Construction of a genome-anchored, high-density genetic map for melon (Cucumis melo L.) and identification of Fusarium oxysporum f. sp. melonis race 1 resistance QTL

A wild melon reference genome provides novel insights into the domestication of a key gene responsible for melon fruit acidity

KEY MESSAGE

A wild melon reference genome elucidates the genomic basis of fruit acidity domestication. Structural variants (SVs) have been reported to impose major effects on agronomic traits, representing a significant contributor to crop domestication. However, the landscape of SVs between wild and cultivated melons is elusive and how SVs have contributed to melon domestication remains largely unexplored. Here, we report a 379-Mb chromosome-scale genome of a wild progenitor melon accession "P84", with a contig N50 of 14.9 Mb. Genome comparison identifies 10,589 SVs between P84 and four cultivated melons with 6937 not characterized in previously analysis of 25 melon genome sequences. Furthermore, the population-scale genotyping of these SVs was determined in 1175 accessions, and 18 GWAS signals including fruit acidity, fruit length, fruit weight, fruit color and sex determination were detected. Based on these genotyped SVs, we identified 3317 highly diverged SVs between wild and cultivated melons, which could be the potential SVs associated with domestication-related traits. Furthermore, we identify novel SVs affecting fruit acidity and proposed the diverged evolutionary trajectories of CmPH, a key regulator of melon fruit acidity, during domestication and selection of different populations. These results will offer valuable resources for genomic studies and genetic improvement in melon.

QTL mapping of resistance to Pseudoperonospora cubensis clade 2, mating type A1, in Cucumis melo and dual-clade marker development

This is the first identification of QTLs underlying resistance in Cucumis melo to an isolate of Pseudoperonospora cubensis identified as Clade 2/mating type A1. Pseudoperonospora cubensis, causal organism of cucurbit downy mildew (CDM), causes severe necrosis and defoliation on Cucumis melo (melon). A recombinant inbred line population (N = 169) was screened against an isolate of P. cubensis (Clade 2/mating type A1) in replicated greenhouse and growth chamber experiments. SNPs (n = 5633 bins) identified in the RIL population were used for quantitative trait loci (QTL) mapping. A single major QTL on chromosome 10 (qPcub-10.3-10.4) was consistently associated with resistance across all experiments, while a second major QTL on chromosome 8 (qPcub-8.3) was identified only in greenhouse experiments. These two major QTLs were identified on the same chromosomes (8 and 10) but in different locations as two major QTLs (qPcub-8.2 and qPcub-10.1) previously identified for resistance to P. cubensis Clade 1/mating type A2. Kompetitive allele-specific PCR (KASP) markers were developed for these four major QTLs and validated in the RIL population through QTL mapping. These markers will provide melon breeders a high-throughput genotyping toolkit for development of melon cultivars with broad tolerance to CDM.

Graph-based pangenomics maximizes genotyping density and reveals structural impacts on fungal resistance in melon

The genomic sequences segregating in experimental populations are often highly divergent from the community reference and from one another. Such divergence is problematic under various short-read-based genotyping strategies. In addition, large structural differences are often invisible despite being strong candidates for causal variation. These issues are exacerbated in specialty crop breeding programs with fewer, lower-quality sequence resources. Here, we examine the benefits of complete genomic information, based on long-read assemblies, in a biparental mapping experiment segregating at numerous disease resistance loci in the non-model crop, melon (Cucumis melo). We find that a graph-based approach, which uses both parental genomes, results in 19% more variants callable across the population and raw allele calls with a 2 to 3-fold error-rate reduction, even relative to single reference approaches using a parent genome. We show that structural variation has played a substantial role in shaping two Fusarium wilt resistance loci with known causal genes. We also report on the genetics of powdery mildew resistance, where copy number variation and local recombination suppression are directly interpretable via parental genome alignments. Benefits observed, even in this low-resolution biparental experiment, will inevitably be amplified in more complex populations.

Exploring Potential Mechanisms of Fludioxonil Resistance in Fusarium oxysporum f. sp. melonis

Melon Fusarium wilt (MFW), which is caused by Fusarium oxysporum f. sp. melonis (FOM), is a soil-borne disease that commonly impacts melon cultivation worldwide. In the absence of any disease-resistant melon cultivars, the control of MFW relies heavily on the application of chemical fungicides. Fludioxonil, a phenylpyrrole fungicide, has been shown to have broad-spectrum activity against many crop pathogens. Sensitivity analysis experiments suggest that fludioxonil has a strong inhibitory effect on the mycelial growth of FOM isolates. Five fludioxonil-resistant FOM mutants were successfully generated by repeated exposure to fludioxonil under laboratory conditions. Although the mutants exhibited significantly reduced mycelial growth in the presence of the fungicide, there initially appeared to be little fitness cost, with no significant difference (p < 0.05) in the growth rates of the mutants and wild-type isolates. However, further investigation revealed that the sporulation of the fludioxonil-resistant mutants was affected, and mutants exhibited significantly (p < 0.05) reduced growth rates in response to KCl, NaCl, glucose, and mannitol. Meanwhile, molecular analysis of the mutants strongly suggested that the observed fludioxonil resistance was related to changes in the sequence and expression of the FoOs1 gene. In addition, the current study found no evidence of cross-resistance between fludioxonil and any of the other fungicides tested. These results indicate that fludioxonil has great potential as an alternative method of control for FOM in melon crops.

Quantitative Trait Loci Mapping of Resistance to Powdery Mildew Race 1 in a Recombinant Inbred Line Population of Melon

Powdery mildew, caused by the fungus Podosphaera xanthii, is one of the most important diseases of melon. Although there are several pathogenic races of P. xanthii, race 1 is the predominant race in South Carolina and in other parts of the United States. We used a densely genotyped recombinant inbred line melon population for traditional quantitative trait loci (QTL) mapping, to identify two major (qPx1-5 and qPx1-12) and two minor (qPx1-4 and qPx1-10) QTLs (named according to race - chromosome number) associated with resistance to P. xanthii race 1. QTL mapping of disease severity in multiple tissues (hypocotyl, cotyledons, true leaves, and stems) identified the same genetic basis of resistance in all tissue types. Whole-genome resequencing of the parents was used for marker development across the major QTLs and functional annotation of single nucleotide polymorphisms (SNPs) for candidate gene analysis. Kompetitive allele-specific PCR (KASP) markers were tightly linked to the QTL peaks of qPx1-5 (pm1-5_25329892, pm1-5_25461503 and pm1-5_25625375) and qPx1-12 (pm1-12_22848920 and pm1-12_22904659) in the population and will enable efficient marker-assisted introgression of powdery mildew resistance into improved germplasm. Candidate genes were identified in both major QTL intervals that encode putative R genes with missense mutations between the parents. The candidate genes provide targets for future breeding efforts and a fundamental examination of resistance to powdery mildew in melon.

QTL mapping of resistance to Pseudoperonospora cubensis clade 1, mating type A2, in Cucumis melo

This is the first identification of QTLs underlying resistance to Pseudoperonospora cubensis in Cucumis melo using a genetically characterized isolate. Pseudoperonospora cubensis, causal organism of cucurbit downy mildew (CDM), is one of the largest threats to cucurbit production in the eastern USA. Currently, no Cucumis melo (melon) cultivars have significant levels of resistance. Additionally, little is understood about the genetic basis of resistance in C. melo. Recombinant inbred lines (RILs; N = 169) generated from a cross between the resistant melon breeding line MR-1 and susceptible cultivar Ananas Yok'neam were phenotyped for CDM resistance in both greenhouse and growth chamber studies. A high-density genetic linkage map with 5,663 binned SNPs created from the RIL population was utilized for QTL mapping. Nine QTLs, including two major QTLs, were associated with CDM resistance. Of the major QTLs, qPcub-10.1 was stable across growth chamber and greenhouse tests, whereas qPcub-8.2 was detected only in growth chamber tests. qPcub-10.1 co-located with an MLO-like protein coding gene, which has been shown to confer resistance to powdery mildew and Phytophthora in other plants. This is the first screening of C. melo germplasm with a genetically characterized P. cubensis isolate.

High-Density GBS-Based Genetic Linkage Map Construction and QTL Identification Associated With Yellow Mosaic Disease Resistance in Bitter Gourd (Momordica charantia L.)

Yellow mosaic disease (YMD) in bitter gourd (Momordica charantia) is a devastating disease that seriously affects its yield. Although there is currently no effective method to control the disease, breeding of resistant varieties is the most effective and economic option. Moreover, quantitative trait locus (QTL) associated with resistance to YMD has not yet been reported. With the objective of mapping YMD resistance in bitter gourd, the susceptible parent "Punjab-14" and the resistant parent "PAUBG-6" were crossed to obtain F₄ mapping population comprising 101 individuals. In the present study, the genotyping by sequencing (GBS) approach was used to develop the genetic linkage map. The map contained 3,144 single nucleotide polymorphism (SNP) markers, consisted of 15 linkage groups, and it spanned 2415.2 cM with an average marker distance of 0.7 cM. By adopting the artificial and field inoculation techniques, F_4:5 individuals were phenotyped for disease resistance in Nethouse (2019), Rainy (2019), and Spring season (2020). The QTL analysis using the genetic map and phenotyping data identified three QTLs qYMD.pau_3.1, qYMD.pau_4.1, and qYMD.pau_5.1 on chromosome 3, 4, and 5 respectively. Among these, qYMD.pau_3.1, qYMD.pau_4.1 QTLs were identified during the rainy season, explaining the 13.5 and 21.6% phenotypic variance respectively, whereas, during the spring season, qYMD.pau_4.1 and qYMD.pau_5.1 QTLs were observed with 17.5 and 22.1% phenotypic variance respectively. Only one QTL qYMD.pau_5.1 was identified for disease resistance under nethouse conditions with 15.6% phenotypic variance. To our knowledge, this is the first report on the identification of QTLs associated with YMD resistance in bitter gourd using SNP markers. The information generated in this study is very useful in the future for fine-mapping and marker-assisted selection for disease resistance.

Genome-wide identification, characterization, and expression analysis related to autotoxicity of the GST gene family in Cucumis melo L

Glutathione S-transferase (GST) plays an important role in plant resistance to biotic and abiotic stresses. In this paper, the characteristics of melon GST gene family members were analyzed from a genome-wide perspective. Forty-nine GSTs were identified in melon genome, belonging to eight classes. Through the phylogenetic analysis of GST proteins in melon and other plants, it was found that members from the same subfamily in different species clustered together, indicating that the subfamilies of GST have diversified before the divergence within these species. The results of chromosome mapping showed that GSTs were present in all chromosomes except for chromosome 5. Gene replication events played an important role in the expansion and evolution of melon GST gene family. Ten GSTs with significant differential expression were screened in the transcriptome database related to melon autotoxicity stress. The differential expression of these 10 GSTs was detected in roots and leaves of melon seedlings treated with cinnamic acid. The relative expression level of CmGSTU7, CmGSTU10, CmGSTU18, CmGSTF2 and CmGSTL1 in roots of melon seedlings was significantly higher than that in control group. It suggested that the five GSTs might play an important role in cinnamic acid mediated autotoxicity stress in melon. The results of this paper were helpful to reveal the evolution and functional succession of GST family and further understand the response of GST to autotoxicity stress in melon.

Fine-mapping of a major QTL (Fwr1) for fusarium wilt resistance in radish

A major radish QTL (Fwr1) for fusarium wilt resistance was fine-mapped. Sequence and expression analyses suggest that a gene encoding a serine/arginine-rich protein kinase is a candidate gene for Fwr1. Fusarium wilt resistance locus 1 (Fwr1) is a major quantitative trait locus (QTL) mediating the resistance of radish inbred line 'B2' to Fusarium oxysporum, which is responsible for fusarium wilt. We previously detected Fwr1 on radish linkage group 3 (i.e., chromosome 5). In this study, a high-resolution genetic map of the Fwr1 locus was constructed by analyzing 354 recombinant F₂ plants derived from a cross between 'B2' and '835', the latter of which is susceptible to fusarium wilt. The Fwr1 QTL was fine-mapped to a 139.8-kb region between markers FM82 and FM87 in the middle part of chromosome 5. Fifteen candidate genes were predicted in this region based on a sequence comparison with the 'WK10039' radish reference genome. Additionally, we examined the time-course expression patterns of these predicted genes following an infection by the fusarium wilt pathogen. The ORF4 expression level was significantly higher in the resistant 'B2' plants than in the susceptible '835' plants. The ORF4 sequence was predicted to encode a serine/arginine-rich protein kinase and includes SNPs that result in nonsynonymous mutations, which may have important functional consequences. This study reveals a novel gene responsible for fusarium wilt resistance in radish. Further analyses of this gene may elucidate the molecular mechanisms underlying the fusarium wilt resistance of plants.

QTL Mapping and Marker Development for Tolerance to Sulfur Phytotoxicity in Melon (Cucumis melo)

Elemental sulfur is an effective, inexpensive fungicide for many foliar pathogens, but severe phytotoxicity prohibits its use on many melon varieties. Sulfur phytotoxicity causes chlorosis and necrosis of leaf tissue, leading to plant death in the most sensitive lines, while other varieties have little to no damage. A high-density, genotyping-by-sequencing (GBS)-based genetic map of a recombinant inbred line (RIL) population segregating for sulfur tolerance was used for a quantitative trait loci (QTL) mapping study of sulfur phytotoxicity in melon. One major (qSulf-1) and two minor (qSulf-8 and qSulf-12) QTL were associated with sulfur tolerance in the population. The development of Kompetitive Allele-Specific PCR (KASP) markers developed across qSulf-1 decreased the QTL interval from 239 kb (cotyledons) and 157 kb (leaves) to 97 kb (both tissues). The markers were validated for linkage to sulfur tolerance in a set of melon cultivars. These KASP markers can be incorporated into melon breeding programs for introgression of sulfur tolerance into elite melon germplasm.

Revealing Distinctions in Genetic Diversity and Adaptive Evolution Between Two Varieties of Camellia sinensis by Whole-Genome Resequencing

Camellia sinensis var. sinensis (CSS) and C. sinensis var. assamica (CSA) are the two most economically important tea varieties. They have different characteristics and geographical distribution. Their genetic diversity and differentiation are unclear. Here, we identified 18,903,625 single nucleotide polymorphisms (SNPs) and 7,314,133 insertion-deletion mutations (indels) by whole-genome resequencing of 30 cultivated and three wild related species. Population structure and phylogenetic tree analyses divided the cultivated accessions into CSS and CSA containing 6,440,419 and 6,176,510 unique variations, respectively. The CSS subgroup possessed higher genetic diversity and was enriched for rare alleles. The CSA subgroup had more non-synonymous mutations and might have experienced a greater degree of balancing selection. The evolution rate (dN/dS) and KEGG enrichment indicated that genes involved in the synthesis and metabolism of flavor substances were positively selected in both CSS and CSA subpopulations. However, there are extensive genome differentiation regions (2959 bins and approximately 148 M in size) between the two subgroups. Compared with CSA (141 selected regions containing 124 genes), the CSS subgroup (830 selected regions containing 687 genes) displayed more selection regions potentially related to environmental adaptability. Fifty-three pairs of polymorphic indel markers were developed. Some markers were located in hormone-related genes with distinct alleles in the two cultivated subgroups. These identified variations and selected regions provide clues for the differentiation and adaptive evolution of tea varieties. The newly developed indel markers will be valuable in further genetic research on tea plants.