Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2016
Publication Date: 2/17/2017
Citation: Alexander, L.W. 2017. Production of triploid Hydrangea macrophylla via unreduced gamete breeding. HortScience. 52:221-224.
Interpretive Summary: Hydrangea varieties with resistance to pests, disease, and drought would significantly reduce the environmental and budgetary footprint associated with Hydrangea production. In an effort to improve these traits, we have produced diploid and triploid H. macrophylla from controlled crossed of diploid parents. Triploid H. macrophylla plants were significantly more compact with thicker stems and larger leaves than diploid plants. We established a link between ploidy and stomate size, showing that triploids have significantly larger stomata than diploids. As in other species, changes in plant form and stomatal density may lead to increased disease and drought resistance in Hydrangea. These results have wide-ranging impacts on Hydrangea breeding improvement such as the ability to create a “bridge” for breeding cultivars and species of differing ploidy levels, and the ability to change the ploidy level of popular cultivars. Superior plants from this population will be released as cultivars that will provide nursery growers an effective way to reduce time, cost, and environmental impact of production.
Technical Abstract: Hydrangea macrophylla (Thunb.) Ser., florist’s or bigleaf hydrangea, is the most economically important member of the Hydrangea genus, which accounted for 73,000,000 in US nursery sales in 2007. Diploid and triploid cultivars exist and there is some evidence triploidy leads to larger plant and floral structures. The diploid cultivar, H. macrophylla ‘Trophee’, was previously shown to have a bimodal pollen size distribution which may be indicative of unreduced gametes. We used H. macrophylla ‘Trophee’ as a parent in a series of crosses with other diploid H. macrophylla cultivars. The objective of this study was to evaluate five reciprocal full-sibling H. macrophylla families for ploidy and phenotype, determine the impact of ploidy on phenotype, and determine the efficacy of unreduced gamete breeding. Diploids, triploids, and a single tetraploid were found in the offspring pool with peak means of 51.2 ± 1.5, 72.7 ± 1.8, and 88.5, respectively. All offspring from crosses with ‘Trophee’ as the female parent were diploid as expected. The full-sibling family with ‘Trophee’ as the male parent contained 94% triploids, supporting the hypothesis that the bimodal pollen size distribution of ‘Trophee’ reflects the presence of unreduced gametes. Diploids and triploids were not significantly different in plant height (p=0.58), stem width (p=0.99), or inflorescence size (p=0.67). The single tetraploid in the offspring pool was significantly shorter (p=.025) and had significantly narrower stems (p=.038) than the diploids and triploids. Triploids had significantly larger stomata (8.29±1.4 µM2) than diploids (5.5 µM2) or tetraploids (6.0 µM2), where tetraploids had a higher number of stomata per unit area (144) compared to diploids (76 ± 24) and triploids (45 ± 16, p<0.001). These results establish a link between ploidy and phenotype in plants of similar genetic background and support the efficacy of unreduced gametes in polyploidy breeding. We also report the first production of a tetraploid Hydrangea macrophylla using traditional controlled pollination breeding.