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  • Author or Editor: Mark W. Farnham x
  • Journal of the American Society for Horticultural Science x
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A collection of collard (Brassica oleracea L., Acephala group) germplasm, including 13 cultivars or breeding lines and 5 landraces, was evaluated using randomly amplified polymorphic DNA (RAPD) markers and compared to representatives of kale (Acephala group), cabbage (Capitata group), broccoli (Italica group), Brussels sprouts (Gemmifera group), and cauliflower (Botrytis group). Objectives were to assess genetic variation and relationships among collard and other crop entries, evaluate intrapopulation variation of open-pollinated (OP) collard lines, and determine the potential of collard landraces to provide new B. oleracea genes. Two hundred nine RAPD bands were scored from 18 oligonucleotide decamer primers when collard and other B. oleracea entries were compared. Of these, 147 (70%) were polymorphic and 29 were specific to collard. Similarity indices between collard entries were computed from RAPD data and these ranged from 0.75 to 0.99 with an average of 0.83. Collard entries were most closely related to cabbage (similarity index = 0.83) and Brussels sprouts entries (index = 0.80). Analysis of individuals of an OP cultivar and landrace indicated that intrapopulation genetic variance accounts for as much variation as that observed between populations. RAPD analysis identified collard landraces as unique genotypes and showed them to be sources of unique DNA markers. The systematic collection of collard landraces should enhance diversity of the B. oleracea germplasm pool and provide genes for future crop improvement.

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Using anther culture to generate doubled-haploid (DH) homozygous lines for use as parents in F1 hybrid crosses has become a common practice in breeding broccoli (Brassica oleracea L. Italica Group). During anther culture and subsequent embryogenesis and plant regeneration, polyploidization of microspore-derived embryos may not occur or it may occur accompanied by a doubling, tripling, quadrupling, octupling, or irregular polyploidization of the genome. Thus regenerants from the process can be haploids, diploids, triploids, tetraploids, octaploids, or aneuploids. The objectives of this research were to 1) conduct repeat cycles of broccoli anther culture using a group of F1 hybrids as anther donors and develop populations of regenerants; 2) analyze resulting populations using DNA flow cytometry and determine the influence of F1 source on frequency of different ploidy levels among regenerants; and 3) compare seed set in broccoli inbreds developed in a traditional selfing program compared to seed set in DH broccoli derived from anther culture. In two cycles (1994 and 1995) of anther culture, anther-derived populations of regenerants were developed using the F1 hybrids `Marathon', `Everest', `High Sierra', and `Futura' as sources of anthers. In 1994, `Everest', `High Sierra', and `Futura' yielded populations that included 2% to 7% haploids, 53% to 56% diploids, 32% to 38% tetraploids, and 5% to 6% other types. `Marathon'-derived regenerants were 5% haploid, 78% diploid, 15% tetraploid, and 2% other, showing significantly more diploids. In 1995, `Marathon' regenerants again included significantly more diploids and fewer tetraploids than those derived from other F1 sources, confirming that the genotype of the anther source affects the frequency of a particular ploidy level among regenerants derived from culture. In manual self-pollinations of 1994 regenerants, only diploids and rare tetraploids set seed. When plants that set no seed were discounted, seed production following manual self pollinations of 1995 regenerants was not significantly different from that of traditional inbreds derived from the same F1 sources.

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Broccoli (Brassica oleracea L., Italica Group) has been recognized as a source of glucosinolates and their isothiocyanate metabolites that may be chemoprotective against human cancer. A predominant glucosinolate of broccoli is glucoraphanin and its cognate isothiocyanate is sulforaphane. Sulforaphane has been shown to be a potent inducer of mammalian detoxication (Phase 2) enzyme activity and to inhibit chemical-induced tumorigenesis in animal models. Little is known about phenotypic variation in broccoli germplasm for Phase 2 enzyme (e.g., quinone reductase) induction potential. Thus, this study was undertaken to evaluate: 1) quinone reductase induction potential (QRIP) diversity among a population of broccoli inbreds; 2) QRIP levels in selected lines; 3) correlation of QRIP with other horticultural characteristics; and 4) QRIP expression in a sample of synthesized hybrids. In 1996, 71 inbreds and five hybrid checks (all field-grown), ranged from a QRIP of nearly zero to 150,000 units/g fresh weight (FW) (mean of 34,020 units/g FW). These values were highly correlated with methylsulphinylalkyl glucosinolate (MSAG; primarily glucoraphanin) concentrations that ranged from 0.04 to 2.94 μmol.g-1 FW. A select subset of lines evaluated in 1996 were reevaluated in 1997. QRIP and MSAG values in this second year were similar to and correlated with those observed in 1996 (r = 0.73, P < 0.0001 and r = 0.79, P < 0.0001, respectively). In addition, both QRIP and MSAG concentration were highly correlated with days from transplant to harvest. Average F1 hybrid values for QRIP and MSAG in 1997 fell typically between their parental means, but were often closer to the mean of the low parent. Results of this study indicate that divergent QRIP expression can effectively be used to select enhanced inbred lines to use in development of value-added hybrids. Evidence is also provided that there is a significant genetic component to both QRIP and MSAG concentration, and that selection for either one may provide an effective means for developing broccoli hybrids with enhanced chemoprotective attributes. Chemical names used: 4-methylsulphinylbutyl glucosinolate (glucoraphanin) and 4-methylsulphinylbutyl isothiocyanate (sulforaphane).

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Breeders of cole crops (Brassica oleracea L.) have an interest in utilizing current and emerging PCR-based marker systems to differentiate elite germplasm. However, until efficiency and cost-effectiveness are determined, most breeders are hesitant to change methods. In this study, our goal was to compare simple sequence repeat (SSR), amplified fragment-length polymorphism (AFLP), and sequence-related amplified polymorphism (SRAP) marker systems for their effectiveness in differentiating a diverse population of 24 elite broccoli (B. oleracea Italica Group) inbreds. Published SSR primer sequences for Brassica L. species were used along with AFLP and SRAP primer combinations. Several SSR primers failed to amplify DNA in the broccoli population, but all AFLP and SRAP primer combinations produced multiple bands. Twenty-nine percent of the SSR primers were monomorphic, while most of the remaining primers detected only one or two differences among inbreds. AFLP and SRAP methods produced multiple differences per primer in almost every case. Phenetic analysis revealed that the type of marker affected the classification of the genotypes. All three marker systems were able to successfully differentiate between the 24 elite inbreds, however, AFLPs and SRAPs were more efficient, making them better alternatives than SSRs over other established methods for fingerprinting B. oleracea inbreds.

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Broccoli (Brassica oleracea L., Italica Group) is a good vegetable source of Ca and Mg, two critical minerals in human nutrition. Studies have shown that bioavailability of Ca from broccoli is comparable to that from milk. Thus, broccoli is an important alternative source of Ca in segments of the population that consume limited amounts of dairy products. Essentially nothing is known about the genetic influence on variation in Ca or Mg concentration of broccoli heads. Thus, the goal of this research was to examine variation in Ca, and also Mg concentrations, in a collection of USDA inbreds and commercial F1 hybrids. In 1996 and 1997 field studies, significant differences among inbred entries and among hybrid entries were observed for Ca and Mg concentrations of broccoli heads. With hybrids and inbreds, mean head Ca concentrations were ≈3.0 mg·g-1 dry weight (DW), and entries with lowest and highest Ca concentrations differed >2-fold. Mean Mg concentrations of hybrid heads was 2.3 mg·g-1 DW (range 1.8 to 2.6) and 2.8 mg·g-1 DW (range 2.2 to 3.7) in 1996 and 1997, respectively. Inbred lines had mean head Mg concentrations of 2.0 and 2.6 mg·g-1 DW in the two respective years and ranges in concentration were similar as for hybrids. Analysis of variance indicated significant environment and entry by environment effects for Ca and Mg concentrations of hybrids. With inbreds, a significant entry by environment effect for Ca concentration and environment effect for Mg concentration was also observed. Significant environment and entry by environment effects indicate that the environmental influence on phenotypic expression of Ca and Mg concentrations may complicate genetic improvement of head mineral concentration.

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Downy mildew, incited by the biotrophic fungal parasite, Peronospora parasitica (Pers. Fr.) Fr., is one of the most destructive diseases of broccoli (Brassica oleracea L., Italica Group) and other related crop species throughout the world. Cultivation of resistant cultivars is the most desirable control method because it provides a practical, long-term, and environmentally benign means of limiting damage from this disease. The commercial hybrid cultivar, Everest, has been shown previously to contain a high level of downy mildew resistance. Doubled-haploid (DH) lines developed from that hybrid were also shown to exhibit a similar, high level of resistance at the three- to four-leaf stage. To determine the mode of inheritance of this true leaf resistance, the resistant DH line was crossed to a susceptible line (derived from `Marathon') to produce an F1 hybrid. Subsequently, F2 and backcross (BC) populations were developed from the hybrid. In addition, a DH population of ≈100 lines was developed from the same F1 used to create the F2 and BC. All populations were evaluated for response to artificial inoculation with P. parasitica at the three- to four-leaf stage. F1 plants were resistant like the resistant parent and F2 populations segregated approximately nine resistant to seven susceptible. Using the resistant parent as recurrent parent, BC populations contained all resistant plants, while the BC to the susceptible parent fit a 1 resistant: 3 susceptible segregation ratio. These results can be explained by a model with two complementary dominant genes. This model was confirmed by the DH population that segregated ≈1:3, resistant to susceptible. Due to the dominant nature of this resistance, controlling genes should be easily incorporated into F1 hybrids and used commercially to prevent downy mildew.

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Downy mildew, caused by the fungal parasite Peronospora parasitica (Pers.: Fr.) Fr., is a destructive disease of Brassica oleracea L. crops, including broccoli (B. oleracea, Italica Group). The development and deployment of downy mildew resistant broccoli cultivars is a priority for breeders and producers. Identification of genetic markers linked to downy mildew resistance genes should facilitate selection for resistance and pyramiding of resistance genes into cultivars. The objectives of this study were to 1) identify RAPD markers linked to a single dominant gene for resistance in broccoli, 2) clone and sequence the linked RAPD markers, and 3) develop and evaluate SCAR markers as screening tools for resistance. Bulked segregant analysis led to the identification of eight linked RAPD markers following a screen of 848 decamers. Two of the linked RAPD fragments, UBC359620 and OPM16750, were converted to dominant SCAR markers linked in coupling to the resistance locus at 6.7 and 3.3 cM, respectively. The SCAR marker based on UBC359620 sequence exhibited less accuracy (94%) than the original RAPD (96%) in differentiating resistant and susceptible plants, but the accuracy (97%) of the OPM16750-SCAR was not different than the original RAPD. These SCAR markers are among the first genetic markers found linked to a gene conferring cotyledon-stage downy mildew resistance in B. oleracea. Results of this work provide breeders with useful information and tools for the systematic development of resistant cultivars.

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