SCAR Markers for Practical Application of Marker-assisted Selection in Persimmon (Diospyros kaki Thunb.) Breeding

Marker-Assisted Selection in Breeding for Fruit Trait Improvement: A Review

Breeding fruit species is time-consuming and expensive. With few exceptions, trees are likely the worst species to work with in terms of genetics and breeding. Most are characterized by large trees, long juvenile periods, and intensive agricultural practice, and environmental variability plays an important role in the heritability evaluations of every single important trait. Although vegetative propagation allows for the production of a significant number of clonal replicates for the evaluation of environmental effects and genotype × environment interactions, the spaces required for plant cultivation and the intensity of work necessary for phenotypic surveys slow down the work of researchers. Fruit breeders are very often interested in fruit traits: size, weight, sugar and acid content, ripening time, fruit storability, and post-harvest practices, among other traits relevant to each individual species. The translation of trait loci and whole-genome sequences into diagnostic genetic markers that are effective and affordable for use by breeders, who must choose genetically superior parents and subsequently choose genetically superior individuals among their progeny, is one of the most difficult tasks still facing tree fruit geneticists. The availability of updated sequencing techniques and powerful software tools offered the opportunity to mine tens of fruit genomes to find out sequence variants potentially useful as molecular markers. This review is devoted to analysing what has been the role of molecular markers in assisting breeders in selection processes, with an emphasis on the fruit traits of the most important fruit crops for which examples of trustworthy molecular markers have been developed, such as the MDo.chr9.4 marker for red skin colour in apples, the CCD4-based marker CPRFC1, and LG3_13.146 marker for flesh colour in peaches, papayas, and cherries, respectively.

Genomics-based discrimination of 2n gamete formation mechanisms in polyploids: a case study in nonaploid Diospyros kaki 'Akiou'

Unreduced gametes (2n gametes), possessing double the haploid genome, whatever ploidy that happens to be, are a common source of ploidy variation in plant populations. First and second division restitution (FDR and SDR) are the dominant mechanisms of 2n gamete production; all else being equal, FDR gametes have a higher degree of heterozygosity, thus they are advantageous in breeding. The discrimination of these mechanisms from the consequence of hybridization is challenging, especially in higher polyploids, and usually requires information on centromere location. In this study, we propose a genotyping-based strategy to uncover the mechanisms of 2n gamete formation in progeny that has a higher ploidy than its parents. Simulation of 2n gamete production revealed that FDR and SDR pathways can be discriminated based on allele transmission patterns alone without information on centromere location. We applied this strategy to study the formation mechanism of a nonaploid Diospyros kaki ‘Akiou', which was bred via hybridization between D. kaki hexaploid cultivars. The result demonstrated that ‘Akiou' was derived from the fertilization of a normal female gamete by a 2n male gamete and that this 2n gamete was produced through FDR. Consequently, the distinct duplex transmission pattern in the FDR gamete enabled us to infer the genomic characteristics of polyploid persimmon. The method could be tested only for the plant being polypoid, which allows for the ability to discriminate causes of 2n gamete formation using allele dosage in progeny, and will be useful in future studies of polyploid genomics.

Epigenetic Flexibility Underlies Somaclonal Sex Conversions in Hexaploid Persimmon

Epigenetic regulation adds a flexible layer to genetic variations, potentially enabling long-term, but reversible, changes to a trait, while maintaining genetic information. In the hexaploid Oriental persimmon (Diospyros kaki), genetically monoecious cultivars bearing male flowers require the Y-encoded small RNA (smRNA) gene, OGI. This gene represses the expression of its autosomal counterpart gene, MeGI, as part of the canonical male production system. However, a D. kaki cultivar, Saijo, which lacks the OGI gene and originally bears only female flowers, occasionally produces somaclonal mutant male and revertant female (RF) branches. In this study, we investigated the mechanisms underlying these somaclonal sex conversions in persimmon. Specifically, we aimed to unravel how a genetically female tree without the OGI gene can produce male flowers and RF flowers. Applying multi-omics approaches, we revealed that this noncanonical male production system is basically consistent with the canonical system, in which the accumulation of smRNA targeting MeGI and the considerable DNA methylation of MeGI are involved. The epigenetic status of MeGI on CGN and CHG was synchronized to the genome-wide methylation patterns, both in transition to and from the male production system. These results suggest that the somaclonal sex conversions in persimmon are driven by the genome-wide epigenetic regulatory activities. Moreover, flexibility in the epigenetic layers of long-lived plant species (e.g. trees) is important for overcoming genetic robustness.

The persimmon (Diospyros oleifera Cheng) genome provides new insights into the inheritance of astringency and ancestral evolution

Persimmon (Diospyros kaki) is an oriental perennial woody fruit tree whose popular fruit is produced and consumed worldwide. The persimmon fruit is unique because of the hyperaccumulation of proanthocyanidins during fruit development, causing the mature fruit of most cultivars to have an astringent taste. In this study, we obtained a chromosome-scale genome assembly for 'Youshi' (Diospyros oleifera, 2n = 2x = 30), the diploid species of persimmon, by integrating Illumina sequencing, single-molecule real-time sequencing, and high-throughput chromosome conformation capture techniques. The assembled D. oleifera genome consisted of 849.53 Mb, 94.14% (799.71 Mb) of which was assigned to 15 pseudochromosomes, and is the first assembled genome for any member of the Ebenaceae. Comparative genomic analysis revealed that the D. oleifera genome underwent an ancient γ whole-genome duplication event. We studied the potential genetic basis for astringency development (proanthocyanidin biosynthesis) and removal (proanthocyanidin insolublization). Proanthocyanidin biosynthesis genes were mainly distributed on chromosome 1, and the clustering of these genes is responsible for the genetic stability of astringency heredity. Genome-based RNA-seq identified deastringency genes, and promoter analysis showed that most of their promoters contained large numbers of low oxygen-responsive motifs, which is consistent with the efficient industrial application of high CO2 treatment to remove astringency. Using the D. oleifera genome as the reference, SLAF-seq indicated that 'Youshi' is one of the ancestors of the cultivated persimmon (2n = 6x = 90). Our study provides significant insights into the genetic basis of persimmon evolution and the development and removal astringency, and it will facilitate the improvement of the breeding of persimmon fruit.

Analysis of Genetic Diversity and Development of a SCAR Marker for Green Tea (Camellia sinensis) Cultivars in Zhejiang Province: The Most Famous Green Tea-Producing Area in China

Camellia sinensis (L.) O. Kuntze is one of the most important non-alcoholic beverage crops in Asian and African countries. In recent years, many green tea cultivars have been released and played an important role in improving the production and quality of tea trees. The objectives of this study were to assess the genetic diversity of the eighteen main green tea cultivars in Zhejiang Province-the most famous green tea-producing area of China-using start codon-targeted (SCoT) markers and to develop a specific sequence-characterized amplified region (SCAR) marker for application in cultivar diagnosis. Thirty-one SCoT primers produced 264 loci, 226 of which were polymorphic. The genetic similarity coefficients among these green tea cultivars ranged from 0.587 to 0.814, indicating that a high level of genetic diversity was present. Both a UPGMA dendrogram and a PCoA plot grouped the tea cultivars into three groups. The partitioning of groups in the UPGMA and PCoA was similar, and much of the clustering was highly consistent with the classification of tea cultivars according to their genetic backgrounds. A unique SCoT band, SCoT4-1649, specific to the tea cultivar 'Yingshuang,' was transformed into a SCAR marker. This SCAR marker is highly useful for the identification and germplasm conservation of green tea cultivars.

Characterization of a gene regulatory network underlying astringency loss in persimmon fruit

Transcriptome analysis of a persimmon population segregating for an astringency trait in fruit suggested central roles for a limited number of transcriptional regulators in the loss of proanthocyanidin accumulation. Persimmon (Diospyros kaki; 2n = 6x = 90) accumulates a large amount of proanthocyanidins (PAs) in its fruit, resulting in an astringent taste. Persimmon cultivars are classified into four types based on the nature of astringency loss and the amount of PAs at maturity. Pollination constant and non-astringent (PCNA)-type cultivars stop accumulating PAs in the early stages of fruit development and their fruit can be consumed when still firm without the need for artificial deastringency treatments. While the PCNA trait has been shown to be conferred by a recessive allele at a single locus (ASTRINGENCY; AST), the exact genetic determinant remains unidentified. Here, we conducted transcriptome analyses to elucidate the regulatory mechanism underlying this trait using developing fruits of an F₁ population segregating for the PCNA trait. Comparisons of the transcriptomes of PCNA and non-PCNA individuals and hierarchical clustering revealed that genes related to the flavonoid pathway and to abiotic stress responses involving light stimulation were expressed coordinately with PA accumulation. Furthermore, coexpression network analyses suggested that three putative transcription factors were central to the PA regulatory network and that at least DkMYB4 and/or DkMYC1, which have been reported to form a protein complex with each other for PA regulation, may have a central role in the differential expression of PA biosynthetic pathway genes between PCNA and non-PCNA.

Persimmon breeding in Japan for pollination-constant non-astringent (PCNA) type with marker-assisted selection

Oriental persimmon (Diospyros kaki) originated in Eastern Asia, and many indigenous cultivars have been developed in China, Japan, and Korea. These cultivars are classified into four groups based on their natural astringency loss on the tree and seed formation: pollination-constant non-astringent (PCNA), pollination-variant non-astringent (PVNA), pollination-constant astringent (PCA), and pollination-variant astringent (PVA). PCNA is the most desirable type because the fruit can be eaten without any postharvest treatment; therefore, one of the goals of our persimmon breeding programs is to release superior PCNA cultivars. The PCNA genotype is recessive to the other three non-PCNA genotypes, and PCNA-type F1 offspring are obtained exclusively from crosses among PCNA genotypes. Moreover, the number of superior PCNA cross-parents have been limited. In the late 1980s, inbreeding depression became obvious, especially in terms of fruit size, tree vigor, and productivity. To mitigate the inbreeding, a backcross program using PCNA [(non-PCNA × PCNA) × PCNA] was started in 1990. This process, however, was inefficient because only 15% of the offspring were PCNA, and all offspring had to be grown to the fruiting stage. Therefore, molecular markers linked to the PCNA locus were developed for discriminating PCNA offspring. A molecular marker linked to Chinese PCNA has also been developed.