Genome size, flowering, and breeding compatibility in Osmanthus

Authors: Alexander, Lisa

Submitted to: Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2022
Publication Date: 1/3/2023
Citation: Alexander, L.W. 2023. Genome size, flowering, and breeding compatibility in Osmanthus. Horticulturae. https://doi.org/10.3390/horticulturae9010056.
DOI: https://doi.org/10.3390/horticulturae9010056

Interpretive Summary: Ornamental flowering trees enjoy worldwide popularity for their impact in the landscape, and few flowering tree species boast the level of fragrance found in Osmanthus (tea olive) species. These species are used prolifically as specimen trees, hedges, and fragrant verges in USDA Hardiness zones 7 - 10 where they thrive. Further breeding improvement is necessary in Osmanthus to increase cold-hardiness and introgress the smooth leaf and upright form of O. fragrans into cultivars adapted for U.S. landscapes. However, breeding improvement is hindered by few available cultivars and a lack of female or perfect flowers. U.S. National Arboretum scientists in McMinnville, TN evaluated thirty-four genotypes representing four species for genome size, floral morphology, pollen viability, and self- and cross-compatibility to provide a roadmap for Osmanthus hybrid breeding. All genotypes were diploid with 2C genome sizes ranging from 2.89 to 3.21 pg. Perfect flowers were observed in all O. armatus and 82% of O. heterophyllus genotypes, while all observed O. fortunei and O. fragrans genotypes had male-only flowers. All genotypes produced viable pollen, and all cross combinations were successful except self-pollinations, providing evidence of a self-incompatibility system in O. armatus and O. heterophyllus. Current breeding efforts in the U.S. should focus on importing or developing cultivars and populations with perfect flowers, lengthening flowering times through photoperiod and/or temperature, and improving germination rates of seed. Extending the range of Osmanthus species into more cold-hardy climates through hybrid breeding would open new opportunities for adoption and use of these species by growers, landscapers, and the public.

Technical Abstract: Extending the range of Osmanthus species into more cold-hardy climates would open new opportunities for adoption and use of these species by growers, landscapers, and the public. Breeding improvement is hindered by few available cultivars and a lack of female or perfect flowers. The objectives of this study were to evaluate floral morphology and pollination biology of Osmanthus species available in the U.S. market. Thirty-four genotypes representing four species were evaluated for genome size, floral morphology, pollen viability, and self- and cross-compatibility. All genotypes were diploid with 2C genome sizes ranging from 2.89 to 3.21 pg. Pollen viability based on staining and in vitro germination ranged from 8% to 98% and 6% to 53%, respectively. Perfect flowers were observed in all O. armatus and 82% of O. heterophyllus genotypes. All observed O. fortunei and O. fragrans genotypes had male-only flowers. Pollen germination was observed on stigmas of 94.2 % of O. armatus and O. heterophyllus flowers collected 24 hours after cross- or self-pollination. There was a significant association between cross type and percentage of flowers with pollen tubes reaching the ovaries; after 72 h, pollen tubes had reached the ovaries of in 67% of intraspecific crosses, 78% of interspecific crosses, and 0% of self crosses ('2 = 26.5, p < 0.001). This study provides evidence of a self-incompatibility system in O. armatus and O. heterophyllus and provides insights into opportunities and challenges for Osmanthus hybrid breeding.