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John R. Stommel and Robert J. Griesbach

Ornamental peppers are a novel and growing segment of the ornamentals industry. Currently available varieties are utilized as pot plants and in bedding plant applications. Utilizing unimproved populations developed from initial crosses with Indian Capsicum land races, germplasm lines with unique gene combinations for multiple fruiting, fruit orientation, leaf pigmentation and leaf variegation were developed and released by USDA-ARS. Via introgression of diverse Capsicum species accessions and heirloom varieties into these populations, more recent efforts seek to exploit abundant genetic variation for fruit shape, size, color and pungency, foliar attributes, and plant growth habit to develop new pepper germplasm for ornamental and dual ornamental/culinary applications. Fruit pungency of selected material may range from mild to extremely pungent. Fruit shape may be round, conical, or lobed. Whereas small fruit size is generally well suited for ornamental applications, ornamental/culinary types exploit larger upright conical or small bell-shaped fruit. Plant foliage may be uniformly green in color, exhibit varying degrees of anthocyanin accumulation, or display variegation. Inheritance of selected attributes, potential barriers to development of select recombinants, and examples of representative advanced selections in the breeding program will be presented.

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David C. Zlesak and Neil O. Anderson

A majority of commercial Lilium hybrids and species do not flower the first year from seed or scales due to an obligate vernalization requirement. The Formosa lily (L. formosanum) is a unique species within the genus Lilium because some genotypes flower from seed the first year without vernalization. The objective of this study is to determine the inheritance of stem emergence, which culminates in flowering, in seed-propagated families without vernalization. Nine L. formosanum genotypes, selected from six populations for obligate or non-obligate vernalization for flowering, were intermated to generate 23 families with 104 seedlings per family. Families were grown in a randomized complete-block design at 21 °C (day/night) and data collected were seedling mortality, stem emergence or rosetting without vernalization, and weeks to emergence. At the end of 44 weeks, rosetted genotypes were vernalized for 8 weeks (4 °C); 100% emerged. We propose this trait is controlled by two genes. For flowering without vernalization to occur, there needs to be at least one dominant allele at one of the loci. Locus Ver 2 has less penetrance than Ver 1. Families segregating for dominant alleles at both Ver 1 and Ver 2 emerged sooner (34.2 weeks) than those segregating for a dominant allele at only Ver 1 (36.1 weeks) or Ver 2 (37.6 weeks). Identification of these genes can aid in the development of uniform, fast-flowering L. formosanum hybrids as well as aid in the introgression of this trait into standard commercial lily classes.

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Tae-Jin Lee, Dermot P. Coyne, Thomas E. Clemente, and Amitava Mitra

Expression of lactoferrin (LF) gene, a cationic iron-binding glycoprotein, was investigated in transgenic tomato plants (Lycopersicon esculentum Mill.). Resistance of the transgenic tomato plants to the pathogen (Ralstonia solanacearum Smith) causing bacterial wilt was also determined. Tomato line F7926-96, susceptible to bacterial wilt, was transformed with Agrobacterium strain C58C1 containing a plasmid construction carrying a modified LF cDNA. The introgression of LF cDNA into the susceptible tomato line was confirmed by Southern blot and the expression of full-length lactoferrin transcript and protein was also detected by northern and western blots, respectively. Based on resistance to kanamycin, a Mendelian segregation for a single locus insertion was observed in the T1 and T2 generations and all T1 and T2 plants resistant to kanamycin showed the single corresponding band of LF cDNA in Southern blot analysis. Two transgenic tomato lines inoculated with 1 × 107 and 1 × 108 colony-forming units (CFU)/mL with Rs isolate NC251 (K60, race 1) exhibited early resistance and subsequent susceptibility, while 44% to 55% of plants survived until maturity (fruit ripening) when inoculated with 1 × 105 CFU/mL in comparison with the fully susceptible tomato line. The latter resistance to bacterial wilt in transgenic tomatoes with the stable Mendelian segregation patterns for the LF gene suggests a potential new approach to consider for control of bacterial wilt of tomato. The possible value of this gene along with other plant genes to control bacterial pathogens is discussed.

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Rafael Fernández-Muñoz, María Salinas, Marta Álvarez, and Jesús Cuartero

Genetics of resistance to Tetranychus urticae Koch and of glandular trichomes of Lycopersicon pimpinellifolium accession TO-937 in a cross between susceptible L. esculentum Mill. `Moneymaker' and resistant TO-937 was studied in a greenhouse experiment. Parents, F1, F2, and two BC1 generations, interspersed with susceptible tomato plants to avoid negative interplot interference, were artificially infested. Mite susceptibility was evaluated by a rating based on plant capacity to support mite reproduction. TO-937, BC1 to TO-937, and F1 were resistant, `Moneymaker' susceptible, and the F2 and the BC1 to `Moneymaker' segregated. Resistance was controlled by a single dominant major locus, but modulated by unknown minor loci. TO-937 presented type IV glandular trichomes, their presence governed by two dominant unlinked loci. Type IV trichome density correlated to resistance; however, a causal relationship between type IV trichomes and mite resistance could not be definitively established. The relatively simple inheritance mode will favor successful introgression of resistance into commercial tomatoes from the close relative L. pimpinellifolium.

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James R. Ballington and Barbara J. Smith

Thirty-three accessions of Fragaria virginiana collected from Mississippi in 1995 were evaluated for horticultural traits and leaf disease resistance at Reidsville, N.C., and strawberry anthracnose resistance (Colletotrichum acutatum and C. fragariae) at Poplarville, Miss., in 1997. The range of variability in berry shape, fruit flesh color, fruit skin toughness, and degree of sunkenness of seeds among accessions indicated probable introgression with F. xananassa in most all accessions. Seventeen of 29 accessions screened for resistance to C. acutatum were resistant, and an additional 10 were tolerant. Overall, these accessions appear to be good additional sources of resistance to this, the prevalent species of anthracnose in the southeastern United States. In addition, the majority of accessions appear to be tolerant-resistant to leaf scorch, leaf blight, and/or powdery mildew. Nine accessions were resistant to all three leaf diseases, and four were resistant to C. acutatum as well as the three foliar diseases. No accessions were resistant to C. fragariae and only five were tolerant. All five accessions tolerant to C. fragariae were also either resistant or tolerant to C. acutatum but the converse was not true.

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J.R. Bohac

Sweetpotato, Ipomoea batatas is in the morning glory family, Convolvulaceae, genus Ipomoea, group Batatas. It has many wild Ipomoea relatives that serve as a reservoir of many needed pest and stress-resistance genes. A major barrier to introgression of useful genes is the ploidy gap—sweetpotato is a hexaploid and wild Ipomoeas are diploids and tetraploids. The wild species can be successfully crossed using 2n pollen or by first increasing ploidy by colchicine treatment. The ploidy of such hybrid offpsring can be determined by DNA flow cytometry. My objective was to develop a technique to determine DNA content in Ipomoea and values for DNA content for the major Ipomoea species using the EPIC flow cytometer with a UV detector. Nuclei were extracted and pretreated with cellulase and pectolyase before staining with propidium iodide (PI). A highly linear relationship was found between the DNA content determined by DNA flow cytometry and the ploidy of the closest sweetpotato relatives as determined by chromosome counts. These species were diploid I. trifida, tetraploid I. batatas, and hexaploid I. batatas. DNA content was most similar among other diploid Ipomoea species in the group Batatas and was significantly different in other Ipomoeas not in group Batatas.

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Bob Bors and J. Alan Sullivan

Fragaria vesca has been introgressed into F. ×ananassa in the form of decaploids and synthetic octoploids. As F. vesca is self-incompatible and crosses with most diploid Fragaria species when used as a female parent, it could serve as a bridge for introgression of additional genetic material. A primary goal of this study was to screen selections of F. vesca for interspecific crossability among diploid species. The F. vesca collection included 10 cultivars of the alpine strawberry, F. vesca var. semperflorens, as well as 30 wild runnering types gathered from around the world. The following diploid species were represented by one to three genotypes each: F. viridis, F. nubicola, F. nipponica, F. nilgerens, F. iinumae, F. daltoniana, F. gracilis, as well as two unnamed species from China. Fragaria vesca was used as the female parent and the other species provided the pollen. Crossing took place in the greenhouse, with one pollination occurring during the “popcorn” or “balloon” stage. Germination was performed in vitro using cut achenes shortly after fruit ripening. The alpine strawberry cultivars were easier to cross than wild selections of F. vesca. Their continuous blooming habit combined with higher positioning of flowers allowed for easier and perhaps less-damaging emasculation. Crossability, as measured by seed set and germination, was more variable in wild-type F. vesca and generally lower than alpine strawberry cultivars.

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Zhan'ao Deng, Fred G. Gmitter Jr., Shunyuan Xiao, and Shu Huang

Citrus tristiza virus (CTV) is the most-significant viral pathogen of citrus in the world. Rapid decline of trees on sour orange and stem pitting of grapefruit and sweet orange, two diseases induced by CTV, severely jeopardize citrus production worldwide. It is recognized that all future rootstocks should be resistant to this virus, and scion resistance to stem pitting stains is desirable. To facilitate introgression of the CTV resistance gene from Poncirus trifoliata and development of CTV-resistant varieties in citrus, gene mapping projects have been initiated and more than a dozen RAPD markers have been identified with tight linkage to the resistance gene. As part of our efforts to use marker-assisted selection with a large number of crosses, and ultimately to accomplish map-based cloning of the CTV resistance gene, we have been converting the most tightly linked RAPD markers into SCAR (sequence characterized amplified region) markers by cloning, sequencing the marker fragments, and designing locus-specific primers. One codominant and several dominant SCARs have been developed thus far. The updated progress and utilization of these SCARs in marker-assisted selection and possibly in characterization of a BAC library will be presented and discussed.

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Vance Baird

Prunus, which includes peach/nectarine, almond, apricot, cherry, and plum, is a large and economically important genus in the family Rosaceae. The size and long generation time of these tree crops have hampered improvement through classical breeding and long-term selection programs. With the advent of DNA-based molecular diagnostics, an exciting era in germplasm improvement has dawned. Efforts are underway, notably in the United States (e.g., California, Michigan, North Carolina, and South Carolina) and the European Community (e.g., England, France, Italy, and Spain), to apply the tools of molecular mapping and marker-assisted selection to this important genus. The objective of these projects is to develop molecular linkage maps of sufficient marker density to tag phenotypic trait loci of agronomic importance. These include traits controlled by single genes (e.g., flower color, compatibility, flesh color, pest resistance), as well as more-complex, quantitative traits (e.g., cold hardiness, tree architecture, sugar content). An immediate outcome of these mapping efforts has been the development of DNA “fingerprints,” allowing for the discrimination of cultivars—both scion and rootstock. The maps will be used by breeders and molecular biologists to monitor gene introgression from wild species into elite lines, for marker-assisted selection of desired trait combinations, and for map-based cloning of specific genes. The molecular markers used in these mapping projects include RFLPs, RAPDs, and microsatellites. Each has their appropriate applications and advantages depending upon the resources at hand and the project's specific goals.

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Hong Y. Yang, Schuyler S. Korban, Jutta Kruger, and Hanna Schmidt

Apple scab, caused by Venturia inaequalis (Cke.) Wint., is the most serious disease of apple trees. Resistance to V. inaequalis, derived from the small-fruited species Malus floribunda 821, is determined by a major dominant gene Vf. Our major objective is to identify RAPD markers linked to the Vf gene. The approach in this paper is based on the introgression of the Vf gene from M. floribunda into commercial cultivars. Almost 200 random sequence decamer-primers have been used to screen a pair of bulked samples and the donor parent M. floribunda clone 821 for markers linked to the Vf gene conferring resistance to apple scab. A single primer has been identified which generated a PCR fragment, OPK16/1300, from the donor parent M. floribunda clone 821 and the scab-resistant selections/cultivars bulk, but not from the scab-susceptible recurrent parent bulk. Co-segregation analysis using a segregating apple progeny and polymorphism analysis of individual scab-resistant Coop selections/cultivars have confirmed that this marker is linked to the scab-resistance gene Vf. OPK16/1300 has since been cloned and sequenced. Sequence-specific primers of 25 oligonucleotides based on the marker have been synthesized and used to screen further M. floribunda clone 821, scab-susceptible apple cultivars, scab-resistant apple cultivars, and scab-resistant Coop selections. The sequence-specific primers have identified polymorphisms of OPK16/1300 based on the presence or absence of a single band.