Rapid identification of fruit length loci in cucumber (Cucumis sativus L.) using next-generation sequencing (NGS)-based QTL analysis

Quantitative Trait Loci Mapping and Comparative Transcriptome Analysis of Fruit Weight (FW) in Watermelon (Citrullus lanatus L.)

The watermelon (Citrullus lanatus L.) holds substantial economic value as a globally cultivated horticultural crop. However, the genetic architecture of watermelon fruit weight (FW) remains poorly understood. In this study, we used sh14-11 with small fruit and N14 with big fruit to construct 100 recombinant inbred lines (RILs). Based on whole-genome resequencing (WGR), 218,127 single nucleotide polymorphisms (SNPs) were detected to construct a high-quality genetic map. After quantitative trait loci (QTL) mapping, a candidate interval of 31-38 Mb on chromosome 2 was identified for FW. Simultaneously, the bulked segregant analysis (BSA) in the F2 population corroborated the identification of the same interval, encompassing the homologous gene linked to the known FW-related gene fas. Additionally, RNA-seq was carried out across 11 tissues from sh14-11 and N14, revealing expression profiles that identified 1695 new genes and corrected the annotation of 2941 genes. Subsequent differential expression analysis unveiled 8969 differentially expressed genes (DEGs), with 354 of these genes exhibiting significant differences across four key developmental stages. The integration of QTL mapping and differential expression analysis facilitated the identification of 14 FW-related genes, including annotated TGA and NAC transcription factors implicated in fruit development. This combined approach offers valuable insights into the genetic basis of FW, providing crucial resources for enhancing watermelon cultivation.

Molecular and genetic regulations of fleshy fruit shape and lessons from Arabidopsis and rice

Fleshy fruit shape is an important external quality trait influencing the usage of fruits and consumer preference. Thus, modification of fruit shape has become one of the major objectives for crop improvement. However, the underlying mechanisms of fruit shape regulation are poorly understood. In this review we summarize recent progress in the genetic basis of fleshy fruit shape regulation using tomato, cucumber, and peach as examples. Comparative analyses suggest that the OFP-TRM (OVATE Family Protein - TONNEAU1 Recruiting Motif) and IQD (IQ67 domain) pathways are probably conserved in regulating fruit shape by primarily modulating cell division patterns across fleshy fruit species. Interestingly, cucumber homologs of FRUITFULL (FUL1), CRABS CLAW (CRC) and 1-aminocyclopropane-1-carboxylate synthase 2 (ACS2) were found to regulate fruit elongation. We also outline the recent progress in fruit shape regulation mediated by OFP-TRM and IQD pathways in Arabidopsis and rice, and propose that the OFP-TRM pathway and IQD pathway coordinate regulate fruit shape through integration of phytohormones, including brassinosteroids, gibberellic acids, and auxin, and microtubule organization. In addition, functional redundancy and divergence of the members of each of the OFP, TRM, and IQD families are also shown. This review provides a general overview of current knowledge in fruit shape regulation and discusses the possible mechanisms that need to be addressed in future studies.

Mapping and identification of CsSF4, a gene encoding a UDP-N-acetyl glucosamine-peptide N-acetylglucosaminyltransferase required for fruit elongation in cucumber (Cucumis sativus L.)

The short fruit length phenotype in sf4 is caused by a SNP in Csa1G665390, which encodes an O-linked N-acetylglucosamine (GlcNAc) transferase in cucumber. Cucumber fruit is an excellent resource for studying fruit morphology due to its fast growth rate and naturally abundant morphological variations. The regulatory mechanisms underlying plant organ size and shape are important and fundamental biological questions. In this study, a short-fruit length mutant, sf4, was identified from an ethyl methanesulfonate (EMS) mutagenesis population derived from the North China-type cucumber inbred line WD1. Genetic analysis indicated that the short fruit length phenotype of sf4 was controlled by a recessive nuclear gene. The SF4 locus was located in a 116.7-kb genomic region between the SNP markers GCSNP75 and GCSNP82 on chromosome 1. Genomic and cDNA sequences analysis indicated that a single G to A transition at the last nucleotide of Csa1G665390 intron 21 in sf4 changed the splice site from GT-AG to GT-AA, resulting in a 42-bp deletion in exon 22. Csa1G665390 is presumed to be a candidate gene, CsSF4 that encodes an O-linked N-acetylglucosamine (GlcNAc) transferase (OGT). CsSF4 was highly expressed in the leaves and male flowers of wild-type cucumbers. Transcriptome analysis indicated that sf4 had alterations in expression of many genes involved in hormone response pathways, cell cycle regulation, DNA replication, and cell division, suggesting that cell proliferation-associated gene networks regulate fruit development in cucumber. Identification of CsSF4 will contribute to elucidating the function of OGT in cell proliferation and to understanding fruit elongation mechanisms in cucumber.

Molecular mapping of genomic regions and identification of possible candidate genes associated with gynoecious sex expression in bitter gourd

Bitter gourd is an important vegetable crop grown throughout the tropics mainly because of its high nutritional value. Sex expression and identification of gynoecious trait in cucurbitaceous vegetable crops has facilitated the hybrid breeding programme in a great way to improve productivity. In bitter gourd, gynoecious sex expression is poorly reported and detailed molecular pathways involve yet to be studied. The present experiment was conducted to study the inheritance, identify the genomic regions associated with gynoecious sex expression and to reveal possible candidate genes through QTL-seq. Segregation for the gynoecious and monoecious sex forms in the F₂ progenies indicated single recessive gene controlling gynoecious sex expression in the genotype, PVGy-201. Gynoecious parent, PVGy-201, Monoecious parent, Pusa Do Mausami (PDM), and two contrasting bulks were constituted for deep-sequencing. A total of 10.56, 23.11, 15.07, and 19.38 Gb of clean reads from PVGy-201, PDM, gynoecious bulk and monoecious bulks were generated. Based on the ΔSNP index, 1.31 Mb regions on the chromosome 1 was identified to be associated with gynoecious sex expression in bitter gourd. In the QTL region 293,467 PVGy-201 unique variants, including SNPs and indels, were identified. In the identified QTL region, a total of 1019 homozygous variants were identified between PVGy1 and PDM genomes and 71 among them were non-synonymous variants (SNPS and INDELs), out of which 11 variants (7 INDELs, 4 SNPs) were classified as high impact variants with frame shift/stop gain effect. In total twelve genes associated with male and female gametophyte development were identified in the QTL-region. Ethylene-responsive transcription factor 12, Auxin response factor 6, Copper-transporting ATPase RAN1, CBL-interacting serine/threonine-protein kinase 23, ABC transporter C family member 2, DEAD-box ATP-dependent RNA helicase 1 isoform X2, Polygalacturonase QRT3-like isoform X2, Protein CHROMATIN REMODELING 4 were identified with possible role in gynoecious sex expression. Promoter region variation in 8 among the 12 genes indicated their role in determining gynoecious sex expression in bitter gourd genotype, DBGy-1. The findings in the study provides insight about sex expression in bitter gourd and will facilitate fine mapping and more precise identification of candidate genes through their functional validation.

Genome-Wide Association Analysis of Fruit Shape-Related Traits in Areca catechu

The areca palm (Areca catechu L.) is one of the most economically important palm trees in tropical areas. To inform areca breeding programs, it is critical to characterize the genetic bases of the mechanisms that regulate areca fruit shape and to identify candidate genes related to fruit-shape traits. However, few previous studies have mined candidate genes associated with areca fruit shape. Here, the fruits produced by 137 areca germplasms were divided into three categories (spherical, oval, and columnar) based on the fruit shape index. A total of 45,094 high-quality single-nucleotide polymorphisms (SNPs) were identified across the 137 areca cultivars. Phylogenetic analysis clustered the areca cultivars into four subgroups. A genome-wide association study that used a mixed linear model identified the 200 loci that were the most significantly associated with fruit-shape traits in the germplasms. In addition, 86 candidate genes associated with areca fruit-shape traits were further mined. Among the proteins encoded by these candidate genes were UDP-glucosyltransferase 85A2, the ABA-responsive element binding factor GBF4, E3 ubiquitin-protein ligase SIAH1, and LRR receptor-like serine/threonine-protein kinase ERECTA. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis showed that the gene that encoded UDP-glycosyltransferase, UGT85A2, was significantly upregulated in columnar fruits as compared to spherical and oval fruits. The identification of molecular markers that are closely related to fruit-shape traits not only provides genetic data for areca breeding, but it also provides new insights into the shape formation mechanisms of drupes.

Genetic Characterization of the Partial Disease Resistance of Rice to Bacterial Panicle Blight and Sheath Blight by Combined QTL Linkage and QTL-seq Analyses

Bacterial panicle blight (BPB) and sheath blight (SB) are major diseases of rice and few cultivars have shown a high level of resistance to these diseases. A recombinant inbred line (RIL) population developed from the U.S. cultivars Jupiter (moderately resistant) and Trenasse (susceptible) was investigated to identify loci associated with the partial disease resistance to BPB and SB. Disease phenotypes in BPB and SB, as well as the days-to-heading (DTH) trait, were evaluated in the field. DTH was correlated to BPB and SB diseases, while BPB was positively correlated to SB in the field trials with this RIL population. Genotyping was performed using Kompetitive Allele Specific PCR (KASP) assays and whole-genome sequence (WGS) analyses. Quantitative trait locus (QTL) mapping and bulk segregant analysis using a set of WGS data (QTL-seq) detected a major QTL on the upper arm of chromosome 3 for BPB, SB, and DTH traits within the 1.0-1.9 Mb position. Additional QTLs associated with BPB and SB were also identified from other chromosomes by the QTL-seq analysis. The QTLs identified in this study contain at least nine candidate genes that are predicted to have biological functions in defense or flowering. These findings provide an insight into the complex nature of the quantitative resistance to BPB and SB, which may also be closely linked to the flowering trait.

Primary mapping of quantitative trait loci regulating multivariate horticultural phenotypes of watermelon (Citrullus lanatus L.)

Watermelon fruits exhibit a remarkable diversity of important horticultural phenotypes. In this study, we initiated a primary quantitative trait loci (QTL) mapping to identify the candidate regions controlling the ovary, fruit, and seed phenotypes. Whole genome sequencing (WGS) was carried out for two differentiated watermelon lines, and 350 Mb (96%) and 354 Mb (97%) of re-sequenced reads covered the reference de novo genome assembly, individually. A total of 45.53% non-synonymous single nucleotide polymorphism (nsSNPs) and 54.47% synonymous SNPs (sSNPs) were spotted, which produced 210 sets of novel SNP-based cleaved amplified polymorphism sequence (CAPS) markers by depicting 46.25% co-dominant polymorphism among parent lines and offspring. A biparental F_2:3 mapping population comprised of 100 families was used for trait phenotyping and CAPS genotyping, respectively. The constructed genetic map spanned a total of 2,398.40 centimorgans (cM) in length and averaged 11.42 cM, with 95.99% genome collinearity. A total of 33 QTLs were identified at different genetic positions across the eight chromosomes of watermelon (Chr-01, Chr-02, Chr-04, Chr-05, Chr-06, Chr-07, Chr-10, and Chr-11); among them, eight QTLs of the ovary, sixteen QTLs of the fruit, and nine QTLs of the seed related phenotypes were classified with 5.32-25.99% phenotypic variance explained (PVE). However, twenty-four QTLs were identified as major-effect and nine QTLs were mapped as minor-effect QTLs across the flanking regions of CAPS markers. Some QTLs were exhibited as tightly localized across the nearby genetic regions and explained the pleiotropic effects of multigenic nature. The flanking QTL markers also depicted significant allele specific contributions and accountable genes were predicted for respective traits. Gene Ontology (GO) functional enrichment was categorized in molecular function (MF), cellular components (CC), and biological process (BP); however, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were classified into three main classes of metabolism, genetic information processing, and brite hierarchies. The principal component analysis (PCA) of multivariate phenotypes widely demonstrated the major variability, consistent with the identified QTL regions. In short, we assumed that our identified QTL regions provide valuable genetic insights regarding the watermelon phenotypes and fine genetic mapping could be used to confirm them.

Identification of Bottle Gourd (Lagenaria siceraria) OVATE Family Genes and Functional Characterization of LsOVATE1

The OVATE gene family is a class of conserved transcription factors that play significant roles in plant growth, development, and abiotic stress, and also affect fruit shape in vegetable crops. Bottle gourd (Lagenaria siceraria), commonly known as calabash or gourd, is an annual climber belonging to the Cucurbitaceae family. Studies on bottle gourd OVATE genes are limited. In this study, we performed genome-wide identification of the OVATE gene family in bottle gourd, and identified a total of 20 OVATE family genes. The identified genes were unevenly distributed across 11 bottle gourd chromosomes. We also analyzed the gene homology, amino acid sequence conservation, and three-dimensional protein structure (via prediction) of the 20 OVATE family genes. We used RNA-seq data to perform expression analysis, which found 20 OVATE family genes to be differentially expressed based on spatial and temporal characteristics, suggesting that they have varying functions in the growth and development of bottle gourd. In situ hybridization and subcellular localization analysis showed that the expression characteristics of the LsOVATE1 gene, located on chromosome 7 homologous to OVATE, is a candidate gene for affecting the fruit shape of bottle gourd. In addition, RT-qPCR data from bottle gourd roots, stems, leaves, and flowers showed different spatial expression of the LsOVATE1 gene. The ectopic expression of LsOVATE1 in tomato generated a phenotype with a distinct fruit shape and development. Transgenic-positive plants that overexpressed LsOVATE1 had cone-shaped fruit, calyx hypertrophy, petal degeneration, and petal retention after flowering. Our results indicate that LsOVATE1 could serve important roles in bottle gourd development and fruit shape determination, and provide a basis for future research into the function of LsOVATE1.

Morphological and Genetic Diversity of Cucumber (Cucumis sativus L.) Fruit Development

Cucumber (Cucumis sativus L.) fruits, which are eaten at an immature stage of development, can vary extensively in morphological features such as size, shape, waxiness, spines, warts, and flesh thickness. Different types of cucumbers that vary in these morphological traits are preferred throughout the world. Numerous studies in recent years have added greatly to our understanding of cucumber fruit development and have identified a variety of genetic factors leading to extensive diversity. Candidate genes influencing floral organ establishment, cell division and cell cycle regulation, hormone biosynthesis and response, sugar transport, trichome development, and cutin, wax, and pigment biosynthesis have all been identified as factors influencing cucumber fruit morphology. The identified genes demonstrate complex interplay between structural genes, transcription factors, and hormone signaling. Identification of genetic factors controlling these traits will facilitate breeding for desired characteristics to increase productivity, improve shipping, handling, and storage traits, and enhance consumer-desired qualities. The following review examines our current understanding of developmental and genetic factors driving diversity of cucumber fruit morphology.

Identification of a major QTL, Parth6.1 associated with parthenocarpic fruit development in slicing cucumber genotype, Pusa Parthenocarpic Cucumber-6

Parthenocarpy is an extremely important trait that revolutionized the worldwide cultivation of cucumber under protected conditions. Pusa Parthenocarpic Cucumber-6 (PPC-6) is one of the important commercially cultivated varieties under protected conditions in India. Understanding the genetics of parthenocarpy, molecular mapping and the development of molecular markers closely associated with the trait will facilitate the introgression of parthenocarpic traits into non-conventional germplasm and elite varieties. The F1, F2 and back-crosses progenies with a non-parthenocarpic genotype, Pusa Uday indicated a single incomplete dominant gene controlling parthenocarpy in PPC-6. QTL-seq comprising of the early parthenocarpy and non-parthenocarpic bulks along with the parental lines identified two major genomic regions, one each in chromosome 3 and chromosome 6 spanning over a region of 2.7 Mb and 7.8 Mb, respectively. Conventional mapping using F2:3 population also identified two QTLs, Parth6.1 and Parth6.2 in chromosome 6 which indicated the presence of a major effect QTL in chromosome 6 determining parthenocarpy in PPC-6. The flanking markers, SSR01148 and SSR 01012 for Parth6.1 locus and SSR10476 and SSR 19174 for Parth6.2 locus were identified and can be used for introgression of parthenocarpy through the marker-assisted back-crossing programme. Functional annotation of the QTL-region identified two major genes, Csa_6G396640 and Csa_6G405890 designated as probable indole-3-pyruvate monooxygenase YUCCA11 and Auxin response factor 16, respectively associated with auxin biosynthesis as potential candidate genes. Csa_6G396640 showed only one insertion at position 2179 in the non-parthenocarpic parent. In the case of Csa_6G405890, more variations were observed between the two parents in the form of SNPs and InDels. The study provides insight about genomic regions, closely associated markers and possible candidate genes associated with parthenocarpy in PPC-6 which will be instrumental for functional genomics study and better understanding of parthenocarpy in cucumber.

Identification of single major QTL and candidate gene(s) governing hull-less seed trait in pumpkin

The hull-less pumpkin (Cucurbita pepo) seed does not require de-hulling before use for human consumption, as a result highly preferred by the oil, nut, and baking industries. In hull-less seeds, a single recessive gene is responsible for the absence of outer thick seed coat layers; however, the genomic region and gene(s) controlling the trait are unclear to date. In this study, four crosses attempted to derive F₂ and backcross populations confirmed the single recessive gene inheritance of hull-less seed trait in populations adapted to the sub-tropical climate. The candidate genomic region for hull-less seed trait was identified through the BSA-QTLseq approach using bulks of F_2:3 progenies from a cross of HP111 (hulled) and HLP36 (hull-less). A novel genomic region on chromosome 12 ranging from 1.80 to 3.86 Mb was associated with the hull-less seed trait. The re-sequencing data identified a total of 396 SNPs within this region and eight were successfully converted into polymorphic KASP assays. The genotyping of segregating F₂ (n = 160) with polymorphic KASP assays resulted in a 40.3 cM partial linkage map and identified Cp_3430407 (10 cM) and Cp_3498687 (16.1 cM) as flanking markers for hull-less locus (Cphl-1). These flanking markers correspond to the 68.28 kb region in the reference genome, and the marker, Cp_3430407 successfully predicted the genotype in 93.33% of the C. pepo hull-less germplasm lines, thus can be used for marker-assisted selection in parents polymorphic for the hull-less seed trait. The Cphl-1-linked genomic region (2.06 Mb) encompasses a total of 182 genes, including secondary cell wall and lignin biosynthesis-related transcriptional factors viz., "NAC" (Cp4.1LG12g04350) and "MYB" (Cp4.1LG12g03120). These genes were differentially expressed in the seeds of hulled and hull-less genotypes, and therefore could be the potential candidate genes governing the hull-less seed trait in pumpkin.

Serial-Omics and Molecular Function Study Provide Novel Insight into Cucumber Variety Improvement

Cucumbers are rich in vitamins and minerals. The cucumber has recently become one of China’s main vegetable crops. More specifically, the adjustment of the Chinese agricultural industry’s structure and rapid economic development have resulted in increases in the planting area allocated to Chinese cucumber varieties and in the number of Chinese cucumber varieties. After complete sequencing of the “Chinese long” genome, the transcriptome, proteome, and metabolome were obtained. Cucumber has a small genome and short growing cycle, and these traits are conducive to the application of molecular breeding techniques for improving fruit quality. Here, we review the developments and applications of molecular markers and genetic maps for cucumber breeding and introduce the functions of gene families from the perspective of genomics, including fruit development and quality, hormone response, resistance to abiotic stress, epitomizing the development of other omics, and relationships among functions.

Mapping of genetic loci controlling fruit linked morphological traits of melon using developed CAPS markers

BACKGROUND

Fruit morphology traits are important commercial traits that directly affect the market value. However, studying the genetic basis of these traits in un-explored botanical groups is a fundamental objective for crop genetic improvement through marker-assisted breeding.

METHODS AND RESULTS

In this study, a quantitative trait loci (QTLs) mapping strategy was used for dissecting the genomic regions of fruit linked morphological traits by single nucleotide polymorphism (SNP) based cleaved amplified polymorphism sequence (CAPS) molecular markers. Next-generation sequencing was done for the genomic sequencing of two contrasted melon lines (climacteric and non-climacteric), which revealed 97% and 96% of average coverage over the reference melon genome database, respectively. A total of 57.51% non-synonymous SNPs and 42.49% synonymous SNPs were found, which produced 149 sets of codominant markers with a 24% polymorphism rate. Total 138-F₂ derived plant populations were genotyped for linkage mapping and composite interval mapping based QTL mapping exposed 6 genetic loci, positioned over distinct chromosomes (02, 04, 08, 09, and 12) between the flanking intervals of CAPS markers, which explained an unlinked polygenic architecture in genome. Three minor QTLs of fruit weight (FWt2.1, FWt4.1, FWt9.1), one major QTL of fruit firmness (FrFir8.1), one major QTL of fruit length (FL12.1), and one major QTL of fruit shape (FS12.1) were determined and collectively explained the phenotypic variance from 5.64 to 15.64%. Fruit phenotypic correlation exhibited the significant relationship and principal component analysis also identified the potential variability. Multiple sequence alignments also indicated the significant base-mutations in the detected genetic loci, respectively.

CONCLUSION

In short, our illustrated genetic loci are expected to provide the reference insights for fine QTL mapping and candidate gene(s) mining through molecular genetic breeding approaches aimed at developing the new varieties.

Genetic and Molecular Regulation Mechanisms in the Formation and Development of Vegetable Fruit Shape

Vegetable crops have a long history of cultivation worldwide and rich germplasm resources. With its continuous development and progress, molecular biology technology has been applied to various fields of vegetable crop research. Fruit is an important organ in vegetable crops, and fruit shape can affect the yield and commercialization of vegetables. In nature, fruits show differences in size and shape. Based on fruit shape diversity, the growth direction and coordination mechanism of fruits remain unclear. In this review, we discuss the latest research on fruit shape. In addition, we compare the current theories on the molecular mechanisms that regulate fruit growth, size, and shape in different vegetable families.

Genome-wide association study of the candidate genes for grape berry shape-related traits

BACKGROUND

In the breeding of new horticultural crops, fruit shape is an important selection characteristic. A variety of fruit shapes appeared during the gradual process of selection and domestication. However, few studies have been conducted on grape berry shape, especially studies related to mining candidate genes. To discover candidate genes related to grape berry shape, the present study first took the berry shape parameters analyzed by Tomato Analyzer as the target traits and used a genome-wide association analysis to analyze candidate genes.

RESULTS

In total, 122 single-nucleotide polymorphism (SNP) loci had significant correlations with multiple berry shape traits in both years, and some candidate genes were further mined. These genes were mainly related to LRR receptor-like serine/threonine-protein kinase (At1g05700 and At1g07650), transcription factors (GATA transcription factor 23-like, transcription factor VIP1, transcription initiation factor TFIID, and MADS-box transcription factor 6), ubiquitin ligases (F-box protein SKIP19 and RING finger protein 44), and plant hormones (indole-3-acetic acid-amido synthetase GH3.6 and ethylene-responsive transcription factor ERF061). In addition, some important SNP loci were associated with multiple berry-shape traits. The study further revealed some genes that control multiple traits simultaneously, indicating that these berry shape traits are subject to the coordinated regulation of some genes in controlling berry shape.

CONCLUSIONS

In the present work, we identified interesting genetic determinants of grape berry shape-related traits. The identification of molecular markers that are closely related to these berry-shape traits is of great significance for breeding specific berry-shaped grape varieties.

Identification of yield-related genes through genome-wide association: case study of weeping forsythia, an emerging medicinal crop

KEY MESSAGE

This study identified candidate genes related to fruit yield for an emerging medicinal crop, weeping forsythia.

BACKGROUND

The genetic basis of crop yield is an agricultural research hotspot. Identifying the genes related to yield traits is the key to increase the yield. Weeping forsythia is an emerging medicinal crop that currently lacks excellent varieties. The genes related to fruit yield in weeping forsythia have not been identified.

OBJECTIVE

Thus, we aimed to screen the candidate genes related to fruit yield of weeping forsythia by using genome-wide association analysis.

METHODS

Here, 60 samples from the same field and source of weeping forsythia were collected to identify its yield-related candidate genes. Association analysis was performed on the variant loci and the traits related to yield, i.e., fruit length, width, thickness, and weight.

RESULTS

Results from admixture, neighbor-joining, and kinship matrix analyses supported the non-significant genetic differentiation of these samples. Significant association was found between 2 variant loci and fruit length, 8 loci and fruit width, 24 loci and fruit thickness, and 13 loci and fruit weight. Further search on the 20 kb up/downstream of these variant loci revealed 1 gene related to fruit length, 16 genes related to fruit width, 12 genes related to fruit thickness, and 13 genes related to fruit weight. Among which, 4 genes, namely, WRKY transcription factor 35, salicylic acid-binding protein, auxin response factor 6, and alpha-mannosidase were highly related to the fruit development of weeping forsythia.

CONCLUSION

This study identify four candidate genes related to fruit development, which will provide useful information for the subsequent molecular-assisted and genetic breeding of weeping forsythia.

Fine mapping and identification of the candidate gene BFS for fruit shape in wax gourd (Benincasa hispida)

Non-synonymous mutations in the BFS gene, which encodes the IQD protein, are responsible for the shape of wax gourd fruits. Fruit shape is an important agronomic trait in wax gourds. Therefore, in this study, we employed bulked segregant analysis (BSA) to identify a candidate gene for fruit shape in wax gourds within F₂ populations derived by crossing GX-71 (long cylindrical fruit, fruit shape index = 4.56) and MY-1 (round fruit, fruit shape index = 1.06) genotypes. According to BSA, the candidate gene is located in the 17.18 Mb region on chromosome 2. Meanwhile, kompetitive allele-specific PCR (KASP) markers were used to reduce it to a 19.6 Kb region. Only one gene was present within the corresponding region of the reference genome, namely Bch02G016830 (designated BFS). Subsequently, BFS was sequenced in six wax gourd varieties with different fruit shapes. Sequence analysis revealed two non-synonymous mutations in the round wax gourd and one non-synonymous mutation in the cylindrical wax gourd. Quantitative real‑time PCR (qRT-PCR) analysis further showed that the expression of BFS in round fruits was significantly higher than in long cylindrical fruits at the ovary formation stage. Therefore, BFS is a candidate gene for determination wax gourd shape. The predicted protein encoded by the BFS gene belongs to the IQ67-domain protein family, which have the structural characteristics of scaffold proteins and coordinate Ca²⁺ CaM signaling from the membrane to the nucleus. Ultimately, two derived cleaved amplified polymorphic sequence (dCAPS) markers were developed to facilitate marker-assisted selection for wax gourds breeding.

Long-read genome assembly and genetic architecture of fruit shape in the bottle gourd

The bottle gourd (Lagenaria siceraria, Cucurbitaceae) is an important horticultural crop exhibiting tremendous diversity in fruit shape. The genetic architecture of fruit shape variation in this species remains unknown. We assembled a long-read-based, high-quality reference genome (ZAAS_Lsic_2.0) with a contig N50 value over 390-fold greater than the existing reference genomes. We then focused on dissection of fruit shape using a one-step geometric morphometrics-based functional mapping approach. We identified 11 quantitative trait loci (QTLs) responsible for fruit shape (fsQTLs), reconstructed their visible effects and revealed syntenic relationships of bottle gourd fsQTLs with 12 fsQTLs previously reported in cucumber, melon or watermelon. Homologs of several well-known and newly identified fruit shape genes, including SUN, OFP, AP2 and auxin transporters, were comapped with bottle gourd QTLs.

CsKTN1 for a katanin p60 subunit is associated with the regulation of fruit elongation in cucumber (Cucumis sativus L.)

We identified a short fruit3 (sf3) mutant in cucumber. Map-based cloning revealed that CsKTN1 gene encodes a katanin p60 subunit, which is associated with the regulation of fruit elongation. Fruit length is an important horticultural trait for both fruit yield and quality of cucumber (Cucumis sativus L.). Knowledge on the molecular regulation of fruit elongation in cucumber is very limited. In this study, we identified and characterized a cucumber short fruit3 (sf3) mutant. Histological examination indicated that the shorter fruit in the mutant was due to reduced cell numbers. Genetic analysis revealed that the phenotype of the sf3 mutant was controlled by a single gene with semi-dominant inheritance. By map-based cloning and Arabidopsis genetic transformation, we showed that Sf3 was a homolog of KTN1 (CsKTN1) encoding a katanin p60 subunit. A non-synonymous mutation in the fifth exon of CsKTN1 resulted in an amino acid substitution from Serine in the wild type to Phenylalanine in the sf3 mutant. CsKTN1 expressed in all tissues of both the wild type and the sf3 mutant. However, there was no significant difference in CsKTN1 expression levels between the wild type and the sf3 mutant. The hormone quantitation and RNA-seq analysis suggested that auxin and gibberellin contents are decreased in sf3 by changing the expression levels of genes related with auxin and gibberellin metabolism and signaling. This work helps understand the function of the katanin and the molecular mechanisms of fruit growth regulation in cucumber.

Identification of quantitative trait loci for powdery mildew resistance in highly resistant cucumber (Cucumis sativus L.) using ddRAD-seq analysis

Powdery mildew, caused by Podosphaera xanthii (syn. Sphaerotheca fuliginea ex Fr. Poll.), is one of the most economically important foliar diseases in cucumber (Cucumis sativus L.). Cucumber parental line 'Kyuri Chukanbohon Nou 5 Go', developed from weedy cucumber line CS-PMR1, is highly resistant to powdery mildew and is promising breeding material. We performed quantitative trait locus (QTL) analysis using double-digest restriction-site-associated DNA sequencing (ddRAD-Seq) in a population from a cross between 'Kyuri Chukanbohon Nou 5 Go' and the Japanese native cultivar 'Kaga-aonaga-fushinari', which is susceptible to powdery mildew. The resistance of the population and its parents was evaluated using leaf disc assays and image analysis. We detected one major QTL on Chr. 5 that was effective at both 20°C and 25°C and one minor QTL on Chr. 1 effective at 20°C. We detected two additional QTLs in subpopulation: one on Chr. 3 effective at 20°C and one on Chr. 5 effective at both 20°C and 25°C in a position different from the major QTL. The resistance alleles at all four QTLs were contributed by 'Kyuri Chukanbohon Nou 5 Go'. The results of this study can be used to develop practical DNA markers tightly linked to genes for powdery mildew resistance.

Identification of a major-effect QTL associated with pre-harvest sprouting in cucumber (Cucumis sativus L.) using the QTL-seq method

Background

Cucumber (Cucumis sativus L.) is cultivated worldwide, and it is essential to produce enough high-quality seeds to meet demand. Pre-harvest sprouting (PHS) in cucumber is a critical problem and causes serious damage to seed production and quality. Nevertheless, the genetic basis and molecular mechanisms underlying cucumber PHS remain unclear. QTL-seq is an efficient approach for rapid quantitative trait loci (QTL) identification that simultaneously takes advantage of bulked-segregant analysis (BSA) and whole-genome resequencing. In the present research, QTL-seq analysis was performed to identify QTLs associated with PHS in cucumber using an F₂ segregating population.

Results

Two QTLs that spanned 7.3 Mb on Chromosome 4 and 0.15 Mb on Chromosome 5 were identified by QTL-seq and named qPHS4.1 and qPHS5.1, respectively. Subsequently, SNP and InDel markers selected from the candidate regions were used to refine the intervals using the extended F₂ populations grown in the 2016 and 2017 seasons. Finally, qPHS4.1 was narrowed to 0.53 Mb on chromosome 4 flanked by the markers SNP-16 and SNP-24 and was found to explain 19–22% of the phenotypic variation in cucumber PHS. These results reveal that qPHS4.1 is a major-effect QTL associated with PHS in cucumber. Based on gene annotations and qRT-PCR expression analyses, Csa4G622760 and Csa4G622800 were proposed as the candidate genes.

Conclusions

These results provide novel insights into the genetic mechanism controlling PHS in cucumber and highlight the potential for marker-assisted selection of PHS resistance breeding.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07548-8.

QTL-seq analysis of powdery mildew resistance in a Korean cucumber inbred line

QTL mapping and RT-PCR analyses identified the CsGy5G015660 as a strong powdery mildew resistance candidate gene and natural variation of CsGy5G015660 allele was observed using 115 core germplasm. Powdery mildew (PM) is among the most serious fungal diseases encountered in the cultivation of cucurbits. The development of PM-resistant inbred lines is thus of considerable significance for cucumber breeding programs. In this study, we applied bulked segregant analysis combined with QTL-seq to identify PM resistance loci using F₂ population derived from a cross between two Korean cucumber inbred lines, PM-R (resistant) and PM-S (susceptible). Genome-wide SNP profiling using bulks of the two extreme phenotypes identified two QTLs on chromosomes 5 and 6, designated pm5.2 and pm6.1, respectively. The two PM resistance loci were validated using molecular marker-based classical QTL analysis: pm5.2 (30% R² at LOD 11) and pm6.1 (11% R² at LOD 3.2). Furthermore, reverse transcriptase-PCR analyses, using genes found to be polymorphic between PM-R and PM-S, were conducted to identify the candidate gene(s) responsible for PM resistance. We found that transcripts of the gene CsGy5G015660, encoding a putative leucine-rich repeat receptor-like serine/threonine-protein kinase (RPK2), showed specific accumulation in PM-R prior to the appearance of disease symptoms, and was accordingly considered a strong candidate gene for PM resistance. In addition, cleaved amplified polymorphic sequence markers from CsGy5G015660 were developed and used to screen 35 inbred lines. Natural variation in the CsGy5G015660 allele was also observed based on analysis of a core collection of 115 cucumber accessions. Our results provide new genetic insights for gaining a better understanding of the genetic basis of PM resistance in cucumber, and pave the way for further utilization in cucumber PM resistance breeding programs.

Quantitative Trait Loci Mapping for Earliness, Fruit, and Seed Related Traits Using High Density Genotyping-by-Sequencing-Based Genetic Map in Bitter Gourd (Momordica charantia L.)

Bitter gourd (Momordica charantia L.) is an important vegetable crop having numerous medicinal properties. Earliness and yield related traits are main aims of bitter gourd breeding program. High resolution quantitative trait loci (QTLs) mapping can help in understanding the molecular basis of phenotypic variation of these traits and thus facilitate marker-assisted breeding. The aim of present study was to identify genetic loci controlling earliness, fruit, and seed related traits. To achieve this, genotyping-by-sequencing (GBS) approach was used to genotype 101 individuals of F₄ population derived from a cross between an elite cultivar Punjab-14 and PAUBG-6. This population was phenotyped under net-house conditions for three years 2018, 2019, and 2021. The linkage map consisting of 15 linkage groups comprising 3,144 single nucleotide polymorphism (SNP) markers was used to detect the QTLs for nine traits. A total of 50 QTLs for these traits were detected which were distributed on 11 chromosomes. The QTLs explained 5.09-29.82% of the phenotypic variance. The highest logarithm of the odds (LOD) score for a single QTL was 8.68 and the lowest was 2.50. For the earliness related traits, a total of 22 QTLs were detected. For the fruit related traits, a total of 16 QTLs and for seed related traits, a total of 12 QTLs were detected. Out of 50 QTLs, 20 QTLs were considered as frequent QTLs (FQ-QTLs). The information generated in this study is very useful in the future for fine-mapping and marker-assisted selection for these traits in bitter gourd improvement program.

Localization of quantitative trait loci for cucumber fruit shape by a population of chromosome segment substitution lines

Cucumber fruit shape, a significant agronomic trait, is controlled by quantitative trait loci (QTLs). Feasibility of chromosome segment substitution lines (CSSLs) is well demonstrated to map QTLs, especially the minor-effect ones. To detect and identify QTLs with CSSLs can provide new insights into the underlying mechanisms regarding cucumber fruit shape. In the present study, 71 CSSLs were built from a population of backcross progeny (BC4F2) by using RNS7 (a round-fruit cucumber) as the recurrent parent and CNS21 (a long-stick-fruit cucumber) as the donor parent in order to globally detect QTLs for cucumber fruit shape. With the aid of 114 InDel markers covering the whole cucumber genome, 21 QTLs were detected for fruit shape-related traits including ovary length, ovary diameter, ovary shape index, immature fruit length, immature fruit diameter, immature fruit shape index, mature fruit length, mature fruit diameter and mature fruit shape index, and 4 QTLs for other traits including fruit ground and flesh color, and seed size were detected as well. Together our results provide important resources for the subsequent theoretical and applied researches on cucumber fruit shape and other traits.

Recent progress on the molecular breeding of Cucumis sativus L. in China

Molecular breeding of Cucumis sativus L. is based on traditional breeding techniques and modern biological breeding in China. There are opportunities for further breeding improvement by molecular design breeding and the automation of phenotyping technology using untapped sources of genetic diversity. Cucumber (Cucumis sativus L.) is an important vegetable cultivated worldwide. It bears fruits of light fragrance, and crisp texture with high nutrition. China is the largest producer and consumer of cucumber, accounting for 70% of the world's total production. With increasing consumption demand, the production of Cucurbitaceae crops has been increasing yearly. Thus, new cultivars that can produce high-quality cucumber with high yield and easy cultivation are in need. Conventional genetic breeding has played an essential role in cucumber cultivar innovation over the past decades. However, its progress is slow due to the long breeding period, and difficulty in selecting stable genetic characters or genotypes, prompting researchers to apply molecular biotechnologies in cucumber breeding. Here, we first summarize the achievements of conventional cucumber breeding such as crossing and mutagenesis, and then focus on the current status of molecular breeding of cucumber in China, including the progress and achievements on cucumber genomics, molecular mechanism underlying important agronomic traits, and also on the creation of high-quality multi-resistant germplasm resources, new variety breeding and ecological breeding. Future development trends and prospects of cucumber molecular breeding in China are also discussed.

QTL-seq for identification of loci associated with resistance to Phytophthora crown rot in squash

Phytophthora capsici Leonian, the causal agent of foliar blight, root rot, fruit rot and crown rot syndromes in squash (Cucurbita moschata), is a devastating pathogen worldwide. Resistance to Phytophthora crown rot in University of Florida breeding line #394-1-27-12 (C. moschata) is conferred by three independent dominant genes (R1R2R3). Availability of DNA markers linked to R1R2R3 genes would allow efficient breeding for Phytophthora crown rot resistance through marker-assisted selection (MAS). The goal of the current study was to identify quantitative trait loci (QTLs) associated with resistance to Phytophthora crown rot in an F₂ population (n = 168) derived from a cross between #394-1-27-12 (R) and Butter Bush (S) using QTL-seq bulk segregant analysis. Whole-genome resequencing of the resistant (n = 20) and susceptible (n = 20) bulk segregants revealed ~900,000 single nucleotide polymorphisms distributed across C. moschata genome. Three QTLs significantly (P < 0.05) associated with resistance to Phytophthora crown rot were detected on chromosome 4 (QtlPC-C04), 11 (QtlPC-C11) and 14 (QtlPC-C14). Several markers linked to these QTLs are potential targets for MAS against Phytophthora crown rot in C. moschata. The present study reports the first QTLs associated with Phytophthora crown rot resistance in C. moschata.

Genetic mapping of green curd gene Gr in cauliflower

Gr5.1 is the major locus for cauliflower green curd color and mapped to an interval of 236 Kbp with four most likely candidate genes. Cauliflower with colored curd enhances not only the visual appeal but also the nutritional value of the crop. Green cauliflower results from ectopic development of chloroplasts in the normal white curd. However, the underlying genetic basis is unknown. In this study, we employed QTL-seq analysis to identify the loci that were associated with green curd phenotype in cauliflower. A F2 population was generated following a cross between a white curd (Stovepipe) and a green curd (ACX800) cauliflower plants. By whole-genome resequencing and SNP analysis of green and white F2 bulks, two QTLs were detected on chromosomes 5 (Gr5.1) and 7 (Gr7.1). Validation by traditional genetic mapping with CAPS markers suggested that Gr5.1 represented a major QTL, whereas Gr7.1 had a minor effect. Subsequent high-resolution mapping of Gr5.1 in the second large F2 population with additional CAPS markers narrowed down the target region to a genetic and physical distance of 0.3 cM and 236 Kbp, respectively. This region contained 35 genes with four of them representing the best candidates for the green curd phenotype in cauliflower. They are LOC106295953, LOC106343833, LOC106345143, and LOC106295954, which encode UMP kinase, DEAD-box RNA helicase 51-like, glutathione S-transferase T3-like, and protein MKS1, respectively. These findings lay a solid foundation for the isolation of the Gr gene and provide a potential for marker-assisted selection of the green curd trait in cauliflower breeding. The eventual isolation of Gr will also facilitate better understanding of chloroplast biogenesis and development in plants.

Genetic architecture of fruit size and shape variation in cucurbits: a comparative perspective

The Cucurbitaceae family hosts many economically important fruit vegetables (cucurbits) such as cucumber, melon, watermelon, pumpkin/squash, and various gourds. The cucurbits are probably best known for the diverse fruit sizes and shapes, but little is known about their genetic basis and molecular regulation. Here, we reviewed the literature on fruit size (FS), shape (FSI), and fruit weight (FW) QTL identified in cucumber, melon, and watermelon, from which 150 consensus QTL for these traits were inferred. Genome-wide survey of the three cucurbit genomes identified 253 homologs of eight classes of fruit or grain size/weight-related genes cloned in Arabidopsis, tomato, and rice that encode proteins containing the characteristic CNR (cell number regulator), CSR (cell size regulator), CYP78A (cytochrome P450), SUN, OVATE, TRM (TONNEAU1 Recruiting Motif), YABBY, and WOX domains. Alignment of the consensus QTL with candidate gene homologs revealed widespread structure and function conservation of fruit size/shape gene homologs in cucurbits, which was exemplified with the fruit size/shape candidate genes CsSUN25-26-27a and CsTRM5 in cucumber, CmOFP1a in melon, and ClSUN25-26-27a in watermelon. In cucurbits, the andromonoecy (for 1-aminocyclopropane-1-carboxylate synthase) and the carpel number (for CLAVATA3) loci are known to have pleiotropic effects on fruit shape, which may complicate identification of fruit size/shape candidate genes in these regions. The present work illustrates the power of comparative analysis in understanding the genetic architecture of fruit size/shape variation, which may facilitate QTL mapping and cloning for fruit size-related traits in cucurbits. The limitations and perspectives of this approach are also discussed.

Next-generation sequencing identified genomic region and diagnostic markers for resistance to bacterial wilt on chromosome B02 in peanut (Arachis hypogaea L.)

Bacterial wilt, caused by Ralstonia solanacearum, is a devastating disease affecting over 350 plant species. A few peanut cultivars were found to possess stable and durable bacterial wilt resistance (BWR). Genomics-assisted breeding can accelerate the process of developing resistant cultivars by using diagnostic markers. Here, we deployed sequencing-based trait mapping approach, QTL-seq, to discover genomic regions, candidate genes and diagnostic markers for BWR in a recombination inbred line population (195 progenies) of peanut. The QTL-seq analysis identified one candidate genomic region on chromosome B02 significantly associated with BWR. Mapping of newly developed single nucleotide polymorphism (SNP) markers narrowed down the region to 2.07 Mb and confirmed its major effects and stable expressions across three environments. This candidate genomic region had 49 nonsynonymous SNPs affecting 19 putative candidate genes including seven putative resistance genes (R-genes). Two diagnostic markers were successfully validated in diverse breeding lines and cultivars and could be deployed in genomics-assisted breeding of varieties with enhanced BWR.

Molecular Characterisation of a Supergene Conditioning Super-High Vitamin C in Kiwifruit Hybrids

During analysis of kiwifruit derived from hybrids between the high vitamin C (ascorbic acid; AsA) species Actinidia eriantha and A. chinensis, we observed bimodal segregation of fruit AsA concentration suggesting major gene segregation. To test this hypothesis, we performed whole-genome sequencing on pools of hybrid genotypes with either high or low AsA fruit. Pool-GWAS (genome-wide association study) revealed a single Quantitative Trait Locus (QTL) spanning more than 5 Mbp on chromosome 26, which we denote as qAsA26.1. A co-dominant PCR marker was used to validate this association in four diploid (A. chinensis × A. eriantha) × A. chinensis backcross families, showing that the A. eriantha allele at this locus increases fruit AsA levels by 250 mg/100 g fresh weight. Inspection of genome composition and recombination in other A. chinensis genetic maps confirmed that the qAsA26.1 region bears hallmarks of suppressed recombination. The molecular fingerprint of this locus was examined in leaves of backcross validation families by RNA sequencing (RNASEQ). This confirmed strong allelic expression bias across this region as well as differential expression of transcripts on other chromosomes. This evidence suggests that the region harbouring qAsA26.1 constitutes a supergene, which may condition multiple pleiotropic effects on metabolism.

Discovery of genomic regions and candidate genes controlling shelling percentage using QTL-seq approach in cultivated peanut (Arachis hypogaea L.)

Cultivated peanut (Arachis hypogaea L.) is an important grain legume providing high-quality cooking oil, rich proteins and other nutrients. Shelling percentage (SP) is the 2nd most important agronomic trait after pod yield and this trait significantly affects the economic value of peanut in the market. Deployment of diagnostic markers through genomics-assisted breeding (GAB) can accelerate the process of developing improved varieties with enhanced SP. In this context, we deployed the QTL-seq approach to identify genomic regions and candidate genes controlling SP in a recombinant inbred line population (Yuanza 9102 × Xuzhou 68-4). Four libraries (two parents and two extreme bulks) were constructed and sequenced, generating 456.89-790.32 million reads and achieving 91.85%-93.18% genome coverage and 14.04-21.37 mean read depth. Comprehensive analysis of two sets of data (Yuanza 9102/two bulks and Xuzhou 68-4/two bulks) using the QTL-seq pipeline resulted in discovery of two overlapped genomic regions (2.75 Mb on A09 and 1.1 Mb on B02). Nine candidate genes affected by 10 SNPs with non-synonymous effects or in UTRs were identified in these regions for SP. Cost-effective KASP (Kompetitive Allele-Specific PCR) markers were developed for one SNP from A09 and three SNPs from B02 chromosome. Genotyping of the mapping population with these newly developed KASP markers confirmed the major control and stable expressions of these genomic regions across five environments. The identified candidate genomic regions and genes for SP further provide opportunity for gene cloning and deployment of diagnostic markers in molecular breeding for achieving high SP in improved varieties.

Identification of quantitative trait loci governing subgynoecy in cucumber

QTL-seq analysis identified three major QTLs conferring subgynoecy in cucumbers. Furthermore, sequence and expression analyses predicted candidate genes controlling subgynoecy. The cucumber (Cucumis sativus L.) is a typical monoecious having individual male and female flowers, and sex differentiation is an important developmental process that directly affects its fruit yield. Subgynoecy represents a sex form with a high degree of femaleness and would have alternative use as gynoecy under limited resource conditions. Recently, many studies have been reported that QTL-seq, which integrates the advantages of bulked segregant analysis and high-throughput whole-genome resequencing, can be a rapid and cost-effective way of mapping QTLs. Segregation analysis in the F₂ and BC₁ populations derived from a cross between subgynoecious LOSUAS and monoecious BMB suggested the quantitative nature of subgynoecy in cucumbers. Both genome-wide SNP profiling of subgynoecious and monoecious bulks constructed from F₂ and BC₁ plants consistently identified three significant genomic regions, one on chromosome 3 (sg3.1) and another two on short and long arms of chromosome 1 (sg1.1 and sg1.2). Classical QTL analysis using the F₂ confirmed sg3.1 (R² = 42%), sg1.1 (R² = 29%) and sg1.2 (R² = 18%) as major QTLs. These results revealed the unique genetic inheritance of subgynoecious line LOSUAS through two distinct major QTLs, sg3.1 and sg1.1, which mainly increase degree of femaleness, while another QTL, sg1.2, contributes to decrease it. This study demonstrated that QTL-seq allows rapid and powerful detection of QTLs using preliminary generation mapping populations such as F₂ or BC₁ population and further that the identified QTLs could be useful for molecular breeding of cucumber lines with high yield potential.

Identification of genomic regions associated with multi-silique trait in Brassica napus

BACKGROUND

Although rapeseed (Brassica napus L.) mutant forming multiple siliques was morphologically described and considered to increase the silique number per plant, an important agronomic trait in this crop, the molecular mechanism underlying this beneficial trait remains unclear. Here, we combined bulked-segregant analysis (BSA) and whole genome re-sequencing (WGR) to map the genomic regions responsible for the multi-silique trait using two pools of DNA from the near-isogenic lines (NILs) zws-ms (multi-silique) and zws-217 (single-silique). We used the Euclidean Distance (ED) to identify genomic regions associated with this trait based on both SNPs and InDels. We also conducted transcriptome sequencing to identify differentially expressed genes (DEGs) between zws-ms and zws-217.

RESULTS

Genetic analysis using the ED algorithm identified three SNP- and two InDel-associated regions for the multi-silique trait. Two highly overlapped parts of the SNP- and InDel-associated regions were identified as important intersecting regions, which are located on chromosomes A09 and C08, respectively, including 2044 genes in 10.20-MB length totally. Transcriptome sequencing revealed 129 DEGs between zws-ms and zws-217 in buds, including 39 DEGs located in the two abovementioned associated regions. We identified candidate genes involved in multi-silique formation in rapeseed based on the results of functional annotation.

CONCLUSIONS

This study identified the genomic regions and candidate genes related to the multi-silique trait in rapeseed.

Complete resistance to powdery mildew and partial resistance to downy mildew in a Cucumis hystrix introgression line of cucumber were controlled by a co-localized locus

Key message A single recessive gene for complete resistance to powdery mildew and a major-effect QTL for partial resistance to downy mildew were co-localized in a Cucumis hystrix introgression line of cucumber. Downy mildew (DM) and powdery mildew (PM) are two major foliar diseases in cucumber. DM resistance (DMR) and PM resistance (PMR) may share common components; however, the genetic relationship between them remains unclear. IL52, a Cucumis hystrix introgression line of cucumber which has been reported to possess DMR, was recently identified to exhibit PMR as well. In this study, a single recessive gene pm for PMR was mapped to an approximately 468-kb region on chromosome 5 with 155 recombinant inbred lines (RILs) and 193 F₂ plants derived from the cross between a susceptible line 'changchunmici' and IL52. Interestingly, pm was co-localized with the major-effect DMR QTL dm5.2 confirmed by combining linkage analysis and BSA-seq, which was consistent with the observed linkage of DMR and PMR in IL52. Further, phenotype-genotype correlation analysis of DMR and PMR in the RILs indicated that the co-localized locus pm/dm5.2 confers complete resistance to PM and partial resistance to DM. Seven candidate genes for DMR were identified within dm5.2 by BSA-seq analysis, of which Csa5M622800.1, Csa5M622830.1 and Csa5M623490.1 were also the same candidate genes for PMR. A single nucleotide polymorphism that is present in the 3' untranslated region (3'UTR) of Csa5M622830.1 co-segregated perfectly with PMR. The GATA transcriptional factor gene Csa5M622830.1 may be a likely candidate gene for DMR and PMR. This study has provided a clear evidence for the relationship between DMR and PMR in IL52 and sheds new light on the potential value of IL52 for cucumber DMR and PMR breeding program.

Application of Whole Genome Resequencing in Mapping of a Tomato Yellow Leaf Curl Virus Resistance Gene

Tomato yellow leaf curl virus (TYLCV) has significantly impacted the tomato industry around the world, and the use of insecticides and insect nets have not effectively controlled the spread of this pathogen. The tomato line AVTO1227 is highly resistant to TYLCV. In this study, F2 and BC1 populations derived from AVTO1227 and the susceptible line Money maker were used to assess the genetic mechanism underlying TYLCV resistance. We have identified a recessive TYLCV resistance gene, hereby designated as ty-5, which is linked to SlNACI. Genomic DNA pools from resistant and susceptible groups were constructed, and their genomes were resequenced. The ty-5 gene was identified on an interval encompassing the genomic positions 2.22 Mb to 3.19 Mb on tomato chromosome 4. Genotyping using linkage markers further mapped ty-5 within the interval between markers ty5-25 and ty5-29, where only the pelota gene is located. Consequently, pelota was considered as the candidate gene corresponding to ty-5. Two nucleotide transversions within the promoter region and one transversion in exon region of the pelota gene were detected in the parental lines. However, the relative transcript levels of pelota did not significantly differ among the three tomato lines, regardless of TYLCV infection. This study will facilitate marker-assisted breeding for resistance to TYLCV and lay a foundation for the research of the resistance mechanism of ty-5 in tomato.

Genetic mapping reveals a candidate gene (ClFS1) for fruit shape in watermelon (Citrullus lanatus L.)

A 159 bp deletion in ClFS1 gene encoding IQD protein is responsible for fruit shape in watermelon. Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is known for its rich diversity in fruit size and shape. Fruit shape has been one of the major objectives of watermelon breeding. However, the candidate genes and the underlying genetic mechanism for such an important trait in watermelon are unknown. In this study, we identified a locus on chromosome 3 of watermelon genome controlling fruit shape. Segregation analysis in F₂ and BC₁ populations derived from a cross between two inbred lines "Duan125" (elongate fruit) and "Zhengzhouzigua" (spherical fruit) suggests that fruit shape of watermelon is controlled by a single locus and elongate fruit (OO) is incompletely dominant to spherical fruit (oo) with the heterozygote (Oo) being oval fruit. GWAS profiles among 315 accessions identified a major locus designated on watermelon chromosome 3, which was confirmed by BSA-seq mapping in the F₂ population. The candidate gene was mapped to a region 46 kb on chromosome 3. There were only four genes present in the corresponding region in the reference genome. Four candidate genes were sequenced in this region, revealing that the CDS of Cla011257 had a 159 bp deletion which resulted in the omission of 53 amino acids in elongate watermelon. An indel marker was derived from the 159 bp deletion to test the F₂ population and 105 watermelon accessions. The results showed that Cla011257 cosegregated with watermelon fruit shape. In addition, the Cla011257 expression was the highest at ovary formation stage. The predicted protein of the Cla011257 gene fitted in IQD protein family which was reported to have association with cell arrays and Ca²⁺-CaM signaling modules. Clear understanding of the genes facilitating the fruit shape along with marker association selection will be an effective way to develop new cultivars.

GourdBase: a genome-centered multi-omics database for the bottle gourd (Lagenaria siceraria), an economically important cucurbit crop

GourdBase is an integrative data platform for the bottle gourd to examine its multifarious intuitive morphology and annotated genome. GourdBase consists of six main modules that store and interlink multi-omic data: the genome (with transcriptomic data integrated) module, the phenome module, the markers/QTLs module, the maps (genetic, physical and comparative) module, the cultivars module, and the publications module. These modules provide access to various type of data including the annotated reference genome sequence, gene models, transcriptomic data from various tissues, physical and comparative genome maps, molecular markers in different types, phenotypic data for featuring traits including fruit shape and umami taste, and quantitative trait loci (QTLs) that underlie these traits. GourdBase is intuitive, user-friendly and interlinked and is designed to allow researchers, breeders and trained farmers to browse, search and fetch information on interests and assist in genomics-driven studies and breeding. The knowledge base and web interface can be accessed at http://www.gourdbase.cn/.

Rapid identification of a stripe rust resistant gene in a space-induced wheat mutant using specific locus amplified fragment (SLAF) sequencing

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most devastating diseases of wheat. Resistant cultivars are the preferred strategy to control the disease. Space-induced wheat mutant R39 has adult-plant resistance (APR) to Pst. Genetic analysis indicated that a single recessive gene, designated YrR39, was responsible for the APR of R39 to Pst. Bulked segregant analysis (BSA) combined with a SLAF sequencing (SLAF-seq) strategy was used to fine-map YrR39 to a 17.39 Mb segment on chromosome 4B. The region was confirmed by analysis with simple sequence repeat (SSR) markers. A total of 126 genes were annotated in the region and 21 genes with annotations associated with disease response were selected for further qRT-PCR analysis. The candidate gene Traes_4BS_C868349E1 (annotated as an F-box/LRR-repeat protein) was up-regulated after 12, 24, 48, and 96 hours post inoculation with Pst, suggesting it is likely involved in the resistance. The current study demonstrated that BSA combined with SLAF-seq for SNP discovery is an efficient approach for mapping and identifying candidate functional gene.

QTL-seq analysis identifies two genomic regions determining the heading date of foxtail millet, Setaria italica (L.) P.Beauv

Heading date is an important event to ensure successful seed production. Although foxtail millet (Setaria italica (L.) P.Beauv.) is an important foodstuff in semiarid regions around the world, the genetic basis determining heading date is unclear. To identify genomic regions regulating days to heading (DTH), we conducted a QTL-seq analysis based on combining whole-genome re-sequencing and bulked-segregant analysis of an F₂ population derived from crosses between the middle-heading cultivar Shinanotsubuhime and the early-heading cultivar Yuikogane. Under field conditions, transgressive segregation of DTH toward late heading was observed in the F₂ population. We made three types of bulk samples: Y-bulk (early-heading), S-bulk (late-heading) and L-bulk (extremely late-heading). By genome-wide comparison of SNPs in the Y-bulk vs. the S-bulk and the Y-bulk vs. the L-bulk, we identified two QTLs associated with DTH. The first QTL, qDTH2, was detected on chromosome 2 from the Y-bulk and S-bulk comparison. The second QTL, qDTH7, was detected on chromosome 7 from the Y-bulk and L-bulk comparison. The Shinanotsubuhime allele for qDTH2 caused late heading in the F₂ population, whereas the Yuikogane allele for qDTH7 led to extremely late heading. These results suggest that allelic differences in both qDTH2 and qDTH7 determine regional adaptability in S. italica.

A high-density linkage map and QTL mapping of fruit-related traits in pumpkin (Cucurbita moschata Duch.)

Pumpkin (Cucurbita moschata) is an economically worldwide crop. Few quantitative trait loci (QTLs) were reported previously due to the lack of genomic and genetic resources. In this study, a high-density linkage map of C. moschata was structured by double-digest restriction site-associated DNA sequencing, using 200 F2 individuals of CMO-1 × CMO-97. By filtering 74,899 SNPs, a total of 3,470 high quality SNP markers were assigned to the map spanning a total genetic distance of 3087.03 cM on 20 linkage groups (LGs) with an average genetic distance of 0.89 cM. Based on this map, both pericarp color and strip were fined mapped to a novel single locus on LG8 in the same region of 0.31 cM with phenotypic variance explained (PVE) of 93.6% and 90.2%, respectively. QTL analysis was also performed on carotenoids, sugars, tuberculate fruit, fruit diameter, thickness and chamber width with a total of 12 traits. 29 QTLs distributed in 9 LGs were detected with PVE from 9.6% to 28.6%. It was the first high-density linkage SNP map for C. moschata which was proved to be a valuable tool for gene or QTL mapping. This information will serve as significant basis for map-based gene cloning, draft genome assembling and molecular breeding.

Round fruit shape in WI7239 cucumber is controlled by two interacting quantitative trait loci with one putatively encoding a tomato SUN homolog

QTL analysis revealed two interacting loci, FS1.2 and FS2.1, underlying round fruit shape in WI7239 cucumber; CsSUN , a homolog of tomato fruit shape gene SUN , was a candidate for FS1.2. Fruit size is an important quality and yield trait in cucumber, but its genetic basis remains poorly understood. Here we reported QTL mapping results on fruit size with segregating populations derived from the cross between WI7238 (long fruit) and WI7239 (round fruit) inbred cucumber lines. Phenotypic data of fruit length and diameter were collected at anthesis, immature and mature fruit stages in four environments. Ten major-effect QTL were detected for six traits; synthesis of information from these QTL supported two genes, FS1.2 and FS2.1, underlying fruit size variation in the examined populations. Under the two-gene model, deviation from expected segregation ratio in fruit length and diameter among segregating populations was observed, which could be explained mainly by the interactions between FS1.2 and FS2.1, and segregation distortion in the FS2.1 region. Genome-wide candidate gene search identified CsSUN, a homolog of the tomato fruit shape gene SUN, as the candidate for FS1.2. The round-fruited WI7239 had a 161-bp deletion in the first exon of CsSUN, and its expression in WI7239 was significantly lower than that in WI7238. A marker derived from this deletion was mapped at the peak location of FS1.2 in QTL analysis. Comparative analysis suggested the melon gene CmSUN-14, a homolog of CsSUN as a candidate of the fl2/fd2/fw2 QTL in melon. This study revealed the unique genetic architecture of round fruit shape in WI7239 cucumber. It also highlights the power of QTL analysis for traits with a simple genetic basis but their expression is complicated by other factors.

Interval mapping for red/green skin color in Asian pears using a modified QTL-seq method

Pears with red skin are attractive to consumers and provide additional health benefits. Identification of the gene(s) responsible for skin coloration can benefit cultivar selection and breeding. The use of QTL-seq, a bulked segregant analysis method, can be problematic when heterozygous parents are involved. The present study modified the QTL-seq method by introducing a |Δ(SNP-index)| parameter to improve the accuracy of mapping the red skin trait in a group of highly heterozygous Asian pears. The analyses were based on mixed DNA pools composed of 28 red-skinned and 27 green-skinned pear lines derived from a cross between the 'Mantianhong' and 'Hongxiangsu' red-skinned cultivars. The 'Dangshansuli' cultivar genome was used as reference for sequence alignment. An average single-nucleotide polymorphism (SNP) index was calculated using a sliding window approach (200-kb windows, 20-kb increments). Nine scaffolds within the candidate QTL interval were in the fifth linkage group from 111.9 to 177.1 cM. There was a significant linkage between the insertions/deletions and simple sequence repeat markers designed from the candidate intervals and the red/green skin (R/G) locus, which was in a 582.5-kb candidate interval that contained 81 predicted protein-coding gene models and was composed of two subintervals at the bottom of the fifth chromosome. The ZFRI 130-16, In2130-12 and In2130-16 markers located near the R/G locus could potentially be used to identify the red skin trait in Asian pear populations. This study provides new insights into the genetics controlling the red skin phenotype in this fruit.