Pollen competition in onion (Allium cepa L.)

Summary Pollen mixtures with two components, one of which carried a dominant marker gene for red or white bulb skin colour, were used to pollinate flowers on onion umbels from several cultivars. Scoring progenies for the marker revealed that pollen components differed in their ability to effect fertilization, suggesting that gametophytic competition can occur in onions. In many cases, self-pollen appeared to have a competitive advantage over cross-pollen. Both of the male components and the female parent played a part in determining the final ratio obtained from a mixed pollination.Crossed seeds were slightly but significantly heavier than selfed seeds in nine out of ten umbels studied.     

Variation of gynogenesis ability in onion (Allium cepa L.)

Onion varieties from Northern-Europe, Eastern-Europe, Southern-Europe and the U.S. including populations, inbreds, synthetics and clones were tested for their gynogenesis ability for three years: 1993-1995. Embryos were induced in Petri dishes using in vitro culture of flower buds. Embryo rate, regeneration rate, survival rate, embryo quality and ploidy level of regenerants are described. Important genotype and variety effects are pointed out; embryo production and plant regeneration ranged from 0 to 17% and from 0 to 11% of inoculated flowers, respectively. Highest results were obtained with inbreds and synthetics for embryo production as well as for plant regeneration. Populations expressed various responses which were generally low. Geographic origin and year effects were covered by the effects of genetic structure of varieties and specific genotype of donor plant. Among regenerants, 80% were haploid and 13% were spontaneous diploid. No relation has been found between general quality and ploidy level of regenerants. The choice of plant material for successful gynogenesis in onion is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gibberellins as gametocides for the common onion (Allium cepa L.). III. GA4+7 and pollen viability

Summary The gametocidal effect of GA₄₊₇ in onion is strong in respect of pollen production but very weak or absent in respect of pollen viability. Large scale application of GA₄₊₇ for the production of onion hybrid seed is still impossible because of low seed yield and high costsof required chemicals. On a small scale, however. GA₄₊₇ can be used very well to emasculate breeding material.Because of switching over to the gibberellin terminology of plant physiologists, this title slightly deviates from that of the two previous articles.     

Gibberellic acid as a gametocide for the common onion (Allium cepa L.). II. The effect of GA 4/7

Summary The gametocidal effect of Berelex GA 4/7 on onions was investigated. Frequent spraying during the bolting period with relatively high concentrations (0.1 and 0.3%) resulted in a very high percentage of completely male sterile plants in the beginning of the flowering period. The gametocidal effect was accompanied by a considerable reduction in seed production.Differences in reaction to GA 4/7 between the three varieties involved were small.     

Introgression of leaf blight resistance from Allium roylei Stearn into onion (A. cepa L.)

Summary Segregation analysis in BC₁ and F₂ progenies from interspecific hybrids between Allium roylei and A. cepa demonstrated that leaf blight resistance is conditioned by one single dominant gene from A. roylei. This gene, designated Bs ₁ , was unlinked to the genes Pd ₁ and Pd ₂ determining downy mildew resistance. The prospects of exploiting A. roylei as a source for disease resistances in onion breeding are very promising.     

Linkage of downy mildew resistance genes Pd 1 and Pd 2from Allium roylei Stearn in progeny of its interspecific hybrid with onion (A. cepa L.)

Summary Male meiosis of the interspecific hybrid between Allium roylei and A. cepa is undisturbed relative to its parents. Based on meiotic data, A. roylei is concluded to be a closer relative of A. cepa than A. fistulosum. Segregation ratios for downy mildew resistance among BC₁ and F₂ progenies from the F₁ between A. roylei and A. cepa indicate the presence of two dominantly inherited, weakly linked nuclear resistance genes, Pd ₁ and Pd ₂, in A. roylei (recombination frequency 0.32±0.03). Presumably Pd ₁ and Pd ₂are the first genes described in Allium residing in one linkage group. The prospects of exploiting A. roylei in onion breeding seem very promising.     

Pre- and post-fertilization barriers to backcrossing the interspecific hybrid between Allium fistulosum L. and A. cepa L. with A. cepa

Summary To facilitate the introgression of desirable traits of Allium fistulosum into the genome of A. cepa, several accessions of the hybrid between these species were pollinated with A. cepa as the recurrent parent, and in vitro ovary and ovule culture were performed to obtain an increase in the recovery of backcross progeny. Compared to the results obtained from seed development in planta, the increase in the number of backcross progeny was generally very limited, and in some cases even a decrease was found. Raising the sucrose concentration in the ovary culture medium resulted in a higher frequency of ovules developing back seed coats but this was not followed by an increase in the number of backcross progeny obtained. Pollen tube growth of A. cepa was disturbed in the styles of the interspecific hybrids. Per ovule, frequencies of micropylar penetration exceeded frequencies of backcross progeny only to a limited extent. Hence, it was concluded that in the tested interspecific hybrid accessions the attainable gain in viable backcross progeny by the application of in vitro culture techniques is limited by strong pre-fertilization barriers acting at the level of stylar incongruity.     

Geographical distribution of allozyme patterns in shallot (Allium cepa var. ascalonicum Backer) and wakegi onion (A. × wakegi Araki)

Summary Shallot and wakegi were collected in the main islands of Indonesia, and in Japan, Korea, Taiwan, Malaysia, Thailand and Bangladesh. Five isozyme resolutions, phosphoglucomutase (PGM), glutamate oxaloacetate (GOT), glutamate dehydrogenase (GDH), esterase (EST) and peroxidase (POX) were employed for demonstrating inter-and intraspecific differences. A dendrogram separated 189 collected accessions into 25 types of wakegi onion and 18 types of shallot. All accessions of Japan, Korea and Taiwan were determined to be wakegi onion, whereas those of Bangladesh, Malaysia and Thailand were shallot. Twenty-six out of 165 Indonesian accessions indicated wakegi onion distribution in Sumatra, West Java province and in Sulawesi Island. This confirmed that there is mixed-cultivation of the two Allium species with no distinction made between them. Japan and Indonesia had respectively 12 and eight unique types of wakegi onion, while Korea had only one type. West Java showed the most various type of wakegi onion, whereas East Java had many types of shallot. Shallots collected from Bangladesh were distinetly different from those of South East Asian types.     

Chromosome and cytoplasm analyses of somatic hybrids between onion (Allium cepa L.) and garlic (A. sativum L.)

Chromosomes and cytoplasms were analyzed in two lines of a somatic hybrid between onion (Allium cepa L.) and garlic (A. sativum L.). One line of the somatic hybrid had 40 chromosomes and the other 41chromosomes. Genomic in situhybridization successfully revealed the chromosome constitution of the two lines. One line had 20 chromosomes from onion and17 chromosomes from garlic, and the other had 21 chromosomes from onion and 17chromosomes from garlic. Interestingly, both lines had three chimeric chromosomes. PCR-RFLP analyses of chloroplast and mitochondrial DNAs of both lines showed that these were identical to the onion parent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of Allium cepa L. cultivars by means of statistical analysis of C-banded chromosomes

Summary The C-banded karyotypes of six Allium cepa cultivars, including five dry onions and one shallot, were investigated. Chromosome length and banding patterns were statistically analysed. The value of C-banding for distinguishing these Allium cepa cultivars was tested by the application of a discriminant analysis. It appeared that chromosomal differences between cultivars exist, but are too small to be clearly demonstrated with moderate efforts. Large variation within plants and between plants within cultivars, presumably for the greater part caused by the applied method, obscured the variation between the cultivars.     

Independent segregation of two isozyme markers and inter-plant differences in nuclear DNA content in the interspecific backcross (Allium fistulosum L. × A. cepa L.) × A. cepa L.

Summary From backcrosses of three interspecific hybrids (A. fistulosum x A. cepa) with a. cepa 14 diploid and 2 triploid plants were recovered.In this BC1 population introgression of A. fistulosum genetic material into the A. cepa nuclear genome was studied using two isozyme markers: Got-1 and Got-2. Both loci carried two alleles. A. cepa was monomorphic for both markers. A. fistulosum was polymorphic for Got-2. Based on their Got isozyme pattern seven out of the 14 diploid BC1 plants had a recombinant genotype. The loci appeared to be unlinked. Differences were observed in nuclear DNA contents between the diploid BC1 plants, indicating that gametes produced by the interspecific hybrids contained different combinations of chromosomal material from A. cepa and A. fistulosum.Abbreviations Adh alcohol dehydrogenase - Got glutamate oxaloacetate transaminase - Idh isocitrate dehydrogenase - Pgi phosphoglucoisomerase - Pgm phosphoglucomutase     

Breeding for resistance to neck rot (Botrytis allii) in onions (Allium cepa)

Summary For 10 years the method developed by Van der Meer et al, (1970) to test neck rot resistance in onions has been applied in a breeding programme. From experiments with market varieties, inbred lines and half-sib families it is concluded that neck rot resistance as determined by this method is a quantitative character, which shows continuous variation and is in part fixable by selection. The practical importance of this type of neck rot resistance is not yet clear.Report no. 68     

Haploid induction from F1 hybrids between CMS shallot with Allium galanthum cytoplasm and common onion by unpollinated flower culture

The induction of haploid plants from F₁ hybrids between CMS shallot with Allium galanthum cytoplasm and common onion was examined. Starting with 535 unpollinated flowers cultured in B5 medium 25 seedlings from part henogenetic embryos were obtained of which 13 seedlings survived. Eleven seedlings were determined as haploid plants (2n = x = 8) and 2 seedlings were doubled haploid plants (2n = 2x = 16). All haploid and doubled haploid plants preserved chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) from A. galanthum. Segregation in different characters was observed among the haploid plants. The haploid and doubled haploid plants exhibited the different combinations of genes from shallot and common onion. Crossing of the doubled haploid plants with other shallot strains, common onion cultivars or related species may produce excellent F₁ hybrids for bulb production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spontaneous and induced chromosome doubling in gynogenic lines of onion (Allium cepa L.)

The presence of 2,4-dichlorophenoxyacetic acid (2,4-D) or naphthaleneacetic acid (NAA) and 6-benzyladenine (6—BA) in the medium is required to induce an acceptable yield of gynogenic embryos from unfertilized ovary and flower cultures in onion. Four different exposure times of ovary and flower cultures to exogenous growth regulators (15, 30, and 45 days, and the entire culture period) were assayed. The objective was to ascertain the effect of these substances and of their period of application on the formation of gynogenic embryos and on the yield of haploids. An exposure of 15 days was sufficient for ovaries and flowers to be stimulated towards the gynogenic response, whereas, for the remaining period of 30—80 days, the pro-embryos could easily grow on a growth-regulator-free medium. In the gynogenic material obtained, phenotypic differences were visible between genetically independent lines but not between plants within each line, even when they had a different ploidy level (n or 2n). Almost all lines obtained by gynogenesis were sterile. Only a small percentage (1%) became fertile through spontaneous chromosome doubling, and these produced 2—20 seeds each, giving normal plants. The recovery of fertility occurred more often during the generations of bulbils. To exploit this natural propensity towards diploidization in this phase, different amounts and numbers of applications of colchicine were evaluated in two experiments. The treatments corresponding to 10 and 100 mg/1 of colchicine applied for 24h gave the highest number of diploid cells in root tips but no diploidization occurred in the shoot apices. Three days of colchicine treatment at 10 mg/1 produced 46% of plants completely diploid in the shoot apex. The flower fertility of these doubled haploid plants is being evaluated.     

Heritability estimates and response to selection for the pungency and single center traits in onion

Summary Heritabilities of the pungency and single center traits were estimated in onion breeding populations using selection response and half-sib family analyses. Pungency was determined indirectly by measuring enzymatically produced pyruvic acid in individual bulbs. After one generation of selection, pungency was lowered by 8.1% and 8.9% in the populations 90-61-1 and 89-69-8, respectively, and realized heritabilities of 0.21 and 0.51 were estimated.Selection had no effect in lowering the pungency of population 90–62. Heritability estimates calculated through half-sib progeny analysis were 0.53, 0.48, and 0.25 for pungency in the populations 90-61-1, 90–62 and 89-69-8, respectively. The number of single centered onions was increased by 19% and 22% in populations 90–62 and 89-69-8, respectively, after one generation of selection, and the realized heritability estimates were 0.37 and 0.34, respectively.     

Production of Gynogenetic Plants of Onion (Allium cepa L.) after Crossing with Irradiated Pollen

The possibility of producing gynogenetic plants with pollen inactivated by irradiation was explored for the onion (Allium cepa L.). Two successive experiments included the use of cytoplasmic male sterility, recessive nuclear markers in the female plants and pollen irradiation of 150 Gy. Pollinations were achieved with the aid of flies and all seeds were harvested without the use of embryo rescue. Plantlets with recessive characters could be detected and they contained haploid and diploid cells. The development of these plantlets until flowering was observed under agronomic conditions. The use of doubled haploid (DH) plants in onion breeding is discussed.     

Control pollination and pollen management in Sesbania sesban (L.) Merr.

Sesbania sesban was amiable to controlled cross pollination when the emasculation and pollination operations were performed in the morning. A medium of 10% sucrose solution was found to be optimal for in vitro germination of the pollen grains. The pollen is quite tolerant of orthodox storage conditions enabling controlled crosses to be performed among accessions that flower in differing seasons of the year.     

Comparison of different pollen viability assays to evaluate pollen fertility of potato dihaploids

Summary The main obstacle in breeding potato (Solanum tuberosum L.) dihaploids is the severe limitation of male fertility. To determine pollen viability assays that correlate well to fertility in crosses, results of five different pollen viability assays were compared by correlation analysis with fruit and seed set characters in test crosses, and to pollen tube growth in situ (PL-test). The methods used were: staining the pollen cells with carmino acetic acid (CAA-test); in vitro pollen germination (PG-test); and detection of pollen staining rates after incubation with fluoresceine diacetate (FDA-test), 2-3-5-triphenyle tetrazolium chloride (TTC-test), and 5-bromo-4-chloro-3-indolyle--galactoside (X-Gal-test).The results of test crosses and pollen tube growth in situ correlated with the results of all other assays with the following ranking, from highest to lowest: enzyme activity assays (X-Gal-test, FDA-test, TTC-test), in vitro pollen germination (PG-test), and pollen staining by CAA. The newly developed X-Gal-test for monitoring -galactosidase activity showed the least variation of all assays investigated. Thus, this highly reproducible simple procedure is recommended for male fertility screening.Abbreviations B/F Berries obtained per 100 flowers - CAA Carmino acetic acid - FDA Fluoresceine diacetate - PG Pollen germination rate in vitro - PL Pollen tube growth in situ - S/B Seeds per berry - S/F Seeds per pollinated flower - TTC 2-3-5-triphenyle tetrazolium chloride - X-Gal 5-bromo-4-chloro-3-indolyle--galactoside     

Hybrid Tea-rose pollen. II. Inheritance of pollen viability

Summary Pollen viability was evaluated in about 500 seedlings originating from 31 crosses between nine commercial Hybrid Tea-rose varieties. The data indicated that pollen viability was inherited additively.     

Measuring onion cultivars with image analysis using inflection points

Summary The suitability of image analysis was studied to measure bulb characteristics for varietal testing of onions (Allium cepa L.). Eighteen genotypes were used, which covered a whole range of onion shapes, including some quite identical ones. The characteristic height and diameter were measured both by image analysis and by hand. The methods gave comparable results with regard to precision and discriminating power. The discriminating power of the image analysis method could be increased by adding a new characteristic: the relative position of the diameter.For determining the height of the bulb with image analysis two methods are described to locate the top of a bulb. One method is to use the location of the inflection point as the top of the bulb. For most bulb shapes, this point coincides with the top of the bulb as indicated by a crop expert. For ovate shaped onions, however, the inflection point is below the top of the bulb as indicated by the crop expert. The other method is to use the location of the intercept of the tangent at the inflection point with the length axis of the bulb as the top of the bulb. This point is always above the top as indicated by the crop expert. Moreover it seems to perform slightly worse for the discrimination of cultivars. Therefore the location of the inflection point as the top of the bulb is preferred.