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Jack E. Staub, Gennaro Fazio, Thomas Horejsi, Yael Danin-Poleg, Noa Reis and Nurit Katzir

Random amplified polymorphic DNA (RAPD) and simple sequence repeat (SSR) markers were used to characterize genetic relationships among 46 accessions in two C. melo L. subsp. melo (Cantalupensis, Inodorus) and subsp. agrestis (Conomon and Flexuosus) groups. Genetic distance (GD) estimates were made among and between accessions in four melon market classes [Galia, Ogen, Charentais, and Shipper (European and U.S. types)] of Cantalupensis, one market class of Inodorus (Cassaba and Honey Dew), one accession of Conomon, and one accession of Flexuosus by employing three GD estimators; simple matching coefficient, Jaccard's coefficient, and Nei's distance-D. Differences detected among 135 RAPD bands and 54 SSR bands (products of 17 SSR primers) were used to calculate GD. Band polymorphisms observed with 21 RAPD primers and 7 SSR primers was important in the detection of genetic differences. Estimators of GD were highly correlated (P > 0.0001; rs = 0.64 to 0.99) when comparisons were made between estimation methods within a particular marker system. Lower correlations (P > 0.001; rs = 0.17 to 0.40) were detected between marker systems using any one estimator. The GD of the Conomon and Flexuosus accessions was significantly different from the mean GD of all the market classes examined, and market classes were distinguishable from each other. Although lower coefficients of variation can be attained in the estimation of GD when using RAPDs compared to SSRs, the genetic relationships identified using these markers were generally similar. Results of RAPD marker analysis suggest that 80 marker bands were adequate for assessing the genetic variation present in the accessions examined.

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Yosef Burger, Uzi Saar, Nurit Katzir, Harry S. Paris, Yelena Yeselson, Ilan Levin and Arthur A. Schaffer

Fruit sweetness is the major determinant of fruit quality in melons (Cucumis melo L.) and reflects the concentration of the three major soluble sugars, sucrose, glucose, and fructose, present in the fruit flesh. Of these three sugars, sucrose is the prime factor accounting for both the genetic and the environmental variability observed in sugar content of C. melo fruit. Faqqous (subsp. melo var. flexuosus), a cultivar having a low sucrose and total sugar content, was crossed with Noy Yizre'el (subsp. melo var. reticulatus), a cultivar having a high sucrose and total sugar content. F1 plants had a sucrose content averaging slightly higher than that of the low-sucrose parent, indicating that low sucrose content is nearly completely dominant. Segregation in the F2 and backcross progenies indicated that high sucrose accumulation in melon fruit flesh is conferred by a single recessive gene herein designated suc. When the high-sucrose parent was crossed with the moderate-sucrose landrace known as Persia 202 (subsp. melo var. reticulatus), the segregation in the filial and backcross progenies suggested that additional genetic factors affect the amount of sucrose accumulation.

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Yosef Burger, Uzi Sa'ar, Asaph Distelfeld, Nurit Katzir, Yelena Yeselson, Shmuel Shen and Arthur A. Schaffer

The sweet cultivars of Cucumis melo are characterized by high sucrose levels, together with low acid levels in the mature fruit flesh. The trait of high sugar accumulation in C. melo fruit is determined by a single recessive gene, suc. High acid content, conferred by a single dominant gene, So, is found only in C. melo varieties that do not accumulate high levels of sugar and are used for nondessert purposes. We combined the genetic traits of high acid content (low pH) and high sugar levels by crossing the nonsweet, high acid C. melo var. flexuosus, `Faqqous' (So/So, Suc/Suc), with high sugar, low acid C. melo genotypes (so/so, suc/suc) and generating the recombinant genotype So/—, suc/suc. Segregating F2 populations derived from the cross between `Faqqous' and a standard high sugar, low acid line showed that the traits of high sugar and low pH were inherited independently of each other. The accumulation of acid and sugar in the developing fruit of a recombinant high acid, high sugar breeding line, A6, were also temporally independent, with acid accumulation preceding the rise in sucrose levels. The low pH of A6 was correlated with the developmental increase in titratable acidity and particularly of citric acid levels. The combination of increased acidity and high sugar provides the melons with a unique taste due to a sugar to acid ratio not present in sweet C. melo cultivars. These results are discussed in terms of the evolution under domestication of C. melo.