Melon Lines Research Articles

Screening of melon genotypes identifies gummy stem blight resistance associated with Gsb1 resistant loci

Md Zahid Hassan, Arif Hasan Khan Robin, Md Abdur Rahim, Sathishkumar Natarajan, Hoy-Taek Kim, Jong-In Park, and Ill-Sup Nou. J Plant Biotechnol 2018;45:217-27. https://doi.org/10.5010/JPB.2018.45.3.217

A915.34.01.08 - melon line resistant to leafminer (Liriomyza sativae)

Abstract Line A915.34.01.08 is a new source of resistance to Liriomyza sativae in melon. It was obtained by the genealogical method from a heterozygous plant with antibiosis type of resistance found within the accession CNPH 00-915. This line allows making introgression of this resistance in melon lines to develop commercial hybrids.

QTL mapping for melon ( Cucumis melo L.) fruit traits by assembling and utilization of novel SNPs based CAPS markers

Abstract Fruit traits influence consumer’s choice and the market worth of melons. Multiple genes controlled the majority of fruit quality traits and these genes are influenced via differential factors. Cleaved amplified polymorphism sequence (CAPS) molecular markers have been validated as efficient tool for detection of numerous SNPs (single nucleotide polymorphisms). In the current study, genetic linkage map was assembled and QTL discovery was conducted for melon quality traits in F2:3 population, generated from crossing of MR-1 and M4-7. SNPs were detected and converted into CAPS markers from retrieved data of high-throughput resequencing. Two parental lines (MR-1 and M4-7) of melon were analyzed and resequenced for assembling the novel CAPS markers. Totally, 91.54% and 90.00% of assembled sequences covered the reference genome of these two distinct parental lines, respectively. Analysis of assembled genomic data revealed total 2,761,801 SNPs and 7741 CAPS locus. Overall, 410 novels CAPS molecular markers were assembled along with 4-restriction endonuclease and 160 CAPS pairs displayed polymorphic bands of predictable length after screening by the use of PCR and process of enzyme digestion, with 39% rate of polymorphism. Genetic linkage map construction was done by genotyping of these novel CAPS markers within F2:3 population and this map totally spanned 2848.68 cM length with 12 chromosomes and 160 CAPS markers. Total 24 minor QTLs were discovered for fruit weight, firmness, flesh firmness, Brix, fruit length, width and shape with different genetic loci on various chromosomal locations. These newly assembled CAPS markers will be valuable and potent for future breeding and genetic schemes in melon.

Impact of Vat resistance in melon on viral epidemics and genetic structure of virus populations

Cultivar choice is at the heart of cropping systems and resistant cultivars should be at the heart of disease management strategies whenever available. They are the easiest, most efficient and environmentally friendly way of combating viral diseases at the farm level. Among the melon genetic resources, Vat is a unique gene conferring resistance to both the melon aphid Aphis gossypii and the viruses it carries. The ‘virus side’ of this pleiotropic phenotype is seldom regarded as an asset for virus control. Indeed, the effect of Vat on virus epidemics in the field is expected to vary according to the composition of aphid populations in the environment and long-term studies are needed to draw a correct trend. Therefore, the first objective of the study was to re-evaluate the potential of Vat to reduce viral diseases in melon crops. The second objective was to investigate the potential of Vat to exert a selection pressure on virus populations. We monitored the epidemics of Cucurbit aphid-borne yellows virus (CABYV), Cucumber mosaic virus (CMV), Watermelon mosaic virus (WMV) and Zucchini yellow mosaic virus (ZYMV) in two melon lines having a common genetic background, a resistant line (R) and a susceptible line (S), in eight field trials conducted in southeastern France between 2011 and 2015. Vat had limited impact if any on WMV epidemics probably because A. gossypii is not the main vector of WMV in the field, but a favorable impact on CMV, yet of variable intensity probably related to the importance of A. gossypii in the total aphid population. Vat had a significant impact on CABYV epidemics with mean incidence reduction exceeding 50% in some trials. There was no effect of Vat on the structure of virus populations, both for the non-persistent WMV transmitted by numerous aphid species and for the persistent CABYV transmitted predominantly by A. gossypii.

Ultrastructure of compatible and incompatible interactions in phloem sieve elements during the stylet penetration by cotton aphids in melon.

Resistance of the melon line TGR-1551 to the aphid Aphis gossypii is based on preventing aphids from ingesting phloem sap. In electrical penetration graphs (EPGs), this resistance has been characterized with A. gossypii showing unusually long phloem salivation periods (waveform E1) mostly followed by pathway activities (waveform C) or if followed by phloem ingestion (waveform E2), ingestion was not sustained for more than 10 min. Stylectomy with aphids on susceptible and resistant plants was performed during EPG recording while the stylet tips were phloem inserted. This was followed by dissection of the penetrated leaf section, plant tissue fixation, resin embedding, and ultrathin sectioning for transmission electron microscopic observation in order to study the resistance mechanism in the TGR. The most obvious aspect appeared to be the coagulation of phloem proteins inside the stylet canals and the punctured sieve elements. Stylets of 5 aphids per genotype were amputated during sieve element (SE) salivation (E1) and SE ingestion (E2). Cross-sections of stylet bundles in susceptible melon plants showed that the contents of the stylet canals were totally clear and also, no coagulated phloem proteins occurred in their punctured sieve elements. In contrast, electron-dense coagulations were found in both locations in the resistant plants. Due to calcium binding, aphid saliva has been hypothesized to play an essential role in preventing/suppressing such coagulations that cause occlusion of sieves plate and in the food canal of the aphid's stylets. Doubts about this role of E1 salivation are discussed on the basis of our results.

Influence of temperature on powdery mildew development in TGR-1551

Several plants of 'TGR-1551', a Zimbabwean melon accession resistant to races 1, 2 and 5 of Podosphaera xanthii, and the susceptible Spanish cultivar 'Bola de Oro' were inoculated with powdery mildew race 2 at two different temperatures (18-22 and 25-30°C) in order to study the plant-pathogen interaction. Fungal development was evaluated at microscopic level through several parameters such as the number of penetrated points, detected by callose accumulation with the double aniline-blue-calcofluor staining technique, at 48, 72, 96 and 120 h post inoculation (hpi), as well as the number of conidiophores at 120 hpi stained with an ink/acetic acid solution. The melon accessions 'PMR 45' and 'WMR 29', susceptible and resistant to race 2 respectively, were used as control lines at 48 and 120 hpi. At low temperature, 'TGR-1551' showed a non-resistant phenotype since the number of penetration points increased with time, as it is also observed for susceptible lines. Likewise, a high number of conidiophores were detected, although in lower quantities than in the susceptible lines. At high temperature, however, most of fungal conidia deposited on 'TGR-1551' plants stopped its development at germ tube or primary hyphae stage, and no conidiophores were observed at 120 hpi. A similar behavior was recorded for 'WMR 29'. The susceptible melon lines, 'Bola de Oro' and 'PMR 45', showed a great fungal development at both temperatures, being the greatest values observed at high temperature. This environmental factor might be affecting significantly the resistance of several melon genotypes to powdery mildew.

Genome-Wide Single Nucleotide Polymorphism Discovery and the Construction of a High-Density Genetic Map for Melon (Cucumis melo L.) Using Genotyping-by-Sequencing.

Although genotyping-by-sequencing (GBS) enables the efficient and low-cost generation of large numbers of markers, the utility of resultant genotypes are limited, because they are enormously error-prone and contain high proportions of missing data. In this study, we generated single nucleotide polymorphism (SNP) markers for 109 recombinant inbred lines of melon (Cucumis melo L.) using the GBS approach and ordered them according to their physical position on the draft double haploid line DHL92 genome. Next, by investigating associations between these SNPs, we discovered that some segments on the physical map conflict with linkage relationships. Therefore, to filter out error-prone loci, 4,110 SNPs in which we have a high degree of confidence were selected as anchors to test independence with respect to unselected markers, and the resultant dataset was then analyzed using the Full-Sib Family Haplotype (FSFHap) algorithm in the software TASSEL 5.2. On the basis of this analysis, 22,933 loci that have an average rate of missing data of 0.281% were used to construct a genetic map, which spans 1,088.3 cM across 12 chromosomes and has a maximum spacing of 6.0 cM. Use of this high-quality linkage map enabled the identification of several quantitative trait loci (QTL) known to control traits in fruit and validated our approach. This study highlights the utility of GBS markers for the identification of trait-associated QTLs in melon and facilitates further investigation of genome structure.

Mapping Resistance to Alternaria cucumerina in Cucumis melo.

Infection with Alternaria cucumerina causes Alternaria leaf blight (ALB), a disease characterized by lesion formation on leaves, leading to substantial yield and quality losses in Cucumis melo (melon). Although fungicides are effective against ALB, reduction in the frequency of application would be economically and environmentally beneficial. Resistant melon lines have been identified but the genetic basis of this resistance has not been determined. A saturated melon genetic map was constructed with markers developed through genotyping by sequencing of a recombinant inbred line population (F6 to F10; n = 82) derived from single-seed descent of a F2 population from a cross between the ALB-resistant parent MR-1 and the ALB-susceptible parent Ananas Yokneum. The population was evaluated for A. cucumerina resistance with an augmented block greenhouse study using inoculation with the wounded-leaf method. Multiple quantitative trait loci (QTL) mapping identified two QTL that explained 33.9% of variation in lesion area. Several candidate genes within range of these QTL were identified using the C. melo v3.5 genome. Markers linked to these QTL will be used to accelerate efforts to breed melon cultivars resistant to ALB.

Inheritance and QTL mapping of glandular trichomes type I density in Cucumis melo L.

To determine the genetic basis of the density of glandular trichomes type I in melon, a six-generation family and a recombinant inbred lines (F7) population have been tested for the character; both genetic groups have been obtained from a cross between the Zimbabwean melon line TGR-1551 (high density of glandular trichomes) and the Spanish cultivar ‘Bola de Oro’ (low density of glandular trichomes). Classic quantitative genetic analysis indicated an oligogenic model for the inheritance of the character with an important additive component and strong epistatic relationships between loci. The quantitative trait locus (QTL) analyses revealed that the character is controlled by one major QTL, tric11, which explained between 23.8 and 58.7 % of its phenotypic variance; other minor QTLs of weaker effect and dependent of the assayed growing conditions were also found to be involved in the character expression. A codominant marker, the microsatellite ECM63, was found to be linked to the major QTL tric11. The validation of this marker in advanced backcrosses confirmed that the TGR-1551 alleles for this QTL increased glandular trichome density. The validation of the detected QTL was also done in other genetic backgrounds, which indicated that ECM63 could be effective in the selection of plants with high density of glandular trichomes type I.

Identification of a New Race and Development of DNA Markers Associated with Powdery Mildew in Melon

Powdery mildew disease caused by an obligatory parasitic fungus Podosphaera xanthii is a serious problem of melon (Cucumis melo L.) production worldwide. Severity of problem is further associated with emergence of new races over the years. In this study a new race of powdery mildew fungus was discovered from Ibaraki, Japan. The race was different from all other existing races of P. xanthii occurring in Japan. Phenotypic and genetic analysis established the new fungus type as a new race, N5. Ten melon lines were infected with a total of eight fungal races including the new N5 race and it was found that all melon lines had different disease reactions against the new race compared to other seven races. Only four melon genotypes were found resistant out of 42 commercial cultivars and lines were tested. Disease reactions of two sets of F2 populations and one set of backcross population revealed that two separate epistatic gene loci located in two different linkage groups (LG), LG II and LG XII, interact together for the resistant or susceptible reaction of melon lines. A total of six simple sequence repeat (SSR) markers were found polymorphic in melon lines out of 16 tested in response to N5 race. Two different sets of F2 populations between resistant and susceptible melon lines were assessed with two polymorphic SSR markers located in two different groups, LG II and LG XII. SSR genotyping yielded 78% and 94% expected polymerase chain reaction fragments in favor of resistance or susceptibility of F2 populations of CM17187×PMR5 and PMR45×PMR5 of melon lines, respectively.

멜론 흰가루병균 및 식물 호르몬 처리하에서 MLO 유전자군의 발현검정

Powdery mildew disease caused by Podosphaera xanthii is a major concern for Cucumis melo production worldwide. Knowledge on genetic behavior of the related genes and their modulating phytohormones often offer the most efficient approach to develop resistance against different diseases. Mildew Resistance Locus O (MLO) genes encode proteins with seven transmembrane domains that have significant function in plant resistance to powdery mildew fungus. We collected 14 MLO genes from ‘Melonomics’ database. Multiple sequence analysis of MLO proteins revealed the existence of both evolutionary conserved cysteine and proline residues. Moreover, natural genetic variation in conserved amino acids and their replacement by other amino acids are also observed. Real-time quantitative PCR expression analysis was conducted for the leaf samples of P. xanthii infected and phytohormones (methyl jasmonate and salicylic acid) treated plants in melon ‘SCNU1154’ line. Upon P. xanthii infection using 7 different races, the melon line showed variable disease reactions with respect to spread of infection symptoms and disease severity. Three out of 14 CmMLO genes were up-regulated and 7 were down-regulated in leaf samples in response to all races. The up- or down-regulation of the other 4 CmMLO genes was race-specific. The expression of 14 CmMLO genes under methyl jasmonate and salicylic acid application was also variable. Eleven CmMLO genes were up-regulated under salicylic acid treatment, and 7 were up-regulated under methyl jasmonate treatments in C. melo L. Taken together, these stress-responsive CmMLO genes might be useful resources for the development of powdery mildew disease resistant C. melo L.

Tracking of the gene Fom2 and study on the genetic diversity of NB-ARC domain in the number of resistant and sensitive melon cultivars against Fusarium oxysporum f. sp. melonis (race 1)

Vascular wilt caused by Fusarium oxysporum f. sp. melonis (FOM) is one of the most important diseases in melon. Four pathogenic races named: 0, 1, 2, and 1.2 have been identified in population of this fungus based on the pathogenicity pattern on differential varieties. The gene Fom2 has been reported as a key gene for conferring resistance to the fungus race 1. The majority of Iranian melon cultivars are susceptible to race 1. Clarifying the occurrence pattern of Fom2 in the fungus race 1 in sensitive and resistant Iranian melon cultivars in comparison with two standard melon lines could be helpful for the investigation on the role of Fom2 in the cultivars reaction. Since, the full sequence of Fom2 from Melon and I2 from Tomato resistance sources were cloned; it has been shown that the LRR and NB-ARC domains in the coded proteins play critical roles in detection of Avr proteins and R proteins activation as well. Considering to the importance of NB-ARC domain and the lack of functional information in this region of the gene Fom2, this domain was selected for sequencing, identifying possible mutations and to study their possible relationship of mutations with the type of cultivars reaction against fungal races. Six melon cultivars including four Iranian local melon cultivars (Mashhadi, Shahde Shiraz, Khatooni and Khaghani) and two standard melon cultivars (Charentais Fom1 and Charentais Fom2) were used in this study. Two specific primer pairs (PSh20-F/R and PSh20.2-F/R) were designed and used for the tracking of partial and full-length sequence of Fom2. More investigation was performed on the diversity occurrence pattern among cultivars and its conformity with cultivars resistive pattern against the fungus race 1. Our results showed that the partial and the full length sequence of Fom2 had this potential to be tracked in sensitive as well as resistant cultivars. On the other hand, the cultivars reactions were not predictable based on the gene occurrence pattern. However, some studies have already been reported that the linked markers of Fom2 were not useful for the prediction of reaction pattern in Iranian melon cultivars. The partial segments of Fom2 amplified by PSh20-F/R primers were bi-directionally sequenced and the results were submitted in gene bank. Sequence analysis of NB-ARC domain in the six varieties led to detection of 16 polymorphic nucleotide positions, which nine of them appeared in protein levels. Six out of the nine mutations placed in AAA-ATPase subdomain. Although the polymorphic pattern of mutations in nucleotide and amino acid positions were not in accordance with the cultivars resistive pattern but the cultivars could be discriminated based on some other mutations. Due to the switching function of AAA-ATPase sub domain in ATP hydrolysis and activation of R proteins, variation in this region between the resistant and susceptible cultivars could be considerable. It seems the presence of identified mutations at AAA-ATPase subdomain of Fom2 could be considered in cultivars resistive pattern unconformity and it needs to be followed in future studies to clarify the action of detected mutations in this subdomain.

<b>Interference of genotype-by-environment interaction in the selection of inbred lines of yellow melon in an agricultural center in Mossoró-Assu, Brazil

The aims of this study were to i) identify the influence of the GxE interaction in the selection of inbred lines of melon, and ii) test the efficiency of different indexes in the selection of inbred lines with a higher yield potential and better quality fruit. For these purposes, 98 inbred lines of yellow melon and two commercial controls, Vereda and AF-646, were evaluated in a randomized block design with two replications, at the conditions present in Mossoro and Barauna. Analyses of the following traits were performed: fruit yield, mean fruit weight, pulp thickness, pulp firmness and soluble solids content. A complex portion of the GxE interaction was predominantly detected for all traits. The recommendation in this case is to promote selection in each environment. Alternatively, selection based on the average behavior of the inbred lines is the strategy that generates the greatest gains, approaching those obtained through direct selection in each specific environment. The selection indexes aimed at reducing the expression of all traits proved to be best for both locations. The index constructed for Barauna allowed the selection of a greater number of inbred lines with a higher yield potential and better quality fruit and, thus, greater efficiency.

Cucurbit powdery mildews: methodology for objective determination and denomination of races

Cucurbit powdery mildew (CPM), a disease on field and greenhouse cucurbit crops worldwide, is caused most frequently by two obligate erysiphaceous ectoparasites (Golovinomyces orontii s.l., Podosphaera xanthii) that are highly variable in their pathogenicity and virulence. Various independent systems of CPM race determination and denomination are used worldwide, having to date been differentiated on different cultivars or lines of melon (Cucumis melo L.). We briefly review historical perspectives and the current state of understanding of the virulence variation of the two CPM pathogens at the pathogenic race level, their differentiation and their designation. Furthermore, we propose for use by the international CPM research, breeding, seed and production community new tools to enhance research, communication and management of CPM. These tools consist of five components: 1) a set of 21 differential genotypes of Cucumis melo L. for the identification of CPM races; 2) a triple-part, septet code for meaningful, concise designation of CPM races; 3) protocols for maintaining CPM isolates and differential genotypes and for laboratory assays to examine the virulence of CPM isolates; 4) rules and principles of practical application of this system in breeding, seed production and cucurbit growing, including a proposal of a race denomination suitable for practical application; and 5) crucial activities leading to the implementation and running of new tools for CPM research and management. The five components of this package have equal importance, forming a compact system, and none of them can be omitted.

In vitro plant regeneration from cotyledonary explants of Cucumis melo L. var. cantalupensis and genetic stability evaluation using RAPD analysis

An efficient plant regeneration protocol useful in genetic studies for important breeding lines of cantaloupe melons was developed. Three cultivars (Charentais-T, Vedrantais and Isabelle) and three doubled-haploid melon lines (NAD, DH-L2 and DH-L6) were tested for their potential to regenerate under in vitro conditions. Cotyledon explants from 6-day-old seedlings were cultured on three Murashige and Skoog based medium supplemented with different combinations of 6-benzylaminopurine (BAP) and indoleacetic acid (IAA). After 5 weeks of culture all the genotypes were able to regenerate on all media, however the regeneration frequency varied widely among the cultivars and the medium combinations used. The Charentais-T cultivar developed the highest number of shoots/explant on media containing BAP 1.0-IAA 1.0 × 10−2 mg/L (4.1) and BAP 1.2-IAA 1.2 × 10−2 mg/L (4.4) while on medium supplemented with BAP 1.3-IAA 1.3 × 10−2 mg/L the Vedrantais genotype gave the best result (2.4). The work first highlighted a good morphogenetic response of the DH lines; among all the genotypes and combination used, NAD (65.6 %) and DH-L6 (85.4 and 50.0 %) had the highest yield of regenerated plants. The results confirmed that the regeneration competence was strongly affected by genotype. Plantlets with well-developed shoot and root systems were successfully acclimatized and exhibited normal morphology and growth characteristics. In order to confirm genetic stability, a PCR-based RAPD analysis was carried out using 20 decamer-primers. No polymorphic bands were observed among all the plants revealing the genetic homogeneity of the regenerants. Furthermore, the ploidy level of regenerated plants was confirmed by flow cytometry.

The CmACS-7 gene provides sequence variation for development of DNA markers associated with monoecious sex expression in melon (Cucumis melo L.)

Most melon (Cucumis melo L.) breeding lines in South Korea display andromonoecious sex expression, which necessitates laborious hand emasculation during F1 hybrid seed production. Thus, there is a need to develop monoecious sex types in elite germplasm to obviate self-pollination. Sex expression is associated with floral ethylene production, which, in monecious melon plants, is associated with the A locus. Our study was conducted to develop molecular markers for selection of monoecious plants based on sequence variation inherent in the CmACS-7 gene [encoding 1-aminocyclopropane-1-carboxylic acid synthase (ACS)] that is associated with ethylene production. Full-length CmACS-7 sequences were cloned from a monoecious (MO23) and an andromonoecious (AM24) line. The alignment of those CmACS-7 sequences revealed a single nucleotide polymorphism (SNP; C170T) in exon 1 and an 18 bp indel in the 3′-untranslated region (UTR) of between MO23 and AM24, which was then used to develop a cleaved amplified polymorphic sequence (CAPS) (EX1-C170T) and a sequence characterized amplified region (SCAR) marker (T1ex), respectively. The sex expression and the T1ex SCAR-based genotype of 442 F2 plants derived from a MO23 × AM24 cross was determined. Monoecy and andromonoecy segregated in a 3:1 ratio in F2 progeny, where the sex type of 429 plants (13 plants not classified) co-segregated with the SCAR marker, demonstrating that sex expression regulated by CmACS-7 is controlled by a single dominant gene and that it confers monoecy in line MO23. Allelic variation in 112 geographically diverse melon lines for CmACS-7 as accessed by CAPS EX1-C170T and SCAR T1ex markers indicated that the: 1) exon 1 of CmACS-7 is highly conserved and the SNP/sex expression association detected is highly predictable making it potentially useful for marker-based selection of monoecious plants, and; 2) 18 bp indel mutation in the 3′-UTR was present in various lengths depending on different monoecious melon germplasm.

Four sequence positions of the movement protein of Cucumber mosaic virus determine the virulence against cmv1-mediated resistance in melon.

The resistance to a set of strains of Cucumber mosaic virus (CMV) in the melon accession PI 161375, cultivar 'Songwhan Charmi', is dependent on one recessive gene, cmv1, which confers total resistance, whereas a second set of strains is able to overcome it. We tested 11 strains of CMV subgroups I and II in the melon line SC12-1-99, which carries the gene cmv1, and showed that this gene confers resistance to strains of subgroup II only and that restriction is not related to either viral replication or cell-to-cell movement. This is the first time that a resistant trait has been correlated with CMV subgroups. Using infectious clones of the CMV strains LS (subgroup II) and FNY (subgroup I), we generated rearrangements and viral chimaeras between both strains and established that the determinant of virulence against the gene cmv1 resides in the first 209 amino acids of the movement protein, as this region from FNY is sufficient to confer virulence to the LS clone in the line SC12-1-99. A comparison of the sequences of the strains of both subgroups in this region shows that there are five main positions shared by all strains of subgroup II, which are different from those of subgroup I. Site-directed mutagenesis of the CMV-LS clone to substitute these residues for those of CMV-FNY revealed that a combination of four of these changes [the group 64-68 (SNNLL to HGRIA), and the point mutations R81C, G171T and A195I] was required for a complete gain of function of the LS MP in the resistant melon plant.

Inheritance of Some Important Characters to Improve Sweet Melon (Cucumis melo L.) Fruits

The present investigation was carried out during three years of 2012, 2013 and 2014. These experiments were done at Sabaheya Horticultural Research Station, Alexandria, and Fowa area, Kafer El- Sheikh, Egypt. Type of gene action, correlation coefficient and path analysis among all combinations of some important characteristics of sweet melon were studied. A 5X5 half-diallel cross was performed among five pure lines of sweet melon. Additive gene effects were found to be significant for plant length, number of branches / plant, flowering date, maturity date and flesh thickness indicating that the additive gene action played the main role in the inheritance of these traits. The evaluated characteristics of fruit netting, fruit shape index, total soluble solids % and moisture content exhibited insignificant values for the additive gene action. The dominant gene effect was found to be significant for plant length, number of branches per plant, maturity date, fruit netting degree, Total soluble solids % and moisture content indicating the importance of dominant gene effect in the inheritance of this characters. Total yield per plant, phenotypically, correlated with plant length, average fruit number and average fruit weight. Hence, a lot of attention for such relationships in the improvement program of such characters of sweet melon through selection.

Fine mapping of a gene that confers palmately lobed leaf (pll) in melon (Cucumis melo L.)

Leaves serve as the major photosynthetic organs of plants and show considerable diversity in their shapes. Although significant efforts have been focused on exploring the molecular mechanism of leaf development, factors that are responsible for the lobed leaf formation remain unclear. A natural mutant named BM7 in melon (Cucumis melo L.) that consists of palmately lobed leaf offers a unique opportunity to study the underlying mechanism of the lobed leaf shape formation. BM7 spontaneously arose from the melon line Mhy, which has normal round leaf, and was found in a field in China. Genetic analysis showed that the palmately lobed leaf trait was controlled by a single recessive gene, designated pll. Two BC1 populations derived from the cross of (Y8×BM7)×BM7 and (Jiashi×BM7)×BM7 were used to map pll by bulk segregant analysis using simple sequence repeat (SSR) markers. The pll gene was located on linkage group III of melon between SSR markers G69 and 784RS, and the physical distance is 14,631 bp according to the melon genome database. Only one gene, MELO3C010784, was predicted in this region. Gene MELO3C010784 is similar to the AP2-like ethylene-responsive transcription factor ANT, which plays a general role in maintaining meristematic competence in plants. This study is first report of its kind in mapping a gene that confers melon leaf trait and lays the foundation for investigations into the molecular mechanism of palmately lobed leaf formation in melon.

Fine genetic mapping of a locus controlling short internode length in melon (Cucumis melo L.)

Compact and dwarfing vining habits in melon (Cucumis melo L.; 2n = 2x = 24) may have commercial importance since they can contribute to the promotion of concentrated fruit set and can be planted in higher plant densities than standard vining types. A study was designed to determine the genetics of dwarfism associated with a diminutive (short internodes) melon mutant line PNU-D1 (C. melo ssp. cantalupensis). PNU-D1 was crossed with inbred wild-type melon line PNU-WT1 (C. melo ssp. agrestis), and resultant F1 progeny were then self-pollinated to produce an F2 population that segregated as dwarf and vining plant types. Primary stem length of F2 progeny assessed under greenhouse conditions indicated that a single recessive gene, designated mdw1, controlled dwarfism in this population. To identify the chromosomal location associated with mdw1, an simple sequence repeat (SSR)-based genetic linkage map was constructed using 94 F2 progeny. Using 76 SSR markers positioned on 15 linkage groups spanning 462.84 cM, the location of mdw1 was localized to Chromosome 7. Using the putative dwarfing-associated genes, fine genetic mapping of the mdw1 genomic region was facilitated with 1,194 F2 progeny that defined the genetic distance between mdw1 and cytokinin oxidase gene, a candidate gene for compact growth habit (cp) in cucumber, to be 1.7 cM. The candidate gene ERECTA (serin/threonine kinase) and UBI (ubiquitin) were also mapped to genomic regions flanking mdw1 at distances of 0.6 and 1.2 cM, respectively.