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Michael J. Havey and Borut Bohanec

The U.S. Department of Agriculture, Agricultural Research Service, announces the release of onion inbred ‘B8667 A&B’ and synthetic population ‘Sapporo-Ki (SKI) -1 A&B’. Both of these releases represent a cytoplasmic male-sterile (A) line with its maintainer (B) for seed propagation of male-sterile plants. ‘B8667 A&B’ is intended for production of red, long-day, well-storing hybrids. ‘SKI-1’ combines the earliness of the Japanese population ‘Sapporo-Ki’ with maintenance of cytoplasmic male sterility. The long-day synthetic population ‘Onion Haploid-1’ (‘OH-1’) is a joint release of the U.S. Department of Agriculture, Agricultural Research Service, and the University of Ljubljana, Slovenia. ‘OH-1’ is intended to serve as a responsive control for extraction of gynogenic haploids of onion.

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Borut Bohanec, Marijana Jakse, and Michael J. Havey

The production of doubled haploid plants is desirable as an alternative to sexual inbreeding of longer-generation crops. Onion (Allium cepa L.) is a biennial plant and amenable to the production of gynogenic haploids. Although a strong population effect has been observed for gynogenic haploid production, there is no report describing the genetic basis of greater haploid production in onion. We evaluated over years the frequency of haploid production among onion inbreds and identified lines showing significantly (P < 0.01) greater production of haploids. The onion inbreds, B0223B and B2923B, produced the highest mean frequencies of haploids so far reported. Hybrid families from crosses of B2923B with inbreds having relatively low haploid production showed significantly higher haploid production than the low-producing parent and significantly lower haploid production than B2923B. Plants from B0223B and B2923B with established rates of haploid production were testcrossed and/or self-pollinated. The F1 family from B1717A-1 × B2923B-3 showed rates of haploid production slightly greater than the low parent (B1717A-1) and significantly less than the high parent (B2923B-3). Self-pollination of plants from B2923B showing relatively high rates of haploid production generated S1 progenies also producing relatively high frequencies of haploids. Selfed progenies from plant B2923B-6 showed a high mean rate of haploid production (56.8% ± 14.5%) and, more importantly, the highest level of haploid production (82.2%) reported for any single onion plant. These results indicate that relatively high haploid production, at least for B2923B, was quantitatively inherited with dominance towards low production. We suggest S1 family selection as an effective method to increase gynogenic haploid production of onion populations.

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Kristina Košmrlj, Jana Murovec, and Borut Bohanec

Production of hull-less seeds of styrian oil pumpkin (Cucurbita pepo ssp. pepo var. styriaca) is increasing as a result of demand for aromatic seed oil and for other uses. Hybrid cultivars have recently been released but a method for haploid induction has not been established. We focused on the development of a parthenogenetic haploid induction protocol based on pollination with pollen irradiated with X-ray radiation at 0, 50, 100, 150, 200, 300, and 350 Gy. Fruit set largely decreased at 200 Gy, whereas a decline in embryo formation was observed at 100 Gy. Various accessions were tested as the female parent or as the pollen donor and large differences were found. The best parthenogenetic response was found in ‘Turkey #2’ (10.0%), ‘Gleisdorfer Ölkürbis’ (4.4%), and ‘Naked Seed’ (3.9%), whereas ‘GL Opal’ and ‘White Acorn’ were efficient as pollen donors. The ploidy level of 3830 putative parthenogenetic embryos was determined using flow cytometry. Four ploidy levels (n, 2n, 3n, and 4n) were found with the majority being diploid. Interestingly, a significant proportion was determined to be tetraploid and this was clearly correlated with increased radiation delivered to pollen grains. Using selected simple sequence repeat markers on diploid embryos, no spontaneous chromosome doubling could be confirmed. In this study, haploid induction in styrian oil pumpkin was elaborated for the first time. We also showed that X-ray pollen irradiation provides an alternative to gamma radiation treatment, yielding a sufficient percentage of haploid plantlets.

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Borut Bohanec, Marijana Jakše, Predrag Šesek, and Michael J. Havey

Bulbous leek-like plants are a poorly defined group usually assigned to the Allium ampeloprasum complex. Studies were initiated to determine the origin of an unusual bulbous accession received in Shanxi province in China, where it was used in diet as garlic but propagated by seeds, and to genetically compare this accession with morphologically similar plants from Europe. Genetic analyses included karyotypes and genomic in situ hybridization, pollination to leek, genome size determination and nuclear rDNA and plastid DNA polymorphisms. Results revealed that this agriculturally interesting accession from China is a so far unknown variant within tetraploid A. ampeloprasum cultivated taxa. We also observed that great-headed garlic did not share derived states in the chloroplast with leek, revealing that this cultivated plant does not possess the cytoplasm of leek or garlic, while its 1C genome size was 17% bigger than those of studied leek and bulbous-leek accessions.

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Marijana Jakše, Pablo Hirschegger, Borut Bohanec, and Michael J. Havey

Although haploid induction has been used in onions (Allium cepa L.) for over 20 years, several obstacles limit its use in plant breeding programs. To address these limitations, we evaluated the responsiveness of doubled haploid (DH) lines and their selfed progenies, an alternative protocol for chromosome doubling using somatic regeneration of haploid lines, and pollen viability of DH lines. Twenty-one DH lines were self pollinated and tested for haploid induction in the second generation. Among the DH lines, 18 lines showed an average of 20% decrease in gynogenic responsiveness compared with the original lines, while three lines registered an average increase of 5.7%. Using a two-step induction/regeneration procedure, 8,589 somatic regenerants were obtained from 16,170 flower buds from haploid plants, and shoot culture was established. A more laborious procedure using extraction of ovaries in the regeneration stage was found equal to flower bud culture. Chromosome doubling via somatic regeneration was found to be 83% and 100% efficient when the source material was haploid or mixoploid, respectively. Based on the results achieved in this and previous studies, an alternative protocol for chromosome doubling of gynogenic haploids is proposed.