Production of Interspecific Hybrid Plants through Cotyledonary Segment Culture of Embryos Derived from Crosses between Hydrangea macrophylla f. hortensia (Lam.) Rehd. and H. arborescens L.

A sexual incompatibility mechanism prevents reciprocal crosses between Hydrangea macrophylla f. hortensia (Lam.) Rehd. and H. arborescens L. No viable hybrid plants have been produced by conventional hybridization because the progenies exhibit hybrid lethality at a young seedling stage. For circumventing hybrid lethality, we cultured cotyledonary segments of embryos derived from crosses of H. macrophylla and H. arborescens. The cotyledonary segments formed callus. Fifteen callus lines, which regenerated plantlets, were established from crossings of H. macrophylla 'Blue sky'×H. arborescens 'Annabelle'. However, only a single callus line among them regenerated plantlets which could be transplanted to soil. Hybridity of the regenerated plants was examined by morphological, cytological characteristics, and RAPD markers : morphologically, the regenerated plants appeared intermediate to the parents ; the regenerated plants were aneuploids having a chromosome number, 2n=42 ; whereas those of the parents H. macrophylla 'Blue sky' and H. arborescens 'Annabelle' were 2n=52 and 2n=38, respectively ; the RAPD banding patterns of the regenerated plants exhibited the same hybrid bands as the parents. The hybridity of the regenerated plants was probably confirmed by the above methods.