Staining for viability testing, germination and maturation of Sambucus nigra L. pollen in vitro.

Freshly released pollen of black elderberry (Sambucus nigra L.) was incubated under various culture conditions until germination was achieved. Optimal conditions for germination were determined and used for maturation of unicellular microspores in vitro. Staining with 5-diphenyltetrazolium bromide, propidium iodide and iodine potassium iodide was used to assess pollen viability, nuclear phase and maturation, respectively. The germination rate was highest when fresh pollen was agitated at 40 rpm in Petri dishes containing a liquid medium consisting of Brewbaker and Kwack salts, 15% (w/v) sucrose, 500 mg/l MES sodium salt, at pH 5.0; germination reached nearly 70% after only 1 h in culture. Under these conditions, and with addition of 200 mg/l glutamine, 260 mg/l cytidine and 500 mg/l uridine, uninucleate microspores developed into mature pollen at a 12% germination rate. Our report is the first demonstration of maturation of S. nigra microspores in vitro.

Transcriptome sequencing to produce SNP-based genetic maps of onion.

We used the Roche-454 platform to sequence from normalized cDNA libraries from each of two inbred lines of onion (OH1 and 5225). From approximately 1.6 million reads from each inbred, 27,065 and 33,254 cDNA contigs were assembled from OH1 and 5225, respectively. In total, 3,364 well supported single nucleotide polymorphisms (SNPs) on 1,716 cDNA contigs were identified between these two inbreds. One SNP on each of 1,256 contigs was randomly selected for genotyping. OH1 and 5225 were crossed and 182 gynogenic haploids extracted from hybrid plants were used for SNP mapping. A total of 597 SNPs segregated in the OH1 × 5225 haploid family and a genetic map of ten linkage groups (LOD ≥8) was constructed. Three hundred and thirty-nine of the newly identified SNPs were also mapped using a previously developed segregating family from BYG15-23 × AC43, and 223 common SNPs were used to join the two maps. Because these new SNPs are in expressed regions of the genome and commonly occur among onion germplasms, they will be useful for genetic mapping, gene tagging, marker-aided selection, quality control of seed lots, and fingerprinting of cultivars.

Quantitative determination of mosaic GFP gene expression in tobacco.

A specific form of gene silencing that was observed visually as a mosaic distribution of fluorescent and non-fluorescent cells apparently dispersed at random within tissues was found in a few green fluorescent protein (GFP)-transformed tobacco lines. To characterize this event quantitatively, we studied flow cytometric measurements in GFP-expressing and -silenced cells in T1 and T2 progeny of four selected plants. The proportion of silenced cells varied considerably among the T1 lines but with notable genotype differences. Mosaic expression was inherited into the T2 generation in which the majority of progenies tested exhibited a level of silencing similar to that of their T1 parental plants. However, in some T2 progenies segregation, evident as a decrease or increase in the proportion of fluorescent cells, was observed. We discuss several factors, such as copy number, promoter activity or polyploidy, that may be the possible causes of the gene silencing, but none sufficiently explain the appearance of the mosaic distribution.

Chromosome doubling procedures of onion (Allium cepa L.) gynogenic embryos.

A novel approach for chromosome doubling that consists of treating embryos instead of parts of micropropagated plants was investigated. Following 2-year trials, amiprofos-methyl (APM) was found to be superior to oryzalin on the basis of a lower toxicity, and we were able to narrow the range of concentrations of APM. The addition of 2% dimethyl sulphoxide (DMSO) and 1% Triton X-100 to 25 microM APM had no effect in all treatments. A final experiment with 6,658 embryos demonstrated that a 2-day treatment in liquid media supplemented with 50 microM APM was the most successful with respect to chromosome doubling-36.7% of the plants were diploid-but the survival rate was reduced to 52.5% of that of the non-treated control. A 2-day treatment in liquid medium supplemented with 25 microM APM or a 2-day treatment on solid medium with 50 microM APM resulted in the production of diploids at a frequency of 28.9% and 21.3%, respectively. These may represent alternative methods for chromosome doubling since compared to the untreated control these two treatments reduced the survival rate by only about 24%. Final ploidy and fertility of the large proportion of induced mixoploid plants (up to 30.3%) need to be evaluated in further studies.

Ploidy and morphological characteristics of Solanum tuberosum x Solanum phureja hybrids.

In attempt to induce doubled haploids in potato we studied interspecific hybrids between tetraploid Solanum tuberosum cultivars Igor, Jana, Vesna, Romano, Arinda, Fianna, Donald and Vital and Solanum phureja (clone IVP 48). Four out of eight cultivars produced 21 berries in total and 149 seedlings were obtained. Their ploidy was measured using flow cytometry. Analysis revealed 137 tetraploids, 10 triploids and 2 haploids. One haploid, 6 triploids and most of the tetraploids produced tubers. Nine out of 10 triploids were produced in a cross between cv. Igor and S. phureja. The vigour of the haploid plant was weak and produced characteristic long light yellow tubers. Triploid plants were characterized by a dark violet coloration of the stem, which was the same as the coloration of the S. phureja. Tubers had violet skin colour of various intensities and deep eyes. The majority of the tetraploid plants (135) were phenotypically similar to the S. tuberosum, while two plants had a similar violet stem and tubers as the triploids. Triploids were interspecific hybrids and tetraploids were produced by spontaneous chromosome doubling from S. tuberosum gametes. Two tetraploid plants expressing violet coloration might have been interspecific hybrids formed from 2n S. phureja gametes. Further studies are needed to confirm these assumptions.

Determination of aneuploids in hop (Humulus lupulus L.) using flow cytometry.

In order to study the possibility that high-resolution flow cytometry can be used for determination of aneuploids, different genotypes of Humulus lupulus were analyzed. Triploid cultivars are bred by hybridization between diploid and tetraploid lines, and as the result of this process, some aneuploids are occasionally also formed. We analyzed eight triploid cultivars and seven putative aneuploids. Triploid cultivars Cerera, Cicero, Celeia, Cekin, Blisk, Mt. Hood, Huller Bit. and Willamette (3x = 30) were measured for nuclear DNA content using Trifolium repens as reference. No significant differences among peak positions of triploid cultivars (having an average CV value per peak of 1.94%) were found. Measurement of nuclear DNA content was also performed for seven lines: 175/75, 89/113, 89/154, 91/215, 175/17, 89/87 and 91/74 previously determined by chromosome counting to be aneuploids (CV per peak was 1.41%). A statistically lower DNA content was found for line 175/75 and higher values were measured for lines 89/154, 89/113 and 91/215. Repeated chromosome counting revealed that the number of chromosomes in line 175/75 was 29, while lines 89/154, 89/113 and 91/215 possessed 31 chromosomes. The other lines were identified as triploids. We conclude that flow cytometry can be efficiently used for determination of aneuploidy in Humulus lupulus.

Random amplified polymorphic DNA analysis, genome size, and genomic in situ hybridization of triploid viviparous onions

Triploid viviparous onions (Allium cepa L. var. viviparum Metzg. (ALEF.), auct.), (2n = 3x = 24), are known in some countries only as a rare relic crop, while in other parts of the world they are still traditionally or even commercially cultivated. Results indicating an identical random amplified polymorphic DNA (RAPD) banding pattern and the same DNA content (2C = 43.4 pg) establish the high genetic similarity and the unique origin of the Croatian clone Ljutika and the Indian clone Pran. In order to determine the parental Allium species of these natural triploid hybrids, genomic fluorescent in situ hybridization (GISH) was applied. Biotinylated genomic DNAs from six diploid Allium species (A. cepa L., A. fistulosum L., A. roylei Stearn, A. vavilovii M. Pop. et Vved., A. galanthum Kar. et Kir., A. oschaninii O. Fedtsch.) were used as probes in this study. While probes obtained from genomic DNA of A. cepa, A. vavilovii, and A. roylei hybridized to somatic chromosomes of Ljutika probes from A. fistulosum, A. galanthum, and A. oschaninii did not. The DNA probes of A. cepa and A. roylei each completely or predominantly labelled one genome (eight chromosomes). A few chromosomes, the markers of the triploid karyotype, were not completely labelled by any probe applied. Our GISH results indicate that triploid viviparous onions might possess a complex triparental genome organization.

Effect of media components on the gynogenic regeneration of onion (Allium cepa L.) cultivars and analysis of regenerants.

Two separate experiments were performed to analyze the effects of different media on gynogenic regeneration in four onion cultivars. In a two step flower/ovary culture procedure, 2,4-dichlorophenoxyacetic acid added to the induction medium was superior to phenylacetic acid in the highly regenerating cultivar, while the effect of thidiazuron in the regeneration medium was generally optimal in higher (2 mg/l) rather than in lower (0.2 mg/l) concentrations. Gellan-gum was compared to agar solidified media. A higher number of regenerants was achieved on the former, but an undesirable hyperhydricity of regenerants formed on gellan-gum solidified media greatly reduced the final survival of formed embryos. Analysis of the time interval needed for regeneration showed high variability (from 46 to 152 days after inoculation), which was particularly pronounced in genotypes with lower regeneration capacity. Simpler isozyme patterns of regenerants showed that all analysed regenerants of the cultivar with a high regeneration capacity were homozygous, while in the other three cultivars, a considerable percentage (11.1 to 36.4%) of heterozygous regenerants were also detected. Ploidy analysis of the regenerants with simpler isozyme patterns revealed that the majority of lines remain haploid. Identification of 2 homozygous triploid regenerants demonstrated that as in androgenesis, nuclear fusions can occur during gynogenic haploid regeneration.

Gynogenic lines of onion (Allium cepa L.): evidence of their homozygosity.

Haploid induction via gynogenesis offers the possibility of using doubled haploid (DH) inbred lines in onion breeding. A first DH line that originated from the open-pollinated (OP) cultivar 'Dorata di Parma' was obtained after overcoming difficulties associated with the haploidy of the regenerants. Spontaneous chromosome doubling occurs seldom in onion. The first DH line obtained was cloned and selfed to produce sufficient seeds for genetic studies. The homozygosity of the DH gynogenic line was revealed on the basis of the low standard deviations of the bulb traits polar diameter, shape index and weight with respect to those of the S1 line or the OP cultivar. In the DH line, moreover, segregation of RAPD and alpha esterase markers was not noted. Out of four primers revealing polymorphism at 16 ge-netic loci in the OP cultivar 'Dorata di Parma', none produced polymorphism in the DH gynogenic line. The Est-1 locus, homozygous in 22 plants (Est-1 (1/1) in 3 and Est-1 (2/2) in 19) and heterozygous (Est-1 (1/2)) in 11 plants of the OP cultivar, always carried the same alleles in the DH line. We also tested genetic stability during micropropagation of a second halpoid line obtained via gynogenesis from var. 'Senshyu Yellow'. Seventeen plants of this line were tested to detect changes occurring during the tissue culture process. Again no polymorphism was observed. The high genetic homogeneity observed in the two gynogenic lines of onion could be related to the absence of the callus phase during the gynogenic process.