Cross Compatibility Studies in Lilac (Syringa L.)

Lilacs represents a group of ornamental group trees and shrubs in Oleaceae consisting of 22 to 30 species from two distinct centers of diversity, the highlands of East Asia and the Balkan-Carpathian region of Europe. Current phylogenies recognizes five series within Syringa: Pubescentes, Villosae, Ligustrina, Pinnatifoliae, and Syringa. Intraspecific and interspecific hybridization are proven methods for developing cultivars with improved flowering, new foliar phenotypes, and improved habits. However, interseries hybridization has proven more difficult with only successful hybrids produced from crosses between taxa in series Syringa with S. pinnatifolia in series Pinnatifoliae. Though hundreds of improved lilac cultivars have been introduced, fertility and cross compatibility among cultivars, species, and series have yet to be investigated in a formal study.

Over three years, a cross compatibility study was performed on elite cultivars and species in series Syringa, Pubescentes, and Villosae. Interseries, Interspecific, and intraspecific crosses were attempted resulting in a total of 27,749 pollinations with an average of 224 flowers pollinated per cross combination. For each combination, data were recorded on number of inflorescences and flowers pollinated, number of fruits, number of seed, number of seedlings germinated, and number albino seedlings. Fruit and seed were produced from interseries crosses; however, no viable seedlings were recovered. Nearly 2,000 viable seedlings were recovered from interspecific and intraspecific combinations in series Syringa, Pubescentes, and Villosae. All albino progeny were produced from crosses S. pubescens ‘Miss Kim’ as the female parent.

Since genome variation may contribute to cross compatibility, a flow cytometry survey of relative genome sizes was performed on the parent population. Most parents had relative 2C genome sizes between 3.0 and 3.5 pg. Several cultivars in series Syringa including ‘Aucubaefolia’, ‘Agincourt Beauty’, and ‘President Grevy’ had significantly larger genome sizes (4.94 pg, 4.90, 4.87, respectively). In addition, pollen screened from a subset of parents in series Syringa revealed one cultivar that produced 7% unreduced pollen approximately 30% larger than reduced pollen. Further, a single seedling produced from this cultivar had a relative 2C genome size of 5.76 pg, the largest known genome size recorded in lilac.

This study represents the most comprehensive investigation lilac hybridization. Knowledge on cross compatibility relationships, the presence of unreduced gametes, and genome size variation will aid future efforts in new lilac cultivar development.