Pre- and Post-fertilization Barriers in Interspecific Hybridization between Evergreen Azalea Species and Rhododendron uwaense H. Hara & T. Yamanaka

Reciprocal crosses between nine evergreen azalea species and Rhododendron uwaense were performed to clarify the pre- and post-fertilization barriers in this interspecific hybridization for the purpose of obtaining fragrant evergreen azaleas. Unilateral incompatibility appeared in this hybridization. When evergreen azalea species were used as a seed parent, many pollen tubes stopped elongating in a style and no seed could be obtained. The reverse crosses exhibited inhibition of pollen tube penetration into ovules, chlorophyll deficiency in cotyledons, and death of young seedlings in some cross combinations. These pre- and post-fertilization barriers reduced hybridization but did not arrest it completely. As a result, many putative hybrid seedlings could be obtained. RAPD analysis revealed that 10 of the putative hybrid seedlings examined (two vigorous plants selected from respective crosses of R. uwaense #1 x five evergreen azalea species) possessed specific bands derived from both parents. The plastid DNA (ptDNA) of these seedlings was inherited maternally except for seedlings of R. uwaense #1 x R. yedoense var. poukhanense, and the ptDNA of hybrid ones obtained from R. uwaense #1 x R. yedoense var. poukhanense was inherited paternally.