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Article Excerpt
THE ACHENES (seeds) of cultivated or common sunflower (Helianthus annuus L.) are a rich source of oil and an historically important source of food and pigment for Native Americans (Heiser, 1951, 1977). The seeds of unbranched (single-headed) Native American land races, the earliest cultigens, are significantly larger than the seeds of wild populations, partly because unbranched biotypes have larger capitula and seeds than branched biotypes, and partly because sunflower was domesticated by selecting for larger seeds (Ross, 1939; Heiser, 1951, 1977; Dedio, 1980). Shellable, large-seeded, low-oil (confectionery) land races developed in eastern Europe were introduced to North America in 1880 and had seed oil concentrations in the 260 to 280 g [kg.sup.-1] range. Mennonite, an early confectionery cultivar, CM 612 (PI 546351), an inbred directly isolated from Mennonite (Dedio and Rashid, 1991), and numerous other confectionery cultivars and inbred lines have seed oil concentrations in the 240 to 290 g [kg.sup.-1] range (http://www.ars-grin.gov; verified 2 December 2005).
Unshellable, small-seeded, high-oil (oilseed) cultivars were developed between 1920 and 1955 by direct selection for increased seed oil concentration and indirect selection for small seeds and thin pericarps (hulls) in open-pollinated populations (Putt, 1940, 1997; Heiser, 1951, 1977; Pustovoit, 1964). Cultivars producing 380 g [kg.sup.-1] of seed oil may have been developed as early as 1915; however, the seed oil concentrations of cultivars commonly grown between 1900 and 1915 reportedly ranged from 200 to 300 g [kg.sup.-1] (Putt, 1997). Selection in open-pollinated populations increased seed oil concentrations to 430 g [kg.sup.-1] by 1935 and 490 g [kg.sup.-1] by 1955 (Pustovoit, 1964; Heiser, 1977; Seiler, 1985, 1994; Putt, 1997; Seiler and Brothers, 1999), thereby greatly increasing the economic importance of sunflower as an oilseed and further differentiating the confectionery and oilseed market classes (Pustovoit, 1964; Putt, 1997).
Seeds of confectionery and oilseed cultivars are distinguished by differences in shellability, hull color, seed weight and morphology, and kernel-to-pericarp weight ratio, in addition to seed oil concentration (Beard, 1981; Seiler, 1997). The seeds of confectionery cultivars are typically gray or white, black or brown striped, and easily dehulled or shelled, whereas the seed of oilseed cultivars are typically black (dark purple) and difficult to dehull. Seeds of confectionery cultivars are larger and have lower kernel-to-pericarp weight ratios than seeds of oilseed cultivars. Hull color differences are produced by pigments in the epidermal, hypodermal, and phytomelanin layers (Johnson and Beard, 1977; Leon et al., 1996; Miller and Fick, 1997). The epidermis is either unpigmented, solid brown or black, or black- or brown-striped, the hypodermis is either anthocyanin pigmented (black) or unpigmented, and phytomelanin is either present (black) or absent (Johnson and Beard, 1977; Leon et al., 1996; Miller and Fick, 1997). Several hull pigment loci have been identified through phenotypic analyses of mutations (reviewed by Miller and Fick, 1997). The allelism of many of the mutations is not known, and only Hyp, a hypodermis pigment locus, has been genetically mapped (Leon et al., 1996).
The first high-oil cultivars were apparently developed by introgressing allelic diversity from wild populations into open-pollinated, low-oil cultivars (Pustovoit, 1964; Heiser et al., 1969; Semelczi-Kovacs, 1975; Putt, 1997). Selection within domesticated, low-oil populations per se cannot be completely ruled out--land race selection between 1880 and 1915 could have played a role in advancing sunflower as an oilseed (Putt, 1997). Significant genetic variability for seed oil concentration is present in wild sunflower germplasm, e.g., seed oil concentrations ranged from 191 to 355 g [kg.sup.-1] among 340 wild H. annuus populations screened by the USDA (http://www.ars-grin.gov). The upper end of the range was even greater among 93 wild populations of H. anomalus S. F. Blake, H. petiolaris Nutt., H. debilis Nutt., and other taxa--minimum, mean, and maximum seed oil concentrations were 262, 346, and 457 g [kg.sup.-1], respectively.
Once hybrid seed production systems were developed (Leclercq, 1969; Kinman, 1970), the focus in sunflower breeding rapidly shifted to the development of high-oil inbred lines from an assortment of open-pollinated, high-oil populations and cultivars, the progenitors of most of the early high-oil inbred lines on which the hybrid seed industry was initially built (Korell et al., 1992; Cheres and Knapp, 1998). While present-day hybrids commonly produce 440 to 490 g [kg.sup.-1] of seed oil, CM 612 (PI 546351), CM 630 (PI 566828), and other inbred lines producing 500 to 530 g [kg.sup.-1] of seed oil have been described (Dedio and Rashid, 1991, 1994; http://www. ars-grin.gov). Our goal was to identify phenotypic and quantitative trait loci (QTL) underlying genetic variability for pericarp pigments, seed oil concentration, and other seed traits in a recombinant inbred line (RIL) mapping population (Tang et al., 2002) developed from a hybrid between large-seeded, low-oil (RHA280) and small-seeded, high-oil (RHA801) inbred lines developed in the early period of single-cross hybrid breeding in sunflower (Fick et al., 1974a; Roath et al., 1981).
MATERIALS AND METHODS
Genetic Mapping and Phenotyping
Genetic analyses were performed on 173 [F.sub.7] recombinant inbred lines (RILs) developed by single-seed-descent from a cross between RHA280 (an unbranched, confectionery, fertility restorer line) and RHA801 (a branched, oilseed, fertility restorer line) (Tang et al., 2002). RHA280 has large, brown-striped seeds and low seed oil concentrations (Fick et al., 1974a), whereas RHA801 has small, black seeds and high seed oil concentrations (Roath et al., 1981). The RILs were presumed to be segregating for an apical branching gene (B) described by Putt (1964) and the B locus genotypes for RHA280 and RHA801 were presumed to be BB and bb, respectively.
The RILs were field grown at Corvallis, OR, in the summers of 2000 and 2001 in 6.1-m-long rows spaced 0.9 m apart. Two replications of the RILs and parent inbred lines were planted in randomized complete blocks in May of both years. Fifty-six kilograms per hectare of 20N-10P-15K-7S fertilizer was broadcast preplant and incorporated during the primary tillage operation. The seed bed was prepared by disking and harrowing, and seeds were planted 2.54 cm deep. The within-row seeding rate was 40 seeds per 6.1 m. Plants were hand-thinned to produce a within row spacing of one plant per 0.3 m 2 wk postemergence. Capture [bifenthrin = (2-methyl-3-phenyl-phenyl)methyl 3-(2-chloro-3,3,3-trifluoro-prop-1-enyl)-2, 2-dimethyl-cyclopropane-1-carboxylate, 0.11 kg ai [ha.sup.-1]] and Hoelon {diclofop-methyl = methyl (RS)-2-[4-(2,4-dichlorophenoxy)phenoxy]propionate, 2.35 L [ha.sup.-1]} were applied postplant, preemergence mixture for broadleaf and grass weed control and were activated by a minimum of 1.27 to 2.54 cm of water applied through sprinkler irrigation, rainfall, or both. Subsequent to thinning, 100.8 kg per ha of N was applied by banding urea (46N-0-0). The crop was sprinkler irrigated at 2-wk intervals throughout the first 12 wk of the growing season; 1.28 to 2.54 cm of water was applied at each interval. Capitula were harvested in late September of both years, dried in a forced-air gas seed drier for 48 to 72 h at 42 to 44[degrees]C, threshed, and cleaned.
The RILs were phenotyped at the onset of flowering for presence and absence of apical branching (20 plants per RIL per replication were phenotyped). Four to 12 physiologically mature seeds per RIL were visually phenotyped for hypodermal and phytomelanin pigments in the pericarp by dissecting through successive pericarp cell layers (epidermis, hypodermis, and phytomelanin) as described by Leon et al. (1996). Capitula were harvested from 10 individuals per RIL per replication. Primary capitula were harvested from unbranched RILs, whereas primary and two secondary capitula were harvested from branched RILs. Kernel weight (kwt), pericarp weight (pwt), and kernel-to-pericarp weight ratio (kpr = kwt/ pwt) were measured on 10 seeds per RIL per replication produced in 2000. The 10 seeds per RIL were manually dehulled with a scalpel and separated into kernel and hull fractions. Five-gram samples of seeds were randomly drawn from each replication and pooled into a single 10-g sample for seed oil concentration (soc) analyses by nuclear magnetic resonance (Leon et al., 1995). We measured seed length (sl), width (sw), and depth (sd) on 10 randomly sampled seeds per RIL per replication and 100-seed weight (swt) on 100 randomly sampled seeds per RIL per replication.
We constructed a genetic linkage map for the QTL analysis using three phenotypic loci...
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