Production of Polyploid Hydrangea Macrophylla via Unreduced Gamete Breeding

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.