Breeding and Genetics of Lilacs and Hardy Hibiscus Public Deposited


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  • Lilacs are a group of ornamental trees and shrubs in the Oleaceae family consisting of 22 to 30 species. There are six series within genus Syringa: Pubescentes, Villosae, Ligustrae, Ligustrina, Pinnatifoliae, and Syringa. Fertility and cross-compatibility among cultivars, species, and series have yet to be formally investigated. Over three years, a cross-compatibility study was performed using elite cultivars and species of shrub-form lilacs in series Syringa, Pubescentes, and Villosae. We report the success of each of these combinations and the fertility estimates of viable crosses. This study is a comprehensive investigation of lilac hybridization, and the knowledge gained on cross-compatibility will aid future efforts in lilac cultivar development.Genome size variation can be used to investigate biodiversity, genome evolution, and taxonomic relationships among related taxa. In addition, plant breeders use genome size variation to identify parents useful for breeding sterile or improved ornamentals. Reports conflict on genome evolution, base chromosome number, and polyploidy in lilac. Flow cytometry was used to estimate holoploid (2C) genome sizes in series, species, cultivars, and seedlings from parents with three ploidy combinations: 2x x 2x, 2x x 3x, and 3x x 2x. Monoploid (1Cx) genome sizes were calculated by dividing 2C genome size by ploidy, which was confirmed in a subset of taxa using root tip microscopy. Pollen diameter was measured toinvestigate the frequency of unreduced gametes in diploids and triploids. Interploid crosses between ‘Blue Skies’ (2x) and ‘President Grévy’ (3x) produced an aneuploid population with variable 2C genome sizes. One viable seedling was recovered from a cross between ‘President Grévy’ (3x) and ‘Sensation’ (2x). This near pentaploid (5x) seedling had a larger 2C genome size than either parent, and the largest 2C genome size currently reported in lilac. Pollen diameter measurements revealed that ‘Sensation’ produced 8.5% unreduced pollen. Increased ploidy may provide a mechanism for recovering seedlings from incompatible taxa in lilac breeding.Common lilac, Syringa vulgaris, is an important flowering shrub that accounts for a large share of spring sales in the U.S. nursery industry. However, little research has focused on shortening generation time for lilac breeders. In a previous cross-compatibility study, observations revealed that first-year hybrid seedlings undergo a quiescent phase of growth, producing few leaves but an extensive root system. This study investigated the effects of six germination and post-germination treatments of green seed and dry, dehisced seed on seed germination and subsequent growth in lilacs. Green seed extracted 20 weeks after pollination had the highest germination rate and an increase in vegetative growth compared to controls. Our results indicate that green seed sowing may provide a new tool for shortening juvenility and reduced breeding time in common lilac.Remontancy (reblooming) and disease resistance are two important traits in the dwarf lilacs (Syringa pubescens). Marker-assisted selection could prove useful at producing more disease-resistant, floriforous lilacs for future breeders. To aid future efforts at at marker discovery, genotyping-by-sequencing was applied to a bi-parental mapping population from S. meyeri ‘Palibin’ x S. pubescens Bloomerang® which varies for remontancy and resistance to bacterial blight. SNP-based genetic linkage maps were created for each parent, and maps will continue to be improved with further sequence data. Future efforts to phenotype the mapping population will be combined with these findings for marker-trait association.Althea (Hibiscus syriacus) is an ornamental shrub prized for its winter hardiness and large colorful summer flowers. Althea are primarily tetraploids (2n = 4x = 80) with higher level polyploids reported from experiments with spindle-fiberinhibitors. Previous studies report anatomical variation among althea polyploids, including changes in stomata size. The purpose of this study was four-fold. The first was to identify genome size and ploidy variation in althea cultivars via flow cytometry and root tip chromosome counts. The second was to create a ploidy series consisting of 4x, 5x, 6x, and 8x cytotypes using a combination of interploid hybridization and autopolyploid induction via colchicine and oryzalin. The third was to investigate the ploidy series for variation in stomatal guard cell length, stomatal density, and copy number of fluorescent rDNA signals. The fourth was to investigate segregation patterns in rDNA signals in a subset of pentaploid seedlings. Results of this study revealed ploidy differences among available cultivars. Polyploid induction and interploid hybridation were successful for producing a ploidy series that varied in stomata size, stomata density, and number of 5S and 45S rDNA signals. The rDNA loci confirmed ploidy levels in each cytotype of our ploidy series, and random segregation of rDNA loci provides evidence of random chromosome segregation in interploid hybrids of althea.Despite its attractive, ornamental flowers, althea produces capsules with numerous, fertile seeds that germinate and cause a nuisance in production and the home landscape. Breeding for sterile forms of althea has long been a goal for Hibiscus breeders, yet many popular “sterile” cultivars have been reported as weedy. The purpose of this study was to evaluate female and male fertility for tetraploid and hexaploid cultivars, and to evaluate the female fertility of pentaploid seedlings resulting from 4x x 6x and 6x x 4x crosses. Self- and cross-incompatibilities were discovered, as was variation in seeds per capsule and seeds per pollination. In addition, significant differences were found among flower forms (single, semi-double, and double) for fertility estimates. Double-flowered forms had reduced female fertility, which may indicate that breeding for increased petaloid stamen may result in a reduction in female fertility. Previously reported sterile taxa were also found to be fertile, including ‘Aphrodite’, ‘Diana’, ‘Helene’ and ‘Minerva’. Two hexaploids, ‘Pink Giant’ and Raspberry Smoothie™, were found to have reduced female fertility compared to tetraploids. Fertility testcrosses of pentaploid seedlings revealed a reduction in fertility compared to controls. The reduction in fertility ofpentaploids will likely lead to new, near sterile cultivars for the nursery industry. The combination of double flowers with pentaploid cytotypes will likely lead to completely sterile cultivars of althea.Although floral traits are most important for breeders of althea, little is known about their segregation patterns. The objectives of this study were to determine segregation patterns in eyespot presence, flower color, and flower form. Over four years, thousands of flowering seedlings were observed representing F1, F2, and backcross families. Based on our results, we propose that eyespot presence is controlled by a single locus and that a recessive allele called spotless results in a complete elimination of color. The gene controlling spotless is likely located upstream in the flavonoid biosynthetic pathway. We also propose that flowers with white to blush-pink petal body color and a red eyespot are controlled by a single recessive allele called geisha. This trait exhibits incomplete dominance and is under epistatic control by spotless. It is likely located downstream in the delphinidin biosynthetic pathway, responsible for lavender, dark pink, and blue pigments. In addition to color segregation, depth of color irrespective of hue (CIE L*) was also investigated (spotless and geisha seedlings removed). The deepest pigments were measured in crosses among hexaploid ‘Pink Giant’, taxa homozygous dominant for geisha, and taxa heterozygous for geisha. Conversely, the lightest pigments were observed in crosses between taxa homozygous recessive for geisha and taxa heterozygous for geisha. Future efforts at eliminating the geisha allele from a breeding population may allow for quantitative improvement in total anthocyanin production. Observations on petal number inheritance revealed that seedlings produced a continuous distribution of petal numbers between the petal numbers of the two parents, with occasional transgressive segregants. The highest average petal numbers were found in seedlings resulting from the cross of double-flowered taxa. Flower size (petal area), varied significantly among cross combinations and flower forms. The largest petals were observed in the seedlings of single-flowered by double-flowered crosses. Concomitant upregulation or expression of genes controlling laminar growth in stamen may not only result in petaloid stamen, but may also result in increased laminar growth in the true petals, resulting in wider,overlapping petals. However, further work must be undertaken to eliminate environmental effects on flower size estimates.
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