The primary source (S cytoplasm) of cytoplasmic-genic male sterility (CMS) used to produce hybrid-onion (Allium cepa L.) seed traces back to a single plant identified in 1925 in Davis, California. Many open-pollinated populations also possess this cytoplasm, creating an undesirable state of cytoplasmic uniformity. Transfer of cytoplasms from related species into cultivated populations may produce new sources of CMS. In an attempt to diversify the cytoplasms conditioning male sterility, the cytoplasm of Allium galanthum Kar. et Kir. was backcrossed for seven generations to bulb-onion populations. The flowers of galanthum-cytoplasmic populations possess upwardly curved perianth and filaments with no anthers, making identification of male-sterile plants easier than for either S- or T-cytoplasmic male-sterile onion plants. Mean seed yield per bulb of the galanthum-cytoplasmic populations was measured in cages using blue-bottle flies (Calliphora erythrocephala Meig.) as pollinators and was not significantly different from one of two S-cytoplasmic male-sterile F1 lines, a T-cytoplasmic male-sterile inbred line, or N-cytoplasmic male-fertile lines. Male-sterile lines possessing either the S or galanthum cytoplasm were each crossed with populations known to be homozygous dominant and recessive at the nuclear locus conditioning male-fertility restoration of S cytoplasm and progenies were scored for male-fertility restoration. Nuclear restorers of male fertility for S cytoplasm did not condition male fertility for the galanthum-cytoplasmic populations. It is intended that these galanthum-cytoplasmic onion populations be used as an alternative male-sterile cytoplasm for the diversification of hybrid onion seed production.
D.L. Leite and M.J. Havey
Hybrid leek (Allium ampeloprasum) is significantly more uniform and higher yielding than open-pollinated populations. Because leek has perfect flowers, a male-sterility system is required to produce hybrid seed economically. No cytoplasmic male sterility (CMS) has been described in leek. Restriction fragment length polymorphisms (RFLPs) in the chloroplast and mitochondrial genome have correlated with the expression of CMS in many crops. We undertook restriction-enzyme analyses of the chloroplast and mitochondrial DNAs to identify polymorphic organellar genomes among 65 accessions of cultivated leek. Polymorphisms were detected in the chloroplast and mitochondrial genomes. Reciprocal crosses were generated to establish the transmission of the organellar genomes of leek.
M.J. Havey and O.H. Bark
Open-pollinated (OP) onion (Allium cepa L.) cultivars are primarily in normal (N) fertile cytoplasm; however, specific cultivars possess both N and sterile (S) cytoplasm or are exclusively in S cytoplasm. It is unclear whether the presence of S cytoplasm in OP cultivars is due to ancient or recent introduction or both. Restriction-enzyme analysis of the chloroplast DNA established that S cytoplasm has been introgressed into OP onion cultivars since its discovery in 1925. `Valencia Grano' (released in 1927), `New Mexico Early Grano' (1931), `Texas Early Grano (TEG) 502' (1947), and `Temprana' (1979) are in N cytoplasm; S cytoplasm was introduced into the population `TEG 502 PRR', and subsequent selections (`NuMex BR1' and `NuMex Sunlite') are in S cytoplasm. The inbred `TEG 951 C' and `Texas Grano 1015Y' possess a mixture of N and S cytoplasm and, because these two onions originated from self-pollinations of single plants, bulb or seed mixtures or both must have occurred.
James M. Bradeen and Michael J. Havey
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.
Michael J. Havey and William M. Randle
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.
James M. Bradeen and Michael J. Havey
The genus Allium contains about 500 species, several of which have been cultivated for millennia. Despite its long history of cultivation and its worldwide economic importance, little is known phylogenetically about Allium. Identification of the likely progenitor of A. cepa (the bulb onion) will focus future collection efforts on wild germplasm that may be useful in the genetic improvement of the bulb onion. Several classification schemes based on morphological characteristics have been proposed for A. cepa and its presumed closest relatives. None of these schemes has been definitive. Nuclear restriction fragment length polymorphisms (RFLPs) were identified among Allium species in sections Cepa and Phyllodolon. These were used to unbiasedly estimate phylogenetic relationships.
M.J. Havey, J. McCreight, W. Rhodes and G. Taurick
The cucurbits have several-fold size differences in their mitochondrial genomes. Watermelon possesses a relatively small mitochondrial genome of 330 kb. Squash has a larger mitochondrial genome of 840 kb. Cucumber and melon possess huge mitochondrial genomes of 1500 and 2400 kb, respectively. We demonstrated predominately paternal transmission of the mitochondrial genome in cucumber. Squash shows maternal transmission of the chloroplast genome. We generated reciprocal crosses and identified restriction fragment length polymorphisms in the chloroplast and mitochondrial genomes of melon, squash, and watermelon to establish their transmission. Our analyses also revealed that intergenomic transfers contributed to the evolution of extremely large mitochondrial genomes.
Joseph J. King, James M. Bradeen and Michael J. Havey
Nuclear RFLPs were used to estimate relationships among 14 elite commercial inbreds of bulb onion (Allium cepa) from Holland, Japan, and the United States. Variability for known alleles at 75 RFLP loci and 194 polymorphic fragments revealed by 69 anonymous cDNA probes and a clone of alliinase were scored to yield genetically characterized and uncharacterized data sets, respectively. The inbred onion populations possessed more than two alleles at 20 of 43 (46%) codominant RFLP loci. Relationships among the inbreds were estimated by cluster analysis of simple-matching (genetically characterized data) and Jaccard (genetically uncharacterized data) coefficients using the unweighted pair group method and agreed with known pedigrees. RFLPs confidently distinguished among elite inbreds within and between specific market classes. RFLP profiles for virtual hybrids were computer-generated by combining gametic arrays among inbreds of the same market class and analyzed as described above. Allelic and genetically uncharacterized RFLPs confidently distinguished among these hybrids, even though heterozygosity for many markers produced a majority of monomorphic fragments. We randomly sampled decreasing numbers of RFLPs from the complete data sets and calculated simple-matching and Jaccard distances, noting the numbers of probes that were unable to distinguish any two inbreds or hybrids. As few as 10 polymorphic probe-enzyme combinations distinguished among all the inbreds and samples of 20 genetically characterized or 10 genetically uncharacterized clones distinguished all the virtual hybrids. This study demonstrated that the previously reported few RFLPs observed among open-pollinated (OP) onion populations were due to the highly heterozygous nature of the OP population.
G. Schroeck, I.L. Goldman and M.J. Havey
Since the 1930s, more than 130 inbred lines and 60 hybrid cultivars of onion have been released in the public sector in the United States. Other than breeder's reports from the period 1946-1965 and anecdotal information kept by onion workers, no systematic treatment of the pedigree of public onion germplasm releases has been developed. The objective of this research was to collect, characterize, and display the genetic relationships among more than 200 public onion germplasm sources used in the United States since 1931. Pedigree information revealed that most modern onion cultivars in the United States descend from a few open-pollinated populations brought to this country by immigrants. For example, selection in the open-pollinated populations Common Yellow and Silverskin by onion farmers in the eastern U.S. resulted in the formation of Yellow Globe Danvers, which was a precursor to virtually all Eastern storage onion germplasm in the U.S. Open-pollinated populations such Yellow Globe Danvers, Valencia, Sweet Spanish, Bermuda, and Grano formed the foundation germplasm for the first public U.S. onion breeding programs. Findings from this study suggest a relatively narrow germplasm base of public onion germplasm in the United States; however, this narrow pool coexists alongside significant gains through scientific breeding efforts, particularly during the past 75 years.
C.R. Galmarini, I.L. Goldman and M.J. Havey
Solid content is an important characteristic related to onion flavor, texture, and storability and has practical importance for the dehydration industry. Among the salutary effects of Allium vegetables on the cardiovascular system is the inhibition of platelet activity. Platelets play a key role in thrombosis and acute coronary syndromes because they facilitate blood coagulation. Pungency is also an important commercial trait. A 138-point genetic map is being used to identify and estimate the magnitude of quantitative trait loci controlling solid content, pungency, and health-enhancing attributes of onion. QTL controlling pungency, total solids, soluble solids, and antiplatelet activity were estimated using 54 F3 families, derived from the cross between Brigham `Yellow Globe 15-23' (BYG15-23) and `Ailsa Craig' (AC43). The families, the two parents, and controls were evaluated in four environments, at Palmyra or Randolph, Wis., during 1997 and 1998, on muck soils. For the analyzed traits there is evidence of trangressive segregation, the distributions are, in general, skewed towards the BYG 15-23 parent. Our results confirmed the existence of strong phenotypic correlations among the traits under study. QTL data available also suggest the existence of significant correlations between markers and the traits under study. Most of the markers that are significant for pungency and antiplatelet activity are also significant for solids, suggesting that these characteristics may be controlled by the same chromosome regions.