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  • Author or Editor: Michael J. Havey x
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PstI-genomic and cDNA clones revealing mapped restriction fragment-length polymorphisms (RFLP) in cucumber (Cucumis sativus L.) were sequenced in order to ensure that these clones remain available and to determine if any clones showing genetic linkage in cucumber are physically linked in Arabidopsis thaliana. Sequence comparisons using translated searches revealed that 80% of the cucumber cDNA clones showed significant (≤e-20) similarities to Arabidopsis expressed sequence tags (ESTs) or genomic sequences, as opposed to relatively few (32%) of the cucumber genomic clones. Two clones revealing RFLPs linked at 2 cM in cucumber showed significant (≤e-20) similarities to sequences separated by 347,616 basepairs on chromosome 4 of Arabidopsis.

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Restriction fragment length polymorphisms (RFLPs) in the chloroplast and nuclear genome are useful for estimation of phylogenetic relationships. Fifteen mutations at restriction enzyme sites in the chloroplast DNA were discovered. The wild species A. oschaninii and A. vavilovii were identical to A. cepa for all mutations. These species represent sources of wild germplasm closely related to the bulb onion. Nuclear RFLPs are now being used to estimate the genetic distances between accessions of A. oschaninii A. vavilovii, and open-pollinated populations of the cultivated bulb onion.

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The amounts and types of epicuticular waxes on onion (Allium cepa) leaves affect feeding damage by onion thrips (Thrips tabaci), a serious insect pest of onion. This study used gas chromatography mass spectrometry (GCMS) to measure amounts of epicuticular waxes on foliage of two plants from each of 50 plant introductions (PI) and inbred lines with high (waxy) and low (glossy) amounts of wax. Wax amounts on leaves of the same plants were measured twice (once in the greenhouse and once after moving plants outside) and were significantly (P < 0.01) correlated; however, wax amounts on leaves of plants grown in the greenhouse were approximately twice that of the same plants grown outside. Hentriacontanone-16 (H16) was the predominant wax on leaves of all PIs except PI 289689, and amounts of H16 were significantly correlated with amounts of fatty alcohols and total wax. Five plants from 17 of the PIs were grown in the greenhouse, wax amounts measured using GCMS, and results were significantly correlated with earlier evaluations. Results indicate that measurements of waxes on onion foliage should occur under protected conditions to better characterize phenotypic variation, and selection of higher amounts of waxes other than H16 may be effective toward the development onions suffering less thrips damage.

Open Access

Different sources of cytoplasmic male sterility (CMS) are used to produce hybrid onion seed. The most commonly used source of CMS in onion is S cytoplasm (S-CMS), and male fertility is restored by a dominant allele at the nuclear male-fertility restoration locus (Ms). Male-sterile plants possess S cytoplasm and have the homozygous recessive genotype at Ms; seed propagation of male-sterile plants is possible by crossing with a male-fertile maintainer plant or inbred possessing normal (N) male-fertile cytoplasm and the homozygous recessive at the Ms locus (N msms). Some commercially important onion populations possess S-CMS and high frequencies of the dominant Ms allele, eliminating the possibility to develop maintainer lines. An alloplasmic source of CMS (Gal-CMS) was developed by backcrossing the cytoplasm of Allium galanthum into the nuclear background of onion. The advantage of Gal-CMS is that the dominant allele at Ms does not restore male fertility, making this source of CMS useful for the development of male-sterile lines from populations possessing S cytoplasm and dominant allele(s) at Ms. In this research, a single nucleotide polymorphism unique to the cytoplasms of A. galanthum and Gal-CMS was identified, useful to distinguish Gal-CMS from other onion cytoplasms.

Open Access

Phytophthorainfestans is the casual agent of late blight and is a major threat to potato production worldwide. There are no curative control agents available and resistance genes offer promise in controlling late blight. To date, the primary source of late-blight resistance has been from hexaploid (6x) [4 Endosperm Balance Number (EBN)] Solanum demissum. Mexican diploid (2x) (1EBN) Solanum species possess a wealth of late-blight resistances, but have been neglected due to crossing barriers. Manipulation of EBN and ploidies should allow integration of 2x (1EBN) germplasm into cultivated potato. Synteny between late-blight resistance loci from Solanum species of disparate ploidies and EBNs may facilitate the identification of unique resistance alleles and loci. Isolate MSU96 (US8/A2) of P. infestans revealed a late-blight resistance locus (Rpi1) from 2x(1EBN) S. pinnatisectum (PI 253214) that mapped to chromosome seven (MGG 265:977-985). MSU96 was also avirulent on the late-blight differential R9-Hodgson 2573 (LB3), revealing the presence of the avirulence gene for R9 originating from S. demissum. To test the relationship between Rpi1 and R9, we evaluated a family segregating for R9 and revealed that it does not map to chromosome seven. The independent inheritance of R9 and Rpi1 indicates that Rpi1 is a unique resistance locus. We are conducting a variety of crossing schemes to introgress Rpi1 into cultivated potato.

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The bulb onion (Allium cepa L.) is a diploid with an very large nuclear genome of 15300 Mbp/1C (107× arabidopsis, 16× tomato, 6× maize). We developed a low-density genetic map with morphological, RAPD, and RFLP markers to examine genome organization and to study QTL controlling phenotypically correlated bulb quality traits. A mapping population of 58 F3 families was derived from a cross of the inbreds Brigham Yellow Globe 15-23 (BYG) × Alisa Craig 43 (AC). These inbreds are distinct in solids, storability, pungency, and bulb shape. Analysis of 580 RAPD primers detected 53 (9%) polymorphisms between BYG and AC, but only 12 (2%) segregated at expected ratios among F3 families. Using probes from onion cDNA libraries and four restriction enzymes, 214 RFLPs were identified between mapping parents. A 112-point map includes 96 RFLPs, 13 RAPDs, a locus controlling complementary red bulb color, and two loci hybridizing with a clone of the enzyme alliinase (EC 4.4.1.4), which produces the flavors characteristic of Allium species. Duplicated loci were detected by ≈25% of RFLP probes and were unlinked, loosely linked (2 to 30 cM), or tightly linked (<2 cM). This frequency of duplication was comparable to species with polyploid ancestors (paleopolyploids) and was higher than that found in most true diploids. However, the distribution of duplicated loci suggests that, in contrast to whole genome duplications typical of paleopolyploids, the contemporary size and structure of the onion genome may be a product of intrachromosomal duplications (cryptopolyploidy) and subsequent structural rearrangements.

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Cytoplasmic-genic male sterility (CMS) is used to produce hybrid onion seed. For the most widely used source of CMS in onion, male sterility is conditioned by the interaction of sterile (S) cytoplasm and the homozygous recessive genotype at a single nuclear male-fertility restoration locus (Ms). Maintainer lines used to seed-propagate male-sterile lines possess normal fertile (N) cytoplasm and the homozyous recessive genotype at the Ms locus. Presently, it takes 4 to 8 years to establish if maintainer lines can be extracted from an uncharacterized population or family. We previously developed a PCR marker useful to distinguish N and S cytoplasms of onion. To tag the nuclear male-fertility restoration locus (Ms), we evaluated segregation at Ms over at least three environments. Segregations of AFLPs, RAPDs, and RFLPs revealed molecular markers flanking the Ms locus. We are working to convert these linked molecular markers to nonradioactive PCR-based detection. The organellar and nuclear markers were used to select plants from open-pollinated onion populations and determine if the number of test-crosses required to identify maintaining genotypes.

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The production of hybrid-onion seed depends on cytoplasmic male sterility (CMS) systems. The male-sterile line is seed propagated using a normal (N) cytoplasmic maintainer line homozygous recessive at the nuclear male-fertility restoration locus (MS). Because of onion's biennial generation time, 4 to 8 years are required to establish the genotype at the MS locus. The development of maintainer lines would benefit greatly from a genetic marker linked to the MS locus. Such a marker would allow breeders to establish the nuclear genotype in seedlings and flower only those plants that are maintainers (N msms) or plants that can be used to develop maintainers (N MSms), reducing the number of plants to be testcrossed or backcrossed to a sterile line. We evaluated restriction fragment length polymorphisms (RFLPs), random amplification of polymorphic DNA (RAPDs), and amplified fragment length polymorphisms (AFLPs) to tag the chromosome region carrying the MS locus. No RAPDs or RFLPs cosegregated with MS. AFLP markers were identified that phenotypically correlated with restoration of male fertility.

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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.

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