High-Density Linkage Map Construction and QTL Identification in a Diploid Blueberry Mapping Population

Genotyping by sequencing approaches have been widely applied in major crops and are now being used in horticultural crops like berries and fruit trees. As the original and largest producer of cultivated blueberry, the United States maintains the most diverse blueberry germplasm resources comprised of many species of different ploidy levels. We previously constructed an interspecific mapping population of diploid blueberry by crossing the parent F₁#10 (Vaccinium darrowii Fla4B × diploid V. corymbosum W85-20) with the parent W85-23 (diploid V. corymbosum). Employing the Capture-Seq technology developed by RAPiD Genomics, with an emphasis on probes designed in predicted gene regions, 117 F₁ progeny, the two parents, and two grandparents of this population were sequenced, yielding 131.7 Gbp clean sequenced reads. A total of 160,535 single nucleotide polymorphisms (SNPs), referenced to 4,522 blueberry genome sequence scaffolds, were identified and subjected to a parent-dependent sliding window approach to further genotype the population. Recombination breakpoints were determined and marker bins were deduced to construct a high density linkage map. Twelve blueberry linkage groups (LGs) consisting of 17,486 SNP markers were obtained, spanning a total genetic distance of 1,539.4 cM. Among 18 horticultural traits phenotyped in this population, quantitative trait loci (QTLs) that were significant over at least 2 years were identified for chilling requirement, cold hardiness, and fruit quality traits of color, scar size, and firmness. Interestingly, in 1 year, a QTL associated with timing of early bloom, full bloom, petal fall, and early green fruit was identified in the same region harboring the major QTL for chilling requirement. In summary, we report here the first high density bin map of a diploid blueberry mapping population and the identification of several horticulturally important QTLs.

Transcriptome analysis identifies genes related to the waxy coating on blueberry fruit in two northern-adapted rabbiteye breeding populations

BACKGROUND

Blueberry is of high economic value. Most blueberry varieties selected for the fresh market have an appealing light blue coating or "bloom" on the fruit due to the presence of a visible heavy epicuticular wax layer. This waxy layer also serves as natural defense against fruit desiccation and deterioration.

RESULTS

In this study, we attempted to identify gene(s) whose expression is related to the protective waxy coating on blueberry fruit utilizing two unique germplasm populations that segregate for the waxy layer. We bulked RNA from waxy and non-waxy blueberry progenies from the two northern-adapted rabbiteye hybrid breeding populations ('Nocturne' x T 300 and 'Nocturne' x US 1212), and generated 316.85 million RNA-seq reads. We de novo assembled this data set integrated with other publicly available RNA-seq data and trimmed the assembly into a 91,861 blueberry unigene collection. All unigenes were functionally annotated, resulting in 79 genes potentially related to wax accumulation. We compared the expression pattern of waxy and non-waxy progenies using edgeR and identified overall 1125 genes in the T 300 population and 2864 genes in the US 1212 population with at least a two-fold expression difference. After validating differential expression of several genes by RT-qPCR experiments, a candidate gene, FatB, which encodes acyl-[acyl-carrier-protein] hydrolase, emerged whose expression was closely linked to the segregation of the waxy coating in our populations. This gene was expressed at more than a five-fold higher level in waxy than non-waxy plants of both populations. We amplified and sequenced the cDNA for this gene from three waxy plants of each population, but were unable to amplify the cDNA from three non-waxy plants that were tested from each population. We aligned the Vaccinium deduced FATB protein sequence to FATB protein sequences from other plant species. Within the PF01643 domain, which gives FATB its catalytic function, 80.08% of the amino acids were identical or had conservative replacements between the blueberry and the Cucumis melo sequence (XP_008467164). We then amplified and sequenced a large portion of the FatB gene itself from waxy and non-waxy individuals of both populations. Alignment of the cDNA and gDNA sequences revealed that the blueberry FatB gene consists of six exons and five introns. Although we did not sequence through two very large introns, a comparison of the exon sequences found no significant sequence differences between the waxy and non-waxy plants. This suggests that another gene, which regulates or somehow affects FatB expression, must be segregating in the populations.

CONCLUSIONS

This study is helping to achieve a greater understanding of epicuticular wax biosynthesis in blueberry. In addition, the blueberry unigene collection should facilitate functional annotation of the coming chromosomal level blueberry genome.

Generation and analysis of blueberry transcriptome sequences from leaves, developing fruit, and flower buds from cold acclimation through deacclimation

BACKGROUND

There has been increased consumption of blueberries in recent years fueled in part because of their many recognized health benefits. Blueberry fruit is very high in anthocyanins, which have been linked to improved night vision, prevention of macular degeneration, anti-cancer activity, and reduced risk of heart disease. Very few genomic resources have been available for blueberry, however. Further development of genomic resources like expressed sequence tags (ESTs), molecular markers, and genetic linkage maps could lead to more rapid genetic improvement. Marker-assisted selection could be used to combine traits for climatic adaptation with fruit and nutritional quality traits.

RESULTS

Efforts to sequence the transcriptome of the commercial highbush blueberry (Vaccinium corymbosum) cultivar Bluecrop and use the sequences to identify genes associated with cold acclimation and fruit development and develop SSR markers for mapping studies are presented here. Transcriptome sequences were generated from blueberry fruit at different stages of development, flower buds at different stages of cold acclimation, and leaves by next-generation Roche 454 sequencing. Over 600,000 reads were assembled into approximately 15,000 contigs and 124,000 singletons. The assembled sequences were annotated and functionally mapped to Gene Ontology (GO) terms. Frequency of the most abundant sequences in each of the libraries was compared across all libraries to identify genes that are potentially differentially expressed during cold acclimation and fruit development. Real-time PCR was performed to confirm their differential expression patterns. Overall, 14 out of 17 of the genes examined had differential expression patterns similar to what was predicted from their reads alone. The assembled sequences were also mined for SSRs. From these sequences, 15,886 blueberry EST-SSR loci were identified. Primers were designed from 7,705 of the SSR-containing sequences with adequate flanking sequence. One hundred primer pairs were tested for amplification and polymorphism among parents of two blueberry populations currently being used for genetic linkage map construction. The tetraploid mapping population was based on a cross between the highbush cultivars Draper and Jewel (V. darrowii is also in the background of 'Jewel'). The diploid mapping population was based on a cross between an F1 hybrid of V. darrowii and diploid V. corymbosum and another diploid V. corymbosum. The overall amplification rate of the SSR primers was 68% and the polymorphism rate was 43%.

CONCLUSIONS

These results indicate that this large collection of 454 ESTs will be a valuable resource for identifying genes that are potentially differentially expressed and play important roles in flower bud development, cold acclimation, chilling unit accumulation, and fruit development in blueberry and related species. In addition, the ESTs have already proved useful for the development of SSR and EST-PCR markers, and are currently being used for construction of genetic linkage maps in blueberry.