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Frank Cheng, Norman Weeden, and Susan Brown

The ability to pre-screen apple populations for fruit color at an early seedling stage would be advantageous. In progeny of the cross `Rome Beauty' × `White Angel' red/yellow color variation was found to be highly correlated with the genotype at Idh-2, an isozyme locus that was heterozygous in both parents. We postulate that the red/yellow color variation was produced by a single gene linked to I&-2 and also heterozygous in both parents. This population was also screened with over 400 primers to detect randomly amplified polymorphic (RAPD) markers for fruit color. DNA extraction procedures were developed for bark, and DNA was extracted from bark samples and leaves. Red and yellow fruited individuals were examined in bulk. Several markers have been found that are linked to red color. A high density map is being constructed in this region. These markers are being examined in other crosses segregating for fruit color. The application of these markers will be discussed in relation to the inheritance and manipulation of fruit color.

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Frank Suozhan Cheng and Mikeal L. Roose

`Flying Dragon' Poncirus trifoliata L. Raf. is a dwarfing rootstock for citrus. Inheritance of dwarfing ability was studied in a population of open-pollinated seedlings of `Flying Dragon'. Molecular marker genotypes suggest that all seedlings originated from selfing. Progeny seedlings were budded with `Cutter Valencia' orange and planted in the field to evaluate the dwarfing effect of the seedling rootstock. At 5 years after planting, rankit analysis of the frequency distributions of trunk cross-sectional area and canopy volume suggested the presence of two overlapping distributions of 34 dwarf trees and 7 nondwarf. This ratio is consistent with inheritance of rootstock dwarfing as a single dominant gene for which `Flying Dragon' is heterozygous. Two morphological characteristics of `Flying Dragon', curved thorns and twisted trunk growth, were closely linked to, or pleiotropic effects of, the dwarfing gene. Bulked segregant analysis was used to identify three RAPD markers linked to the dwarfing gene. `Flying Dragon' was identical to nondwarfing cultivars of trifoliate orange at 40 homozygous and heterozygous isozyme and RFLP markers; therefore, it is likely that `Flying Dragon' originated as a mutant of a nondwarfing genotype and has not undergone sexual recombination since this event.

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Mikeal L. Roose, Frank Suozhan Cheng, and Claire T. Federici

The `Flying Dragon' cultivar of Poncirus trifoliata L. Raf. is a strongly dwarfing rootstock for Citrus cultivars, reducing canopy volume of 9 year-old `Valencia' orange trees to 1/3 that of trees on standard rootstocks Open-pollinated seed of `Flying Dragon' was screened with isozyme markers to distinguish zygotic from nucellar (apomictic) seedlings. All zygotics had genotypes consistent with an origin by self-pollination. Zygotic seedlings were budded with `Valencia' orange scion and planted in the field. Of 46 progeny evaluated as rootstocks, 35 produced small trees similar to those on nucellar `Flying Dragon' and 11 produced large trees. This ratio is consistent with the 3:1 segregation expected for a single dominant gene. The dwarfing gene was closely linked, or pleiotropic with a gene causing curved thorns and stems. Several RAPD markers close to the dwarfing gene were identified with bulked segregant analysis. `Flying Dragon' apparently originated as a mutation because it had au identical genotype to non-dwarfing strains of trifoliate orange at all 38 isozyme and RFLP markers tested

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Minou Hemmat, Norman F. Weeden, Frank S. Cheng, and S.K. Brown

The positions of over 50 SSR loci and other sequence tagged sites (STSs) have been located on the linkage maps of five apple cultivars (Rome Beauty, White Angel, Golden Delicious, Liberty, McIntosh) and two New York accessions. In most cases, the primers used produced single amplification products, permitting identification of homologous loci in the different cultivars and the precise alignment of the linkage maps generated for each. Based on this information, we present a general linkage map for apple with STS markers on each linkage group. The map consists of 17 linkage groups (equal to the haploid chromosome number for the species) with over 500 markers. The positions of several resistant gene analogues have been located on this linkage map. None of these sequences map near genes conferring resistance to scab or powdery mildew. SSR loci exhibited a tendency to cluster in certain regions of the linkage map. This clustering slightly reduces their effectiveness as genome markers for comparative mapping or germplasm diversity. However, the SSR markers definitely displayed a high level of polymorphism, making them particularly useful for genetic studies.