Diversity among melon (Cucumis melo L.) landraces from the Indo-Gangetic plains of India and their genetic relationship with USA melon cultivars

We report here the first broad genetic characterization of farmer-developed landraces of melon (Cucumis melo L.) from the Indo-Gangetic plains of India, an area overlooked in previous melon genetic diversity analyses of Indian melon germplasm. Eighty-eight landraces from three melon Groups in two subspecies (C. melo subsp. agrestis Momordica Group, and C. melo subsp. melo Cantalupensis Group and Reticulatus Group) were collected from the four agro-ecological regions (six sub-regions) of two northern states of the Indo-Gangetic plains of India, Uttar Pradesh and Uttarakhand. Significant differences were found among the landraces and eight USA Reticulatus Group reference cultivars for 18 plant and fruit traits: no. of primary branches per plant, days to marketable maturity, sex expression, fruit shape, flesh colour, netting, no. of fruit per plant, fruit weight, shelf life, total soluble solids (°Bx), ascorbic acid (mg/100 g), titratable acidity (%), fruit length and diameter, seed cavity length and diameter, flesh thickness, and resistance to Cucumber mosaic virus. The three melon groups differed significantly for 10 of the plant and fruit traits. Cantalupensis Group and Reticulatus Group accessions were andromonoecious, and the Momordica Group was monoecious. Neighbour-joining (NJ) tree and factorial correspondence analysis (FCA) of simple sequence repeat loci also revealed a high level of genetic variability in this germplasm. The 96 melon genotypes clustered into five groups in the NJ tree analysis: the 16 Indian Reticulatus Group accessions and eight USA reference cultivars formed a distinct group; and the 60 Cantalupensis Group accessions clustered in four other groups with the 12 Momordica Group accessions in a distinct subgroup of one of the Cantalupensis groups. The FCA plot largely confirmed the NJ tree with three distinct groups, one for each melon group. The close affinity of the Indian and USA Reticulatus melons was not unexpected, but it is not clear whether it was inherent in the group and maintained as Reticulatus melons moved from India through Central Asia and Europe to North America, or the result of recent intercrossing of Indian landraces with the USA-derived cultivars and selection for a broad range of Reticulatus type melons.     

Variance component analysis of plant architectural traits and fruit yield in melon

Summary  A cross was made between a unique highly branched, early flowering line, U. S. Department of Agriculture (USDA) 846-1 (P_1; 7 to 11 lateral branches), and ‘Topmark’ (P₂; 2 to 4 lateral branches), a U.S. Western Shipping melon, to produce an array of 119 F₃ families. Subsequently, a genetic analysis was conducted at Arlington and Hancock, Wisconsin in 2001 to evaluate the segregating progeny for factors likely involved in yield-formation, including days to anthesis, percentage of plants with early pistillate flowering, primary branch number, fruit number and weight per plant, average weight per fruit, percentage of plants with predominantly crown fruit set, and percentage of plants with early maturing fruit. Although, genotype × environment (G × E) interactions were important for some traits (e.g., fruit number and fruit weight), considerable additive and/or dominance variance was detected for all traits. This research provides critical data associated with highly branched melon germplasm including trait correlations and heritabilies (broad- and narrow-sense ranged between 0.28 and 0.91) that used judiciously will allow the development high yielding melon cultivars with early, basally concentrated fruit suitable for once-over or machine harvesting operations.     

Comparison of isozyme and random amplified polymorphic DNA data for determining intraspecific variation in Cucumis

Variation at isozyme and random amplified polymorphic DNA (RAPD) loci in eight cucumber and seven melon cultivars, breeding lines, and plant introductions were used to determine the utility of these markers for assessing genetic variation among populations of each species. Although dendrograms derived from cluster analyses using species' variation at marker loci were dissimilar, these disparities were consistent with differences in the pedigrees and/or other information (e.g., morphological) known about each accession and species. Empirical estimations of variances associated with each marker type in the cucumber and melon accessions examined indicate that, per band, lower coefficients of variation can be attained in the estimation of genetic difference when using RAPDs compared to isozymes. The disparity between the marker analyses made may be related to the amount of genome coverage characteristic of a particular marker system in a species and its efficiency in sampling variation in a population.     

Evaluation and QTL mapping of resistance to powdery mildew in lettuce

Lettuce (Lactuca sativa) is the major leafy vegetable that is susceptible to powdery mildew disease under greenhouse and field conditions. Quantitative trait loci (QTLs) for resistance to powdery mildew under greenhouse conditions were mapped in an interspecific population derived from a cross between susceptible L. sativa cultivar Salinas and the highly susceptible L. serriola accession UC96US23. Four significant QTLs were detected on linkage groups LG 1 (pm‐1.1), LG 2 (pm‐2.1 and pm‐2.2) and LG 7 (pm‐7.1), each explaining between 35 to 42% of the phenotypic variation. The four QTLs are not located in the documented hotspots of lettuce resistance genes. Alleles for the disease resistance at the four QTLs originated from both parents (two from each), demonstrating that even highly susceptible accessions may provide alleles for resistance to powdery mildew. These QTLs appeared to operate during limited periods of time. Results of the field trials with F_2:3 and F_3:4 families derived from a Soraya (moderately resistant) × Salinas cross demonstrated effective transfer of resistance to powdery mildew in this material. An integrated rating approach was used to estimate relative levels of resistance in 80 cultivars and accessions tested in a total of 23 field and greenhouse experiments. Generally, very low resistance was observed in crisphead‐type lettuces, while the highest relative resistance was recorded in leaf and butterhead types. Comparison of two disease assessment methods (percentage rating and the area under the disease progress steps, AUDPS) for detection of QTLs shows that the two approaches complement each other.     

Generation means analysis of plant architectural traits and fruit yield in melon

Unique architectural phenotypes have the potential for increasing yield in commercial melon (Cucumis melo L.). Therefore, a generation means analysis was conducted to investigate the inheritance of architectural traits (days to anthesis, primary branch number, fruit number and weight, and average weight per fruit). Progeny (F₁, F₂, BC₁P₁ and BC₁P₂) from a cross between US Department of Agriculture (USDA) line, USDA 846‐1 (P₁) and ‘TopMark’ (P₂) were evaluated at Arlington (AR) and Hancock (HCK), Wisconsin in 2001. Significant (P ≤ 0.05) environment effects and genotype × environment interactions (G × E) analyses necessitated analysis by location. Significant differences (P ≤ 0.05) among parents and generations were observed for all traits, and the two parental lines differed significantly for primary branch number, fruit number and average weight per fruit. Additive gene effects were most important in governing primary branch number and fruit number per plant, while dominance and epistatic genetic effects mainly controlled days to anthesis, fruit weight per plant and average weight per fruit. Narrow‐sense heritabilities were 0.62 (AR) for days to anthesis, 0.71 (AR) and 0.76 (HCK) for primary branch number, 0.68 (AR) and 0.70 (HCK) for fruit weight per plant, 0.33 (AR) and 0.45 (HCK) for fruit weight per plant, and 0.06 (AR) and 0.79 (HCK) for average weight per fruit. Estimations of the least number of effective factors for primary branch number were relatively consistent at both AR (approx. 4) and HCK (approx. 2). Results suggest that introgression of yield‐related genes from highly branched melon types (e.g. USDA 846‐1) into US Western Shipping germplasm may aid in the development of high‐yielding cultivars with concentrated fruit set suitable for machine and/or hand‐harvesting operations.     

Virulence variation of cucurbit powdery mildews in the Czech Republic – population approach

Kosman diversity models were applied to analyses of virulence (disease reaction patterns) variation of 115 isolates of two cucurbit powdery mildew (CPM) species, Golovinomyces orontii (Go) and Podosphaera xanthii (Px), collected in the Czech Republic from 2010 through 2012. Diversity within and distances between Go and Px populations and each other in a spatio-temporal context and with regard to original host plant species were analyzed based on virulence patterns of individual isolates on a set of 21 melon (Cucumis melo L.) race differential genotypes. Significant differentiation among the Go and Px pathogen populations was revealed, and the results clearly demonstrate and confirm that the set of differential C. melo genotypes is well composed because of high differentiation capacity. Differentiation of pathogens among years was significant for both species. No significant difference between Go isolates from different host plant species was established due to high variability among Go isolates, but there was significant host-specific differentiation among Px isolates. Differentiation of pathogens among regions was not detected. These results revealed high virulence variation in isolates of Go and Px, and their spatio-temporal fluctuations. High diversity in virulence of Go isolates supports the treatment of Go as a complex of different (sub)species with distinct virulence factors. Similar relationships of selected Go isolates in a UPGMA dendrogram in a previously reported multigene phylogenetic tree support the logic and suitability of the Kosman assignment based approach to population studies of organisms with asexual or mixed modes of reproduction. The approach applied in this study provides a complex view of virulence structures of powdery mildew populations, and when combined with race determination and denomination on melon, it may serve as a base to understandvirulence variation of these CPM species from a spatio-temporal viewpoint.     

Histological aspects of Cucumis melo PI 313970 resistance to Podosphaera xanthii and Golovinomyces cichoracearum

Journal of Plant Diseases and Protection - Cucumis melo accession PI 313970 possesses numerous genes for race-specific resistance to powdery mildew incited by Podosphaera xanthii. It also exhibits...