The development of linkage maps with large numbers of molecular markers has stimulated the search for methods to map genes involved in quantitative traits (QTLs). A promising method, proposed by Lander and Botstein (1989), employs pairs of neighbouring markers to obtain maximum linkage information about the presence of a QTL within the enclosed chromosomal segment. In this paper the accuracy of this method was investigated by computer simulation. The results show that there is a reasonable probability of detecting QTLs that explain at least 5% of the total variance. For this purpose a minimum population of 200 backcross or F2 individuals is necessary. Both the number of individuals and the relative size of the genotypic effect of the QTL are important factors determining the mapping precision. On the average, a QTL with 5% or 10% explained variance is mapped on an interval of 40 or 20 centiMorgans, respectively. Of course, QTLs with a larger genotypic effect will be located more precisely. It must be noted, however, that the interval length is rather variable.

Accuracy of mapping quantitative trait loci in autogamous species.

Cucumber, Cucumis sativus L. (2n = 2x = 14), is an important vegetable crop worldwide. It was the first specialty crop with a publicly available draft genome. Its relatively small, diploid genome, short life cycle, and self-compatible mating system offers advantages for genetic studies. In recent years, significant progress has been made in molecular mapping, and identification of genes and QTL responsible for key phenotypic traits, but a systematic review of the work is lacking. Here, we conducted an extensive literature review on mutants, genes and QTL that have been molecularly mapped or characterized in cucumber. We documented 81 simply inherited trait genes or major-effect QTL that have been cloned or fine mapped. For each gene, detailed information was compiled including chromosome locations, allelic variants and associated polymorphisms, predicted functions, and diagnostic markers that could be used for marker-assisted selection in cucumber breeding. We also documented 322 QTL for 42 quantitative traits, including 109 for disease resistances against seven pathogens. By alignment of these QTL on the latest version of cucumber draft genomes, consensus QTL across multiple studies were inferred, which provided insights into heritable correlations among different traits. Through collaborative efforts among public and private cucumber researchers, we identified 130 quantitative traits and developed a set of recommendations for QTL nomenclature in cucumber. This is the first attempt to systematically summarize, analyze and inventory cucumber mutants, cloned or mapped genes and QTL, which should be a useful resource for the cucurbit research community.

/pdf/molecularly-tagged-genes-and-quantitative-trait-loci-in-45njzidd75.pdf

Molecularly tagged genes and quantitative trait loci in cucumber with recommendations for QTL nomenclature

Heterosis has historically been exploited in plants; however, its underlying genetic mechanisms and molecular basis remain elusive. In recent years, due to advances in molecular biotechnology at the genome, transcriptome, proteome, and epigenome levels, the study of heterosis in vegetables has made significant progress. Here, we present an extensive literature review on the genetic and epigenetic regulation of heterosis in vegetables. We summarize six hypotheses to explain the mechanism by which genes regulate heterosis, improve upon a possible model of heterosis that is triggered by epigenetics, and analyze previous studies on quantitative trait locus effects and gene actions related to heterosis based on analyses of differential gene expression in vegetables. We also discuss the contributions of yield-related traits, including flower, fruit, and plant architecture traits, during heterosis development in vegetables (e.g., cabbage, cucumber, and tomato). More importantly, we propose a comprehensive breeding strategy based on heterosis studies in vegetables and crop plants. The description of the strategy details how to obtain F1 hybrids that exhibit heterosis based on heterosis prediction, how to obtain elite lines based on molecular biotechnology, and how to maintain heterosis by diploid seed breeding and the selection of hybrid simulation lines that are suitable for heterosis research and utilization in vegetables. Finally, we briefly provide suggestions and perspectives on the role of heterosis in the future of vegetable breeding.

/pdf/molecular-basis-of-heterosis-and-related-breeding-strategies-h0v63tvuvk.pdf

Molecular basis of heterosis and related breeding strategies reveal its importance in vegetable breeding.

Cucumber (<italic>Cucumis sativus</italic> L.), an important vegetable, popular worldwide for its crispy texture and special flavor. Plant hormones such as auxin stands out for its dominating function in morpho- and organogenic processes, formation of organs as well as regulation of tropic responses. These developmental processes are entirely or partially dependent on auxin biosynthesis, transport, and signal transduction. In cucumber, auxin not only fine-tunes its morphogenesis but also response to environmental stress. The role of auxin in regulating different organs (root, hypocotyl, shoot, leaf, tendril, flower, and fruit) development in cucumber was reviewed in the present paper. Moreover, the role of auxin in cucumber response to biotic stresses (powdery mildew, downy mildew, and nematode infections) and abiotic stresses (heat, iron, waterlogging, cold, salinity, drought, and heavy metal stresses) was discussed. Finally, we pointed out the blind spots and future research thoughts to extend our understanding of this myriad molecule in cucumber growth and stress biology. 

Involvement of auxin in growth and stress response of cucumber

Vegetables are an integral part of the human diet worldwide. Traditional breeding approaches have been used extensively to develop new cultivars of vegetables with desirable characteristics, includ...

Genomics and Marker-Assisted Improvement of Vegetable Crops

Green flesh color, resulting from the accumulation of chlorophyll, is one of the most important commercial traits for the fruits. The genetic network regulating green flesh formation has been studied in tomato, melon and watermelon. However, little is known about the inheritance and molecular basis of green flesh in cucumber. This study sought to determine the main genomic regions associated with green flesh. Three F2 and two BC1 populations derived from the 9110Gt (cultivated cucumber, green flesh color) and PI183967 (wild cucumber, white flesh color) were used for the green flesh genetic analysis. Two F2 populations of them were further employed to do the map construction and quantitative trait loci (QTL) study. Also, a core cucumber germplasms population was used to do the GWAS analysis. We identified three indexes, flesh color (FC), flesh extract color (FEC) and flesh chlorophyll content (FCC) in three environments. Genetic analysis indicated that green flesh color in 9110Gt is controlled by a major-effect QTL. We developed two genetic maps with 192 and 174 microsatellite markers respectively. Two novel inversions in Chr1 were identified between cultivated and wild cucumbers. The major-effect QTL, qgf5.1, was identified using FC, FEC and FCC index in all different environments used. In addition, the same qgf5.1, together with qgf3.1, was identified via GWAS. Further investigation of two candidate regions using pairwise LD correlations, combined with genetic diversity of qgf5.1 in natural populations, it was found that Csa5G021320 is the candidate gene of qgf5.1. Geographical distribution revealed that green flesh color formation could be due to the high latitude, which has longer day time to produce the photosynthesis and chlorophyll synthesis during cucumber domestication and evolution. We first reported the cucumber green flesh color is a quantitative trait. We detected two novel loci qgf5.1 and qgf3.1, which regulate the green flesh formation in cucumber. The QTL mapping and GWAS approaches identified several candidate genes for further validation using functional genomics or forward genetics approaches. Findings from the present study provide a new insight into the genetic control of green flesh in cucumber.

/pdf/qtl-mapping-and-genome-wide-association-study-reveal-two-164af6v3vv.pdf

QTL mapping and genome-wide association study reveal two novel loci associated with green flesh color in cucumber

Cucumber (Cucumis sativus L.) is an economically important vegetable crop worldwide, but it is sensitive to low temperatures. Cucumber seedlings exposed to long-term low temperature stress (LT), i.e., below 20°C during the day, and 8°C at night, exhibit leaf yellowing, accelerated senescence, and reduced yield, therefore posing a threat to cucumber production. Studying the underlying mechanisms involved in LT tolerance in cucumber seedlings, and developing germplasm with improved LT-tolerance could provide fundamental solutions to the problem. In this study, an F2 population was generated from two parental lines, "CG104" (LT-tolerant inbred line) and "CG37" (LT-sensitive inbred line), to identify loci that are responsible for LT tolerance in cucumber seedlings. Replicated phenotypic analysis of the F2-derived F3 family using a low-temperature injury index (LTII) suggested that the LT tolerance of cucumber seedlings is controlled by multiple genes. A genetic map of 990.8 cM was constructed, with an average interval between markers of 5.2 cM. One quantitative trait loci (QTL) named qLTT5.1 on chromosome 5, and two QTLs named qLTT6.1 and qLTT6.2 on chromosome 6 were detected. Among them, qLTT6.2 accounted for 26.8 and 24.1% of the phenotypic variation in two different experiments. Single-nucleotide polymorphism (SNP) variations within the region of qLTT6.2 were analyzed using two contrasting in silico bulks generated from the cucumber core germplasm. Result showed that 214 SNPs were distributed within the 42-kb interval, containing three candidate genes. Real-time quantitative reverse transcription PCR and sequence analysis suggested that two genes Csa6G445210, an auxin response factor, and Csa6G445230, an ethylene-responsive transmembrane protein, might be candidate genes responsible for LT tolerance in cucumber seedlings. This study furthers the understanding of the molecular mechanism underlying LT tolerance in cucumber seedlings, and provides new markers for molecular breeding.

/pdf/quantitative-trait-loci-mapping-and-candidate-gene-analysis-2oxr9bpe5n.pdf

Quantitative Trait Loci Mapping and Candidate Gene Analysis of Low Temperature Tolerance in Cucumber Seedlings.

Heat stress during cucumber production often leads to sunburn of leaves, growth retardation of stems and roots, fruit malformation, and even plant death, which have a great impact on the fruit quality and yield. However, no studies on the genetic inheritance and quantitative trait locus mapping of heat tolerance in cucumber at the adult stage have been reported yet. In this study, a set of 86 recombinant inbred lines (RILs) derived from “99281” (heat-tolerant) and “931” (heat-sensitive) were used to identify the heat tolerance QTL in summer 2018, 2019, and 2020. Eight-week-old plants were exposed to a natural high temperature environment in the field, and the heat injury index was used to indicate the heat tolerance performance. Genetic analysis showed that the heat tolerance of adult cucumber is quantitatively inherited. One QTL named qHT1.1 on chromosome 1 was identified. It was delimited by Indel 3-3 and Indel 1-15 and explained 59.6%, 58.1%, and 40.1% of the phenotypic variation in 2018, 2019, and 2020, respectively. The efficiency of marker HT-1, which is closely linked to the locus, was tested using 62 cucumber germplasm accessions and was found to have an accuracy of 97.8% in heat sensitive plants. The qHT1.1 was delimited to a 694.5-kb region, containing 98 genes, nine of which may be involved in heat tolerance. Further sequence analysis showed that there are three single-base substitutions within the coding sequences of Csa1G004990. Gene expression analyses suggested that the expression of Csa1G004990 was significantly higher in “99281” than “931” at 14d, 35d, 42d, and 49d after transplanting. This study provides practically useful markers for heat tolerance breeding in cucumber and provides a basis for further identifying heat tolerant genes.

https://www.mdpi.com/2223-7747/10/2/324/pdf

QTL Mapping of Heat Tolerance in Cucumber (Cucumis sativus L.) at Adult Stage.

Quantitative Trait Loci (QTL) analysis of multiple populations in multiple environments revealed that the fsd6.2 locus, which includes the candidate gene Csgl3, controls high fruit spine density in natural cucumbers. GWAS identified a novel locus fsd6.1, which regulates ultra-high fruit spine density in combination with Csgl3, and evolved during cucumber domestication. Fruit spine density, a domestication trait, largely influences the commercial value of cucumbers. However, the molecular basis of fruit spine density in cucumber remains unclear. In this study, four populations were derived from five materials, which included three with low fruit spine density, one with high fruit spine density, and one with ultra-high fruit spine density. Fruit spine densities were measured in 15 environments over a span of 6 years. The distributions were bimodal suggesting that fruit spine density is controlled by a major-effect QTL. QTL analysis determined that the same major-effect QTL, fsd6.2, is present in four populations. Fine mapping indicated that Csgl3 is the candidate gene at the fsd6.2 locus. Phylogenetic and geographical distribution analyses revealed that Csgl3 originated from China, which has the highest genetic diversity for fruit spine density. One novel minor-effect QTL, fsd6.1, was detected in the HR and HP populations derived from the cross between 65G and 02245. In addition, GWAS identified a novel locus that colocates with fsd6.1. Inspection of a candidate region of about 18 kb in size using pairwise LD correlations, combined with genetic diversity and phylogenetic analysis of fsd6.1 in natural populations, indicated that Csa6G421750 is the candidate gene responsible for ultra-high fruit spine density in cucumber. This study provides new insights into the origin of fruit spine density and the evolution of high/ultra-high fruit spine density during cucumber domestication.

Novel loci fsd6.1 and Csgl3 regulate ultra-high fruit spine density in cucumber.

Identification of QTLs controlling low-temperature tolerance during the germination stage in cucumber (Cucumis sativus L.)

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