstruction by identifying the parental origins of marker alleles in sour cherry. A total of 338 sour cherry individuals from five F1 populations were genotyped using the cherry 6K Illumina Infinium® SNP array and phenotyped for bloom date in 3 years. A total of four QTLs were identified on linkage group (G)1, G2, G4, and G5, respectively. For these QTLs, 14 haplotypes constructed for the QTL regions were significantly associated with bloom date, accounting for 10.1–27.9% of the bloom date variation within individual populations. The three most significant haplotypes, which were identified for the G4 (G4-k), G2 (G2-j), and G1 (G1-c) QTLs, were associated with 2.8, 1.8, and 1.0 days bloom delay, respectively. These three haplotypes were also demonstrated to have additive effects on delaying bloom date for both individual and multiple QTLs. These results demonstrate that bloom date is under polygenic control in sour cherry; yet, pyramiding late blooming haplotypes for single and multiple QTLs would be an effective strategy to obtain later blooming offspring

Public Availability of a Genotyped Segregating Population May Foster Marker Assisted Breeding (MAB) and Quantitative Trait Loci (QTL) Discovery: An Example Using Strawberry

Much of the cost associated with marker discovery for marker assisted breeding (MAB) can be eliminated if a diverse, segregating population is generated, genotyped, and made available to the global breeding community. Herein, we present an example of a hybrid, wild-derived family of the octoploid strawberry that can be used by other breeding programs to economically find and tag useful genes for MAB. A pseudo test cross population between two wild species of Fragaria virginiana and F. chiloensis (FVC 11) was generated and evaluated for a set of phenotypic traits. A total of 106 individuals in the FVC 11 were genotyped for 29,251 single nucleotide polymorphisms (SNPs) utilizing a commercially available, genome-wide scanning platform (Affymetrix Axiom IStraw90TW). The marker trait associations were deduced using TASSEL software. The FVC 11 population segregating for daughters per mother, inflorescence number, inflorescence height, crown production, flower number, fruit size, yield, internal color, soluble solids, fruit firmness, and plant vigor. Coefficients of variations ranged from 10% for fruit firmness to 68% for daughters per mother, indicating an underlying quantitative inheritance for each trait. A total of 2,474 SNPs were found to be polymorphic in FVC 11 and strong marker trait associations were observed for vigor, daughters per mother, yield and fruit weight. These data indicate that FVC 11 can be used as a reference population for quantitative trait loci detection and subsequent MAB across different breeding programs and geographical locations

Development and evaluation of a genome-wide 6K SNP array for diploid sweet cherry and tetraploid sour cherry

High-throughput genome scans are important tools for genetic studies and breeding applications. Here, a 6K SNP array for use with the Illumina Infinium® system was developed for diploid sweet cherry (Prunus avium) and allotetraploid sour cherry (P. cerasus). This effort was led by RosBREED, a community initiative to enable marker-assisted breeding for rosaceous crops. Next-generation sequencing in diverse breeding germplasm provided 25 billion basepairs (Gb) of cherry DNA sequence from which were identified genome-wide SNPs for sweet cherry and for the two sour cherry subgenomes derived from sweet cherry (avium subgenome) and P. fruticosa (fruticosa subgenome). Anchoring to the peach genome sequence, recently released by the International Peach Genome Initiative, predicted relative physical locations of the 1.9 million putative SNPs detected, preliminarily filtered to 368,943 SNPs. Further filtering was guided by results of a 144-SNP subset examined with the Illumina GoldenGate® assay on 160 accessions. A 6K Infinium® II array was designed with SNPs evenly spaced genetically across the sweet and sour cherry genomes. SNPs were developed for each sour cherry subgenome by using minor allele frequency in the sour cherry detection panel to enrich for subgenome-specific SNPs followed by targeting to either subgenome according to alleles observed in sweet cherry. The array was evaluated using panels of sweet (n = 269) and sour (n = 330) cherry breeding germplasm. Approximately one third of array SNPs were informative for each crop. A total of 1825 polymorphic SNPs were verified in sweet cherry, 13% of these originally developed for sour cherry. Allele dosage was resolved for 2058 polymorphic SNPs in sour cherry, one third of these being originally developed for sweet cherry. This publicly available genomics resource represents a significant advance in cherry genome-scanning capability that will accelerate marker-locus-trait association discovery, genome structure investigation, and genetic diversity assessment in this diploid-tetraploid crop group

Outcomes of a GoldenGate® validation assay for a set of 144 SNPs screened with 160 cherry accessions to test potential filtering parameters.

<p>Figures shown in <b>bold</b> are for those cases where the crop for which the SNPs were developed matches the crop on which the SNPs were screened.</p>a<p>Of the 96/48 SNPs examined that targeted sweet cherry/sour cherry.</p

Minor allele frequency (MAF) of polymorphic cherry SNPs.

<p>MAF was determined in sweet cherry (n = 50) and sour cherry (n = 37) evaluation panels for 1825 and 2058 polymorphic SNPs, respectively.</p

Workflow for SNP detection, validation, and final choice in development of the RosBREED 6K cherry SNP array v1.

<p>Stage 1 filtered 1.9 million cherry SNPs anchored to the peach genome to almost 40K SNPs. More stringent filtering criteria in Stage 2, guided by a prior validation step with a small SNP subset examined for a range of potential filters, putatively enriched the quality of the remaining 32K SNP pool. Finally, the 6K array SNPs were chosen from among stage 2 SNPs by attempting to achieve even genetic spacing over species genomes and subgenomes with pre-determined proportional allocations, after preferential inclusion of certain SNPs. ADT = Illumina's Assay Design Tool. MAF = minor allele frequency.</p

SNP informativeness in sour cherry for the eight sets of chromosomes based on whether the SNP was derived from polymorphism in sweet cherry or in one of the two sour cherry subgenomes (i.e., <i>avium</i> or <i>fruticosa</i>).

<p>The 330 sour cherry accessions evaluated are listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0048305#pone.0048305.s003" target="_blank">Table S3</a>.</p>a<p>Numbers of SNPs for the subgenomes of sour cherry are split (/) between <i>avium</i> and <i>fruticosa.</i></p>b<p>Numbers in parentheses are totals for RosCOS SNPs derived from sweet cherry and are included in the first number.</p

Estimated physical and genetic distribution of SNPs of the RosBREED cherry 6K SNP array v1 across sweet cherry chromosomes evaluated for 269 sweet cherry accessions.

a<p>Based on the 4214 attempted sweet cherry SNPs.</p>b<p>Based on all 1825 polymorphic SNPs.</p