Melon is as an alternative model to understand fruit ripening due to the coexistence of climacteric and non-climacteric varieties within the same species, allowing the study of the processes that regulate this complex trait with genetic approaches. We phenotyped a population of recombinant inbred lines (RILs), obtained by crossing a climacteric (Védrantais, cantalupensis type) and a non-climcteric variety (Piel de Sapo T111, inodorus type), for traits related to climacteric maturation and ethylene production. Individuals in the RIL population exhibited various combinations of phenotypes that differed in the amount of ethylene produced, the early onset of ethylene production, and other phenotypes associated with ripening. We characterized a major QTL on chromosome 8, ETHQV8.1, which is sufficient to activate climacteric ripening, and other minor QTLs that may modulate the climacteric response. The ETHQV8.1 allele was validated by using two reciprocal introgression line populations generated by crossing Védrantais and Piel de Sapo and analyzing the ETHQV8.1 region in each of the genetic backgrounds. A Genome-wide association study (GWAS) using 211 accessions of the ssp. melo further identified two regions on chromosome 8 associated with the production of aromas, one of these regions overlapping with the 154.1 kb interval containing ETHQV8.1. The ETHQV8.1 region contains several candidate genes that may be related to fruit ripening. This work sheds light into the regulation mechanisms of a complex trait such as fruit ripening.

Introgression lines are valuable germplasm for scientists and breeders, since they ease genetic studies such as QTL interactions and positional cloning as well as the introduction of favorable alleles into elite varieties. We developed a novel introgression line collection in melon using two commercial European varieties with different ripening behavior, the climacteric cantalupensis ‘Védrantais’ as recurrent parent and the non-climacteric inodorus ‘Piel de Sapo’ as donor parent. The collection contains 34 introgression lines, covering 99% of the donor genome. The mean introgression size is 18.16 Mb and ~ 3 lines were obtained per chromosome, on average. The high segregation of these lines for multiple fruit quality traits allowed us to identify 27 QTLs that modified sugar content, altered fruit morphology or were involved in climacteric ripening. In addition, we confirmed the genomic location of five major genes previously described, which control mainly fruit appearance, such as mottled rind and external color. Most of the QTLs had been reported before in other populations sharing parental lines, while three QTLs (EAROQP11.3, ECDQP11.2 and FIRQP4.1) were newly detected in our work. These introgression lines would be useful to perform additional genetic studies, as fine mapping and gene pyramiding, especially for important complex traits such as fruit weight and climacteric ripening.

Melon is an economically important crop with widely diverse fruit morphology and ripening characteristics. Its diploid sequenced genome and multiple genomic tools make this species suitable to study the genetic architecture of fruit traits. With the development of this introgression line population of the elite varieties ‘Piel de Sapo’ and ‘Védrantais’, we present a powerful tool to study fruit morphology and ripening traits that can also facilitate characterization or pyramidation of QTLs in inodorous melon types. The population consists of 36 lines covering almost 98% of the melon genome, with an average of three introgressions per chromosome and segregating for multiple fruit traits: morphology, ripening and quality. High variability in fruit morphology was found within the population, with 24 QTLs affecting six different traits, confirming previously reported QTLs and two newly detected QTLs, FLQW5.1 and FWQW7.1. We detected 20 QTLs affecting fruit ripening traits, six of them reported for the first time, two affecting the timing of yellowing of the rind (EYELLQW1.1 and EYELLQW8.1) and four at the end of chromosome 8 affecting aroma, abscission and harvest date (EAROQW8.3, EALFQW8.3, ABSQW8.3 and HARQW8.3). We also confirmed the location of several QTLs, such as fruit-quality-related QTLs affecting rind and flesh appearance and flesh firmness.

Melon (Cucumis melo L.) has emerged as an alternative model to study fruit ripening due to the coexistence of climacteric and non-climacteric varieties. The previous characterization of a major QTL ETHQV8.1 sufficient to trigger climacteric ripening in a non-climacteric background allowed the identification within the QTL interval of a negative regulator of ripening CmCTR1-like (MELO3C024518), and a putative DNA demethylase CmROS1 (MELO3C024516), the orthologue of DML2, a DNA demethylase regulating fruit ripening in tomato. To understand the role of these genes in climacteric ripening, we generated homozygous CRISPR knockout mutants of CmCTR1-like and CmROS1 in a climacteric genetic background. The climacteric behavior was altered in both loss-of-function mutants in two summer seasons with an advanced ethylene production profile compared to the climacteric wild type, suggesting a role of both genes in climacteric ripening in melon. Single cytosine methylome analyses of the CmROS1 knockout mutant revealed DNA methylation changes in the promoter regions of key ripening genes as ACS1, ETR1 and ACO1, and ripening associated-transcription factors as NAC-NOR, RIN and CNR, suggesting the importance of CmROS1-mediated DNA demethylation for triggering fruit ripening in melon.

Fruit ripening is one of the main processes affecting fruit quality and shelf-life. In melon there are both climacteric and non-climacteric genotypes, making it a suitable species to study fruit ripening. In the current study, in order to fine tune ripening, we have pyramided three climacteric QTLs in the non-climacteric genotype “Piel de Sapo”: ETHQB3.5, ETHQV6.3 and ETHQV8.1. The results showed that the three QTLs interact epistatically, affecting ethylene production and ripening-related traits such as aroma profile. Each individual QTL has a specific role in the ethylene production profile. ETHQB3.5 accelerates the ethylene peak, ETHQV6.3 advances the ethylene production and ETHQV8.1 enhances the effect of the other two QTLs. Regarding aroma, the three QTLs independently activated the production of esters changing the aroma profile of the fruits, with no significant effects in fruit firmness, soluble solid content and fruit size. Understanding the interaction and the effect of different ripening QTLs offers a powerful knowledge for candidate gene identification as well as for melon breeding programs, where fruit ripening is one of the main objectives.