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  • Author or Editor: Michael J. Havey x
  • Journal of the American Society for Horticultural Science x
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Maintainer lines are used to seed propagate male-sterile lines for the development of hybrid onion (Allium cepa L.) cultivars. The identification of maintainer lines would be more efficient with molecular markers distinguishing genotypes at the nuclear male-fertility restoration (Ms) locus. Ms has been mapped to chromosome 2 of onion and linked genetic markers identified. However, linkages between these markers and Ms were detected using F2 or BC1 families at maximum linkage disequilibrium and, for many markers, their efficacy to predict genotypes at Ms in onion populations at or near linkage equilibrium remains unknown. In this research, near isogenic lines homozygous-dominant and -recessive at Ms were developed and screened for 930 single nucleotide polymorphisms (SNPs). Three SNPs tightly linked on chromosome 2 remained in linkage disequilibrium with genotypes at Ms among randomly selected plants from three open-pollinated populations of onion as well as among a collection of inbred lines. These SNPs should be useful for selection of the recessive ms allele to aid in the development of maintainer lines for hybrid onion development.

Free access

The most common bulb colors of onion (Allium cepa) are red, yellow, and white; chartreuse is a relatively rare bulb color conditioned by the homozygous recessive genotype at the G locus. In this research, plants with chartreuse bulbs were crossed with inbreds with yellow bulbs to develop segregating families for genetic mapping of the G locus. For all of 17 F2 families, segregations for yellow vs. chartreuse bulbs fit the expected 3:1 ratio (P > 0.05). DNAs were isolated from one F2 family and genotyped for single nucleotide polymorphisms (SNPs) to produce a genetic map of the G locus and 380 SNPs, of which 119 SNPs have not been previously mapped. Segregations for yellow vs. chartreuse bulbs placed the G locus at the end of chromosome 7 at 6.7 cM from the nearest SNP (isotig28625_2789). This codominant SNP marker linked to the G locus should be useful for introgression of recessive chartreuse bulb color into diverse onion populations for commercial production of this uniquely colored onion.

Open Access

The formation of onion (Allium cepa) bulbs is affected by photoperiod length and onion germplasm is commonly classified as short- (SD), intermediate-, or long-day (LD) types. The objectives of this study were to develop a segregating family from a cross between doubled haploids (DHs) of LD and SD onions and complete genetic analyses of bulb shape and volume, as well as daylength effects on bulbing. DH parents and F1 and F2 progenies were grown in a greenhouse under lengthening days. The diameters of the neck constriction and pseudostems were measured weekly and their ratio was used as the determinant of bulbing. Bulbs were harvested when the foliage collapsed and the diameters and heights of individual bulbs were measured and used to calculate a shape index (diameter divided by height) and bulb volume. Single nucleotide polymorphisms (SNPs) were genotyped and a genetic map of 112 SNPs constructed. Genetic analysis revealed two highly significant quantitative trait loci (QTL) affected bulbing under increasing daylengths, and both QTL showed significant additive effects with no dominance. One highly significant QTL was detected for bulb-shape index and explained 30% of phenotypic variation for bulb shape. Three additional QTL were slightly above the significance threshold, and together these four QTL explained over 50% of the phenotypic variation for bulb shape. No significant QTL were detected for bulb volume. These results reveal that bulb-shape and daylength effects on bulbing are relatively simply inherited, and this research should facilitate introgression of traits between onion populations of different daylength sensitivities and efforts to modify bulb shape.

Open Access

Natural variation exists in onion (Allium cepa L.) for amounts and types of epicuticular waxes on leaves. Wild-type waxy onion possesses copious amounts of these waxes, whereas the foliage of semiglossy and glossy phenotypes accumulates significantly less wax. Reduced amounts of epicuticular waxes have been associated with resistance to onion thrips (Thrips tabaci Lindeman), an important insect pest of onion. A segregating family from the cross of waxy and semiglossy onions was used to map single nucleotide polymorphisms (SNPs) and identify chromosome regions affecting amounts and types of epicuticular waxes as measured by gas chromatography–mass spectrometry. The amount of the primary epicuticular wax on onion leaves, hentriacontanone-16, was controlled by one region on chromosome 5. One region on chromosome 2 affected concentrations of several primary fatty alcohols. Results indicate that the region on chromosome 2 may be associated with the acyl reduction pathway, and the region on chromosome 5 may affect the decarbonylation pathway of epicuticular wax biosynthesis. Because lower amounts of epicuticular waxes are recessively inherited, SNPs tagging regions on chromosomes 2 and 5 will be useful for marker-assisted breeding to vary amounts and types of epicuticular waxes on onion foliage with the goal to develop cultivars resistant to onion thrips.

Free access

Single nucleotide polymorphisms (SNPs) were genotyped using a high-density array and DNAs from individual plants of important onion (Allium cepa L.) populations from major production regions and from the likely progenitor of onion, Allium vavilovii Popov et Vved. Genotypes at 1226 SNPs were used to estimate genetic relationships among these populations and revealed close associations among onions grown in Europe and those in North America, South America, and eastern Asia, supporting paths of introduction from Europe to the Americas and Asia. ‘Nasik Red’ is a population grown on the Indian subcontinent and was divergent from onions of European origin. Frequencies of SNPs among and within populations were used as a measure of informativeness, and 199 commonly polymorphic SNPs were identified distributed across the eight chromosomes of onion. These SNPs will be useful for estimations of relatedness among broader collections of onion populations, mapping of important phenotypes, fingerprinting of inbred lines and hybrids, and quality control of seed lots.

Free access

Development of two-way onion (Allium cepa L.) hybrids is difficult due to poor seed yields on inbred female parents. Seed yield of onion is affected by inbreeding depression and the seed-production environment. A standard diallel was used to estimate combining abilities for seed yield among seven inbred onion lines. Males and hybrids differed significantly (P < 0.05) for seed yields. Combinations of relatively high-by-high seed-yielding inbred parents were not always the best combinations; combinations of medium-by-medium or medium-by-high seed yielders also produced good F1 seed yielders. For the seven inbred lines, significant correlations (P < 0.05) were observed between mean seed yield per bulb and scape height. Parent-offspring regressions revealed no significant relationship between seed yields of randomly selected, open-pollinated bulbs and their S1 families. Results indicate that relative seed yields of individual bulbs after self-pollination cannot be used to predict seed yields of progeny families. However, the seed yield of inbred lines of onion may reflect the potential seed yield of F1 male-sterile lines.

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Cytoplasmic male sterility (CMS) in onion (Allium cepa L.) is conditioned by the interaction of the male-sterile (S) cytoplasm with recessive alleles at a single nuclear male-fertility restoration locus (Ms). In order to seed propagate male-sterile plants (S msms), onion breeders must identify maintainer lines possessing normal (N) male-fertile cytoplasm and homozygous recessive at the Ms locus (N msms). Molecular markers have been identified distinguishing N and S cytoplasms and closely linked to the nuclear Ms locus. In this study, we evaluated testcross progenies from randomly selected N-cytoplasmic plants from three open-pollinated populations for nuclear restoration of male fertility over at least three environments. The Ms locus and linked restriction fragment length polymorphisms (0.9 and 1.7 cM) were at linkage equilibrium in all three open-pollinated onion populations, indicating that these linked markers cannot be used to identify maintaining genotypes in open-pollinated onion populations. However, cytoplasmic evaluations were effective in reducing the number of testcrosses required to identify CMS-maintaining genotypes.

Free access

Hybrid leek is more uniform and higher yielding than open-pollinated cultivars and is presently produced by asexual propagation of a genic male-sterile plant. A cheaper method to produce hybrid leek seed would be a system of cytoplasmic-genic male sterility (CMS). Restriction fragment length polymorphisms (RFLPs) in the organellar genomes have correlated with CMS in many crops. We undertook gel-blot analyses of the chloroplast and mitochondrial DNAs to assess cytoplasmic diversity among 62 accessions of the major cultivated forms of Allium ampeloprasum L. (leek, kurrat, and great-headed garlic). No polymorphisms were detected in the chloroplast genome of leek and kurrat. Three accessions of leek and one of kurrat possessed one or two of seven polymorphic mitochondrial probe-enzyme combinations. Great-headed garlic differed from leek and kurrat for six polymorphisms in the chloroplast genome and for many mitochondrial probe-enzyme combinations. Our analyses revealed few organellar polymorphisms among accessions of leek and kurrat, reducing the probability that selection of polymorphic cytoplasms will reveal CMS in leek.

Free access

A factorial mating design, using three male-sterile F1 lines in testcrosses with a sample of open-pollinated (OP) onion populations, was used to estimate combining abilities and heterosis for bulb yield, size, storage ability, pungency, soluble solids content (SSC), and water loss after 3 months in storage. Samples of testcross bulbs were flowered and scored for fertility to estimate frequencies of the nuclear allele maintaining cytoplasmic male sterility. General combining ability (GCA) estimates for OP populations (males) were significant (P < 0.05) for yield, SSC, and proportion of bulbs with diameters >7.5 cm. GCA estimates for female testers were significant for storage ability and proportion of bulbs with diameters <5.0 cm. Male × female interactions (specific combining ability estimates) were significant for SSC and storage ability. Our analyses did not reveal any storage population from which inbreds would likely yield significantly better with the male-sterile tester lines. Spanish OP populations tended to produce testcrosses with larger bulbs, lower pungency and SSC, and poorer storage ability. Heterosis estimates were most often significant for yield and SSC; less often for pungency, storage ability, and bulb size; and not significant for water loss in storage. Overall, significant GCA estimates indicate that superior onion inbreds and populations may be developed using recurrent-selection strategies that increase the frequency of desirable alleles with additive effects.

Free access

Commercial bulb-onion (Allium cepa L.) growers often complain that hybrids they have grown successfully for a few years fail to perform at the expected level. Inbreds used to produce hybrid-onion seed rarely have been self-pollinated for more than two generations and retain a high level of heterozygosity. Over time, selection, drift, or contamination of inbreds may contribute to disappointing hybrid performance. We identified randomly amplified polymorphic DNA (RAPD) between two inbred onion lines, demonstrated their Mendelian inheritance, and tried to distinguish among and examine changes in independently maintained, publicly released inbred lines of onion. We observed poor agreement between data sets based on genetically characterized and uncharacterized RAPD markers. Our analyses used only genetically characterized RAPD markers and revealed that contamination, in addition to-drift and/or selection, likely contributed to differences among independently maintained, publicly released inbreds. However, RAPD markers were not able to distinguish confidently among four related inbreds. RAPD markers will be useful in Allium genetics and breeding, but identifying and characterizing reliable polymorphisms is critical.

Free access