Identification of Fruit Firmness QTL ff2.1 by SLAF-BSA and QTL Mapping in Melon

Fruit firmness is an important target of melon breeding, as it is associated with shelf life and economic value; however, the precise mechanism determining fruit firmness during fruit ripening remains elusive. In the present study, one hundred forty-four F2 plants and F2-3 families derived from the high-firmness melon line M2-10 and the low-firmness melon line ZT091 were used to identify major quantitative trait loci (QTLs) by specific-locus amplified fragment (SLAF) sequencing with bulked segregant analysis (BSA). Simple sequence repeat (SSR) and cleaved amplified polymorphic sequence (Caps) markers based on the resequencing of parental lines were also used to narrow the associated region to identify candidate genes. Two regions associated with fruit firmness were investigated, including a 4.87 Mb region on chr. 2 and a 28.7 Mb region on chr. 5 of the melon genome. SSR and Caps markers were used to construct a genetic map of the associated regions: QTL ff5.1 was located between CmSSR13509 and CmSSR13423 and explained 38.44% of the observed variation, with an LOD threshold of 17.44; ff2.1 was located between CmSSR07709 and SNP22228 and explained 28.14% of the variation, with an LOD threshold of 3.8, and this region included 106 Kb and 10 candidate genes. Quantitative Real-time PCR (qRT-PCR) was used to investigated the investigate candidate gene expression at 15, 20 and 25 days after pollination (DAP) in the parental lines, and significant expression levels were detected for most of the genes, including four genes of unknown function and MELO3C017519, MELO3C017520, MELO3C017522, MELO3C029506, and MELO3C029520. These results revealed a new QTL, ff2.1, for melon fruit firmness-related gene identification.

A chromosome-level genome of a Kordofan melon illuminates the origin of domesticated watermelons

Wild relatives or progenitors of crops are important resources for breeding and for understanding domestication. Identifying them, however, is difficult because of extinction, hybridization, and the challenge of distinguishing them from feral forms. Here, we use collection-based systematics, iconography, and resequenced accessions of Citrullus lanatus and other species of Citrullus to search for the potential progenitor of the domesticated watermelon. A Sudanese form with nonbitter whitish pulp, known as the Kordofan melon (C. lanatus subsp. cordophanus), appears to be the closest relative of domesticated watermelons and a possible progenitor, consistent with newly interpreted Egyptian tomb paintings that suggest that the watermelon may have been consumed in the Nile Valley as a dessert by 4360 BP. To gain insights into the genetic changes that occurred from the progenitor to the domesticated watermelon, we assembled and annotated the genome of a Kordofan melon at the chromosome level, using a combination of Pacific Biosciences and Illumina sequencing as well as Hi-C mapping technologies. The genetic signature of bitterness loss is present in the Kordofan melon genome, but the red fruit flesh color only became fixed in the domesticated watermelon. We detected 15,824 genome structural variants (SVs) between the Kordofan melon and a typical modern cultivar, “97103,” and mapping the SVs in over 400 Citrullus accessions revealed shifts in allelic frequencies, suggesting that fruit sweetness has gradually increased over the course of watermelon domestication. That a likely progenitor of the watermelon still exists in Sudan has implications for targeted modern breeding efforts.

PCR-Based InDel Marker Associated with Powdery Mildew-Resistant MR-1

Powdery mildew (PM) is a fungal disease occurring in both field and greenhouse conditions worldwide. It infects many plant species and reduces both the productivity and quality of crops. Melon (Cucumis melo L.) is an economically important crop. In order to develop a molecular marker that can be used more conveniently in the development of PM-resistant melon using MR-1 melon resources, the previously reported cleaved amplified polymorphic sequence (CAPS) marker was improved with a length polymorphism PCR marker. Two cleaved CAPS markers—BSA12-LI3ECORI and BSA12-LI4HINFI—associated with BPm12.1, a major quantitative trait locus (QTL) corresponding to the PM resistance of MR-1, have been reported. In this study, we found that in the BSA12-LI3ECORI CAPS marker specifically, a 41 bp deletion was present in the PCR DNA region of the MR-1 melon genome. A new marker capable of distinguishing polymerase chain reaction (PCR) length polymorphism was produced using insertion-deletion (InDel) information in this region. This PCR-based InDel marker distinguished the genotypes of PM-resistant MR-1, PM-susceptible Top Mark, and their F1 progeny. These results suggest that this InDel marker could be used to develop PM-resistant melon varieties based on MR-1.

Melon Genome Regions Associated with TGR-1551-Derived Resistance to Cucurbit yellow stunting disorder virus

Cucurbit yellow stunting disorder virus (CYSDV) is one of the main limiting factors of melon cultivation worldwide. To date, no commercial melon cultivars resistant to CYSDV are available. The African accession TGR-1551 is resistant to CYSDV. Two major quantitative trait loci (QTLs) have been previously reported, both located near each other in chromosome 5. With the objective of further mapping the gene or genes responsible of the resistance, a recombinant inbred line (RIL) population derived from the cross between TGR-1551 and the susceptible cultivar ‘Bola de Oro’ was evaluated for resistance to CYSDV in five different assays and genotyped in a genotyping by sequencing (GBS) analysis. The major effect of one of the two QTLs located on chromosome 5 was confirmed in the multienvironment RIL assay and additionally verified through the analysis of three segregating BC1S1 populations derived from three resistant RILs. Furthermore, progeny test using the offspring of selected BC3 plants allowed the narrowing of the candidate interval to a 700 kb region. The SNP markers identified in this work will be useful in marker-assisted selection in the context of introgression of CYSDV resistance in elite cultivars.

Comparative genomics of muskmelon reveals a potential role for retrotransposons in the modification of gene expression

Melon exhibits substantial natural variation especially in fruit ripening physiology, including both climacteric (ethylene-producing) and non-climacteric types. However, genomic mechanisms underlying such variation are not yet fully understood. Here, we report an Oxford Nanopore-based high-grade genome reference in the semi-climacteric cultivar Harukei-3 (378 Mb + 33,829 protein-coding genes), with an update of tissue-wide RNA-seq atlas in the Melonet-DB database. Comparison between Harukei-3 and DHL92, the first published melon genome, enabled identification of 24,758 one-to-one orthologue gene pairs, whereas others were candidates of copy number variation or presence/absence polymorphisms (PAPs). Further comparison based on 10 melon genome assemblies identified genome-wide PAPs of 415 retrotransposon Gag-like sequences. Of these, 160 showed fruit ripening-inducible expression, with 59.4% of the neighboring genes showing similar expression patterns (r > 0.8). Our results suggest that retrotransposons contributed to the modification of gene expression during diversification of melon genomes, and may affect fruit ripening-inducible gene expression.